# Is there a consensus as to where terrrestrial planet atmospheres in our solar system came from?

I tried to research this and I'm not sure if there is a consensus where the $CO_2$, $N_2$, etc. comes from on terrestrial planets like Venus, Earth and Mars. Possible sources would be accretion from the protoplanetary disk, outgassing (which I would think would require plate tectonics/ molten core) or reactions at the surface caused by stellar radiation. Also, reactions in the atmosphere can alter the composition. Would it be a combination of all of the above and, if so, do we know if one of the processes dominated over the others? Does anybody have a good link that discusses this?

A quite recent study about the origin and development of terrestrial planet atmospheres is the 2015 paper "The Atmospheres of the Terrestrial Planets: Clues to the Origins and Early Evolution of Venus, Earth, and Mars" (Baines et al.) includes a fairly comprehensive analysis of chemical evidence from Earth and from Martian meteorites and observations of the atmosphere of Venus.

The general consensus is that much of the atmospheric gases were brought through comet and asteroid impacts.

The pre-planetary disk would have been centrifugally sorted. Many iron, stone, carbonacious meteorites, watery, ice-iron-rock agglomerates would have brought a very diverse amalgam of different stones, ices, and room temperature gas elements, throughout the formation. CO2 and N2 objects are a bit less stable in direct solar radiation at 1 AU. CO2 and N2 would have pounded into the world in considerable quantities throughout the accretion process, with a wide range of particulate sizes from microns to kilometers in diameter without a marked difference in element type from start to end.

## Introduction to Solar System Astronomy (ASTR 1304 54702)

An introduction to present theories about the structure and evolution of the solar system, compared to other models and theories since antiquity. A survey of the Sun, planets, moons, rings, asteroids, comets and debris in our solar system. The possibility of life in the Universe.

HOUSTON COMMUNITY COLLEGE

COURSE SYLLABUS FOR ASTR 1304 Introduction to Solar System Astronomy

Spring 2018 Class Number 54702

Course Level First Year (Freshman)

Course Semester Credit Hours (SCH) (lecture, lab) 4 (3 lecture, 1 lab)

Total Course Contact Hours 96

Course Length (number of weeks) 16

Instructor contact information (phone number and email address)

Course Description:

Study of the sun and its solar system, including its origin. May or may not include a laboratory. (Cross- listed as PHYS 1404, 1304, & 1104)

HCC Catalog Description

An introduction to present theories about the structure and evolution of the solar system, compared to other models and theories since antiquity. A survey of the Sun, planets, moons, rings, asteroids, comets and debris in our solar system. The possibility of life in the Universe.

Course Prerequisite(s)

Must be placed into GUST 0341 (or higher) in reading and placed into MATH 0308 (or higher). Core curriculum course Credit: 4 (3 lecture, 1 lab) Academic Discipline Program Learning Outcomes 1. Program SLO #1:

1. Demonstrate understanding of the fundamental concepts of Astronomy Demonstrate understanding of the fundamental principles underlying astronomy including concepts and methods of inquiry at an appropriate level. Topics include, but are not limited to, the Scientific Method, The workings of the Solar System, Properties (evolution ) of Stars and Galaxies.

Solve conceptual and numerical problems in Astronomy Solve conceptual and numerical problems through the recognition of the type of problem at hand, analysis of relevant information, proper application of concepts and techniques applying mathematical tools at an appropriate level. Students should demonstrate improvement in problem solving skills as they progress through courses in the program.

Demonstrate appropriate laboratory skills Demonstrate appropriate laboratory skills including proper use of basic measuring devices, interpretation of laboratory directions and analysis of data obtained using appropriate tools, such as graphical/tabular methods using computers.

Develop interpersonal communication skills Demonstrate an ability to work independently and/or as part of a team through participation in laboratory activities as well as assigned projects.

Course Student Learning Outcomes (SLO)

Upon successful completion of this course the student should be able to:

1. Develop an appreciation for the nature of science and the scientific method. 2. Demonstrate an understanding of the modern theories about the origins, structure and evolution of the solar system. 3. Understand properties of planets, and their moons. 4. Apply the scientific method to the study of the universe, and in varying degrees, to the student’s own interest and particular field of work or study.

Learning Objectives (Numbering system linked to SLO)

Upon completion of this course the student should be able to:

1. Compare and contrast the size of the planet Earth to the size of the solar system and the Milky Way Galaxy. 1.2 Distinguish among astronomical unit, light year and parsec. 1.3 Name a few of the constellations, and relate brightness of stars to their size and distance. 1.4 Describe the cycles of the moon and state the conditions for solar and lunar eclipses. 2.1 Explain the difference between heliocentric and geocentric models of the universe. 3.1 Demonstrate knowledge of the basic laws of physics that pertain to the study of the bodies of the solar system. 3.2 Compare and contrast the characteristics of the terrestrial planets, and demonstrate understanding of the causes of their similarities and differences. 3.3 Compare and contrast the characteristics of the jovian planets, and demonstrate understanding of the causes of their similarities and differences. 3.4 List the differences between the terrestrial and jovian planets, and of how those differences came to be. 3.5 Discus the properties of the lesser bodies of the solar system. 4.1 Describe the current best scientific explanation of the origin of the solar system
2. SCANS and/or Core Curriculum Competencies

3. Reading, Speaking/Listening, Critical Thinking, Computer/Information Literacy

HCC Important Dates Spring Semester 2018

January 16, 2018……………………….Spring Semester Classes Begin

April 3……………………………………….Spring 2018 Reg. 16 Wk Last Day to withdraw

May 13……………………………………..Spring 2018 Reg. 16 Wk. semester ends

Note: Course Calendar (Tentative Schedule. May be subject to change as the semester progresses

WEEK #/ CHAPTER QUIZ/TEST

Week 1: Note (Starting 1/21/2018)

Part I – Exploring the Sky

2 – The Sky 4 The Stars 5 The Sky and Its Motion 6 The Cycles of the Sun 7 Astronomical Influences on Earth’s Climate

Quiz 1 on Chapters 1 and 2 (Posted 1/21 – Due 1/27)

3 – Cycles of the Moon • The Changeable Moon • Lunar Eclipses • Solar Eclipses • Predicting Eclipses

Quiz 2 on Chapter 3 (Posted 1/28 – Due 2/3)

4 – The Origin of Modern Astronomy • The Roots of Astronomy • The Copernican Revolution • Planetary Motion • Galileo Galilei • Modern Astronomy

5 – Gravity • Galileo and Newton • Orbital Motion and Tides • Einstein and Relativity

Quiz 3 on Chapters 4 and 5 (Posted 2/4– Due 2/10)

6 – Light and Telescopes 24 Radiation Information from Space 25 Optical Telescopes 26 Special Instruments 27 Radio Telescopes 28 Modern Astronomy

7 – Atoms and Starlight • Atoms • The Interaction of Light and Matter • Solar Spectra

Quiz 4 on Chapter 6 +7 (Posted 2/11– Due 2/17)

8 – The Sun • The Solar Atmosphere • Nuclear Fusion in the Sun • Solar Activity

TEST I on Chapters 1 to 7. 2/23 – 2/24, Online.

Quiz 5 on Chapter 8 (Posted 2/18– Due 2/24)

10 – The Origins of the Solar System • The Great Chain of Origin • A Survey of the Solar System • The Story of Planet Building • Planets Orbiting Other Stars

11 – Earth - The Standard Comparative Planetology • A Travel Guide to the Terrestrial Planets • Earth as a Planet • The Solid Earth • Earth’s Atmosphere

Part IV – The Solar System Quiz 6 on Chapters 10 and 11 (Posted 2/25 – Due 3/3)

12 – The Moon and Mercury: Comparing Airless Worlds • The Moon • Mercury

Quiz 7 on Chapter 12 (Posted 3/4 – Due 3/10)

13 – Comparative Planetology of Venus and Mars • Venus • Mars • The Moons of Mars

No other assignment due to Spring Break

14 – Comparative Planetology of Jupiter and Saturn • A Travel Guide to the Outer Planets • Jupiter • Jupiter’s Family of Moons • Saturn Moons

Quiz 8 on Chapters 13 and 14 (Posted 3/18– Due 3/24)

TEST II on Chapters 8 and 10 to 12 3/23 – 3/24, Online

Week 10 - 11:

15 –Uranus, Neptune and the Dwarf Planets • Uranus • Neptune • The Dwarf Planets

Quiz 9 on Chapters 14 and 15 (Posted 3/25– Due 3/31)

Lab 8 –Atmospheric Retention Quiz (Posted 3/25 – Due 3/31)

Week 12 -13:

16 – Meteorites, Asteroids and Comets 1.5 Meteorites, Asteroids and Meteoroids 1.6 Asteroids 1.7 Comets 1.8 Asteroid and Comet Impacts

Week 14 - 15:

17 - Astrobiology: Life on Other Planets 1.9 The Nature of Life 1.10 Life in the Universe 1.11 Intelligent Life in the Universe

Quiz 10 on Chapters 16 and 17 (Posted 4/1– Due 4/7)

Week 16: Final Examination (Comprehensive) 4/20 - /21 Online

Instructional Methods

Distance Education – Internet Moodle will be used as the Learning Management System.

Student Assignments

Laboratory assignments will include working with a number of virtual laboratories published by the University of Nebraska (NAAP, the Nebraska Astronomy Applets Project) and available on line at http://astro.unl.edu/naap/.

Information about accessing the individual labs and on how to carry out the assignments has been posted on Canvas. Due dates have been posted on the schedule above, and late submissions will not be accepted. There will be no grade for completion of the laboratory guides posted, but you will be rather graded on the basis of a Quiz posted at the end of each laboratory and available online for a week.

On the theoretical side of the course, homework sets will be posted on Eagle Online and will be available online for, also, a week’s time.

Student Assessment(s)

The overall score is based on the following: • Two regular exams 40% • Chapter Homework/Quizzes 20% • Final Exam 40%

Instructor’s Requirements

Exams and Make-up Policy

Examinations will consist of three non-cumulative regular exams (60 %) plus a comprehensive final (20%). Make-up exams will not be given, so make every effort to take the exams on their scheduled dates. In the event that you must miss a regular exam, the grade made on the final exam will count as the grade for the missed exam (for one missed exam only), and the final course grade will be calculated accordingly. If you do not miss any of the regular exams, your lowest exam score will be replaced by your final exam score if the final exam grade is higher. The final exam will be comprehensive, i.e., it will cover all of the material from the whole semester. Also note that the final is compulsory, i.e. no student can be exempted.

During the semester homework assignments will be posted on Eagle Online on chapters covered. The average grade for homework will count toward 20% of the Final Grade. Answers to homework or exams questions copied/pasted from the textbook or Internet sources will receive no credit. Make sure you write answers in your own words. Also note that once due dates for assignments expire, they will not be reopened. So, make it a point not to miss due dates!

Program/Discipline Requirements

At the program level, the Physics Discipline strives to accomplish the Program Learning Outcomes, Student Learning Outcomes, and Learning Objectives as described above. We desire that you receive a challenging and rewarding experience in your Physics classes at HCC which will prepare you well for future physics and related science courses that you may take in the future.

A = 100 – 90……………………………………4 points per semester hour

B = 89 – 80: …………………………………….3 points per semester hour

C = 79 – 70: …………………………………….2 points per semester hour

D = 69 – 60: …………………………………….1 point per semester hour

59 and below = F………………………………..0 points per semester hour FX = Fail to withdraw before the withdrawal date … 0 points per semester hour IP (In Progress) …………………………………………………….0 points per semester hour W(Withdrawn)……………………………………………………..0 points per semester hour I (Incomplete)……………………………………………………….0 points per semester hour AUD (Audit) ………………………………………………………….0 points per semester hour IP (In Progress) is given only in certain developmental courses. The student must re-enroll to receive credit. COM (Completed) is given in non-credit and continuing education courses. To compute grade point average (GPA), divide the total grade points by the total number of semester hours attempted. The grades “IP,” “COM” and “I” do not affect GPA.

See the above descriptions of the exams, quizzes/homework, and final. The course grade is based on these four criteria according to the Assessment section above.

Textbook Information

Textbook The Solar System Author(s): Seeds/Backman Publisher: Cengage Learning Copyright year: © 2016 ISBN-10: 1305120760 | ISBN-13: 9781305120761

Access Student Services Policies on their Web site: http://hccs.edu/student-rights

Disability Support Services (DSS) “Any student with a documented disability (e.g. physical, learning, psychiatric, vision, hearing, etc.) who needs to arrange reasonable accommodations must contact the Disability Services Office at the respective college at the beginning of each semester. Faculty are authorized to provide only the accommodations requested by the Disability Support Services Office.”

If you have any special needs or disabilities which may affect your ability to succeed in college classes or participate in any college programs or activities, please contact the DSS office for assistance. At Southwest College, contact Dr. Becky Hauri, 713-718-7909. Contact numbers for the other HCC colleges are found in the Annual Schedule of Classes, and more information is posted at the HCC web site at Disability Services.

Title IX prohibits discrimination on the basis of sex including pregnancy and parental status in educational programs and activities. If you require an accommodation due to pregnancy please contact an Abilities Services Counselor. The Director of EEO/Compliance is designated as the Title IX Coordinator and Section 504 Coordinator

Students are responsible for conducting themselves with honor and integrity in fulfilling course requirements. Penalties and/or disciplinary proceedings may be initiated by College System officials against a student accused of scholastic dishonesty. “Scholastic dishonesty” includes, but is not limited to, cheating on a test, plagiarism, and collusion.

“Cheating” on a test includes:

• Copying from another student’s test paper • Using materials during a test that are not authorized by the person giving the test • Collaborating with another student during a test without authority • Knowingly using, buying, selling, stealing, transporting, or soliciting in whole or part the • contents of an administered test • Bribing another person to obtain a test that is to be administered.

“Plagiarism” means the appropriation of another’s work and the unacknowledged incorporation of that work in one’s own written work offered for credit.

“Collusion” means the unauthorized collaboration with another person in preparing written work offered for credit

“Students are responsible for conducting themselves with honor and integrity in fulfilling course requirements. Disciplinary proceedings may be initiated by the college system against a student accused of scholastic dishonesty. Penalties can include a grade of "0" or "F" on the particular assignment, failure in the course, academic probation, or even dismissal from the college. Scholastic dishonesty includes, but is not limited to, cheating on a test, plagiarism, and collusion.” In this class, the penalty for willful cheating on exams is a grade of F in the course. This is the standard policy of the Natural Sciences Attendance Policy As stated in the HCC Catalog, all students are expected to attend classes regularly. Students in DE courses must log into their Eagle Online class at least twice a week or they will be counted as absent. Just like an on-campus class, your regular participation is required.

Although it is the responsibility of the student to withdraw officially from a course, the professor also has the authority to block a student from accessing Eagle Online, and/or to withdraw a student for excessive absences or failure to participate regularly. DE students who do not log into their Eagle Online class before the Official Day of Record and complete their Syllabus quiz will be automatically dropped for non-attendance. Completing the DE online orientation does not count as attendance.

Policy Regarding Multiple Repeats of a Course “NOTICE: Students who repeat a course three or more times may soon face significant tuition/fee increases at HCC and other Texas public colleges and universities. If you are considering course withdrawal because you are not earning passing grades, confer with your instructor/counselor as early as possible about your study habits, reading and writing homework, test-taking skills, attendance, course participation, and opportunities for tutoring or other assistance that might be available.”

Last Day for Administrative and Student Withdrawals For 16-weekFall ’16 classes, this date is November 13. Any student who is contemplating withdrawing from the class is urged to see me first! You may be doing better than you think. Either way, I want to be accessible and supportive. I do not believe in "weed out" classes, and I consider you to be much more than just a name or number! Note my office hours above if you need assistance, I'm here to help.

Policy Regarding Withdrawals  Students desiring to withdraw from a class must do so by the above withdrawal date by filling out a withdrawal form at the registrar’s office. After this date, instructors can no longer enter a grade of “W” for the course for any reason.

Distance Education and/or Continuing Education Policies

The Distance Education Student Handbook contains policies and procedures unique to the DE student. Students should have reviewed the handbook as part of the mandatory orientation. It is the student's responsibility to be familiar with the handbook's contents. The handbook contains valuable information, answers, and resources, such as DE contacts, policies and procedures (how to drop, attendance requirements, etc.), student services (ADA, financial aid, degree planning, etc.), course information, testing procedures, technical support, and academic calendars.

Refer to the DE Student Handbook by visiting this link:

Test Bank N/A

Scoring Rubrics Regular exams, Homework sets, and the final will consist of multiple-choice and essay type questions. These are graded in the standard manner. The regular exams will include extra questions for extra credit however, the maximum points earned on any of the tests and exam won’t exceed 100. Sample Assignments N/A Sample Instructional Methods/Activities See the PowerPoint presentations on Eagle Online for an overview of the content of each chapter: EGLS3 -- Evaluation for Greater Learning Student Survey System “At Houston Community College, professors believe that thoughtful student feedback is necessary to improve teaching and learning. During a designated time, you will be asked to answer a short online survey of research-based questions related to instruction. The anonymous results of the survey will be made available to your professors and division chairs for continual improvement of instruction. Look for the survey as part of the Houston Community College Student System online near the end of the term”.

## Contents

So what about Pluto? It's clearly not a gas giant. So is it a terrestrial planet? Or is it a third class? Or do we not know?

Well, it's not listed in the article text but it's present in the image, along with a bunch of other bodies that are mainly ice rather than silicate (and also some that have active hydrospheres, directly contradicting the article text). I don't know the answer myself but we should either find out what the official line is or we should mention that there's ambiguity. Bryan 18:21, 28 Mar 2005 (UTC) Pluto is, at the moment, considered a dwarf planet, and would technically be designated as a terrestial planet. However, there's so much debate over Pluto's status, especially since Sedna was discovered, that I think Bryan's idea of just mentioning an ambiguity is best. Also, I was thinking of maybe adding a list of exo-terrestial-planets, too, if nobody objects? So far, I have: 55 Cancri A e 14 ME Gliese 777 A c 18 ME mu Arae d 14 ME GJ 436 b 21 ME PSR 1257+12 A .020 ME PSR 1257+12 B 4.3 ME PSR 1257+12 C 3.9 ME Gliese 876 d 6-8 ME. I'm not sure how complete this list is, as I took these from Wikipedia's "list of extrasolar planets" and then added Gliese 876 d, which despite being mentioned on most exoplanet-related pages as the lowest-mass non-pulsar exoplanet, is not listed on the "list of extrasolar planets page." Whew. Long sentence, that. ZelmersZoetrop Pluto (and anything that far out) would probably be better classified as a 'Kuiper Object' as technically the Kuiper Belt starts from inside Neptune's orbit. ref-Kuiper_belt. Although the label 'Minor Planet' is adequate, i would hesitate to call Pluto 'Terrestrial' in the truest sense. Ambiguity, as you say is probably the best course for now :) Grey Area 08:44, 29 July 2005 (UTC)

I am doing a project in school. We have to design a planet and the planet that my partner and i designed is completly water except for a few scattered volcanoes. Would animals like penguins and walruses be able to survive on a planet like this? Please answer ASAP.

See planetary habitability to begin with. I think that page will have the most data for you. If your planet is only going to have volcanic islands then its probably not tectonically active, as this is the main continent building process. This means low biodiversity and a lesser chance that animals like penguins and walruses would arise. But nothing is impossible with a hypothetical planet. Perhaps in shallow pools surrounding your islands multi-cellularity arose, and after that the sky's the limit. Perhaps you can revisit the model and add in some continents? Marskell 13:50, 4 April 2006 (UTC)

What is the role that impact cratering had in history on the formation of terrestrial planets? —The preceding unsigned comment was added by 149.169.207.32 (talk • contribs) on 00:17, 1 September 2006.

I'm not sure I understand the question. Cratering itself doesn't affect the formation much it's usually just used as a yardstick to measure how active the geology of a planet or moon is (if it's geologically active, it recycles its surface material, reducing the number of visible craters if it was active but then stopped being so, the crater density can give you a rough idea of how long ago this happened). The material that's delivered in the process of bombardment is, however, important, as it's how a lot of the volatiles posessed by the inner planets got here as the solar system was forming (how much this changed the amount of volatiles is an open question, though). --Christopher Thomas 05:14, 1 September 2006 (UTC)

In the good old days, when I first learned science, all living things were either animals or plants, and there were four terrestrial planets, four gas giants, plus Pluto with a question mark. Those old dichotomies were clearly inadequate. If the definition of terrestrial planet is intrinsic, i.e., depends only on the physical characteristics of the planet itself, then the Moon must be called a terrestrial planet. The definition does not make it clear whether having a rocky surface is a requirement. What if a planet is almost all rock and iron entirely covered with a few miles of ice? What if it's rock and iron, covered with an inpenetrable atmosphere of hydrogen and helium? What about Titan, which is a planet by any intrinsic definition? And what about Io? I suspect that there will be lots of overlapping classes of planets defined in the next generation, but that consensus on this issue will not soon be achieved. Vegasprof 11:19, 30 April 2007 (UTC)

Judging by its low density (2.0g/cm 3 , Ceres should really be called an ice dwarf and not a terrestrial (dwarf) planet.

That density is much lower than that of the Moon, or Jupiter's inner large moons Io and Europa. If Ceres is a terrestrial (dwarf) planet, what about the Moon, Io and Europa??

terrestrial planets include the folowing. Mercury, Earth,Mars, and Venus

For Rocky Planets with Compressed Density, one can us the Rocky Planet Density Equation. Density ( Rocky Planets) = ( 1+Pi ) X 10^ -9 X R^3 + ( 1+ SQRT 2 ) X 10^ -1 X R + 2900 kg/m^3 The First Term is the Tri-Axial Coefficient of Compression. The second Term is the Uni-Axial (Gravitational-Vertical) Coefficient of Compression. The Third term is the Average Density of the Earth and Moon Crustal Materials. The third term can be changed to changing material conditions. For Example. Venus' third term is 2657.05 ( Hot Granitic material ), or Mars' third term is 2941.05 ( Cold Basaltic material ). Ice Planets would have a third term similar to Earth's water Ice, between 900 and 1,000.

The Beauty of this equation is that you only need to know the Radius, and what material is in the crust to obtain a useful Density. All of the other calculations for Volume, Mass, Surface Gravity,Escape Velocity, etc. only require Density, and Radius, and the Density is now just a function of the Radius also. Mike Clark, Golden, Colorado. 63.225.17.34 (talk) 17:21, 16 September 2016 (UTC)

I think a table of compressed and uncompressed densities of the 4 inner planets and the moon would make a nice addition to this page.

Object mean density uncompressed density
Mercury 5.4 g/cm³ 5.3 g/cm³
Venus 5.2 g/cm³ 4.4 g/cm³
Earth 5.5 g/cm³ 4.4 g/cm³
Moon 3.3 g/cm³ 3.3 g/cm³
Mars 3.9 g/cm³ 3.8 g/cm³

I haven't found an authoritative source for these numbers or a formula to relate the density, mass and uncompressed density. So far I've found this source http://geophysics.ou.edu/solid_earth/notes/planets.html#densities but I don't believe it is original.

This is my first Wikipedia addition. Please let me know if there are things I should do to tidy up the addition. I'm still in search of a good source for the uncompressed density calculation. The uncompressed density of Ceres was an assumption based on the trend of compressed to uncompressed densities as the mass decreased.

And what is "uncompressed density"? —Tamfang (talk) 16:50, 27 October 2010 (UTC)

A planet is squeezed by its own gravity: the deeper layers are compressed by the weight of the overlying layers. This increases the density of the planet. The uncompressed density is the (lower) density that the planet would have if this gravitational squeezing did not occur. The reason one would want to estimate a planet's uncompressed density is that this gives a hint about what the planet is made of. A higher uncompressed density suggests a larger abundance of heavier elements such as iron. —Preceding unsigned comment added by 192.172.8.13 (talk) 16:31, 17 January 2011 (UTC)

There must be a very simple formula for calculating the uncompressed density for a planet, from its Mass and Radius. So, I am surprised how different some of the estimates I have read online for the densities of the terrestrial planets are. — Preceding unsigned comment added by 86.129.207.191 (talk) 09:21, 3 March 2017 (UTC)

Tamfang : "So far I've found this source http://geophysics.ou.edu/solid_earth/notes/planets.html#densities but I don't believe it is original." This link is dead. I'm just trying to work on this question in respect of a recent Arxiv paper on Mercury, and while I understand the concept, working out how to calculate it is much harder.

86.129.207.191 : "There must be a very simple formula for calculating the uncompressed density for a planet, from its Mass and Radius." No, that would be the BULK density, not the UNCOMPRESSED density. To calculate the uncompressed density, you'd need to have a model of the structure of the planet (distance from centre versus material), and an equation of state for those material - how much they compress under different pressures.

These class-notes from an astronomy course (by @plutokiller, even!) give some useful information buried in a lot of maths. http://web.gps.caltech.edu/

mbrown/classes/ge131/notes/djs08.pdf He's more concerned with the upper end of planetary masses, where the transition between thin vapour and metal is a little more drawn out, and it's relation to compact object (neutron stars, white dwarfs), but it is relevant. Probably a lot more relevant stuff in the rest of the class notes at http://web.gps.caltech.edu/

From https://www.fossilhunters.xyz/solar-nebula/uncompressed-density-and-bulk-planetary-compositions.html, someone else is thinking on very much the same lines as me : "However, pressure corrections to uncompressed density estimates require detailed knowledge of the internal planetary structure (i.e., details of core, mantle and crust structures), equations of state of the various materials that make up the planet (e.g. bulk moduli and their pressure derivatives) and the thermal structure of the planet." Unfortunately, this appears to be culled from somewhere else, making reference to "Stacey [30] reviewed the question of the equations of state of planetary materials and estimated an internally consistent set of uncompressed densities for the terrestrial planets.", but then gives no list of references. I think this may be a reference to "http://iopscience.iop.org/article/10.1088/0034-4885/68/2/R03/pdf" but I don't have access to that journal, so I can't follow it up any further. I'll look back to the article to see if there is anything referenced that I can add.

AKarley (talk) 01:43, 22 December 2017 (UTC)

Is there a maximum size (allowing for density, composition etc) which a terrestial planet can be? If so what would happen as the limit is reached (assuming that this is smaller than that required to produce a black hole? Jackiespeel (talk) 17:30, 29 July 2008 (UTC)

A terrestrial planet cannot be larger than about 10 Earth masses (or ≈2.2 Earth radii) according to current thinking. Above this limit (maybe earlier) it will attract enough hydrogen to turn into a gas giant planet. But that's still far below the size needed for a black hole, it's not even enough to form a star. --Roentgenium111 (talk) 17:42, 23 June 2011 (UTC)

I don't think the use of this term is appropriate. The planemo article says hardly anyone uses it, so I think it is not right here. Also, why apply the term to only two of the moons, all "rounded bodies" qualify for the term. HarryAlffa (talk) 18:16, 1 June 2009 (UTC)

In the section on extrasolar planets, the term "fusing star" is used and linked to another article ("Solar Nucleosynthesi", I think?). The other article does not explain the term "fusing star". I recommend changing the term "fusing star" for the title of the other article. --Eddylyons (talk) 23:30, 14 August 2009 (UTC)

It refers to a star that's still undergoing fusion (as opposed to stars that have burned out). It's actually much easier to detect planets around neutron stars than around ordinary stars, so the first few extrasolar planets discovered were around "dead" stars. I've tweaked the wording to make this clear. --Christopher Thomas (talk) 23:41, 14 August 2009 (UTC) There's another mention of a fusing star in the article. Would it be better to explain what a fusing star is?--Eddylyons (talk) 02:05, 17 November 2009 (UTC) I don't really see how better to phrase it. A "fusing star" is a "star in which fusion is occurring". This seems pretty clear. You could call it a "main-sequence star", but a) that'd make less sense to someone who wasn't already familiar with stars, and b) that isn't strictly true (red giants aren't on the main sequence but are still undergoing fusion). The closest simplified description would be "star that's still alive". I'd expect to see that phrasing on the Simple English Wikipedia, but I don't feel it's appropriate for the main English Wikipedia. Also, given that "star still undergoing fusion" is used in the paragraph before the one containing "fusing star", I'd think its meaning would be obvious enough from context even if one had trouble seeing what "fusing star" meant in isolation. Is the passage really that unclear as-is? --Christopher Thomas (talk) 02:40, 17 November 2009 (UTC) I see where you're talking about now. Just my opinion, the lay reader (such as myself, admittedly) would be more familiar with the fusing star (or ordinary star as you said above) as the rule not the exception. When I think of star, my mind's eye thinks of a star, not whether it's fusing, dead, neutron or pulsar. I hesitate to use the term "regular" or "normal" star, but that's what I'm getting at. Using the term fusing star makes it sound like it's an exception to what the lay reader would come to expect. It's not tidy, but maybe "ordinary, main-sequence star"? Or ". fusing star (not a pulsar). "? --Eddylyons (talk) 16:21, 17 November 2009 (UTC)

Why does the first sentence of the article regard 'inner planet' as an synonym of 'terrestrial planet'? The fact that all terrestrial planets in our solar system are inner planets (that is, planets between the sun and de asteroid belt) says nothing about the situation in other planetary systems. As a result, the current article is wrong to suggest that alle terrestrial exo-planets are inner planets too. DaMatriX (talk) 22:49, 15 December 2009 (UTC)

The term "inner planet" was added to the intro in Nov 2008. It is easy enough to fix. Though as of 2009 no exo-planets have been confirmed to be terrestrial. --- Kheider (talk) 23:09, 15 December 2009 (UTC)

--MathFacts (talk) 06:30, 20 December 2009 (UTC)

Under Terrestrial_worlds, I believe any required wiggle room can be changed by changing "referred to as geophysical planets" to "referred to as geophysical worlds", if need be. Though I am going to try and think about this for a day. I also did not realize the solar terrestrial planets section contained a discussion about worlds when I made my first edit. Hmm walked into a can of worms? -- Kheider (talk) 10:01, 21 December 2009 (UTC)

I've removed the following section from the article:

Titan showing surface and atmospheric details

Volcanos on Io constantly re-surface the satellite

As mentioned on Kheider's talk page, this appears to be putting too much weight on an unofficial, informal forum post, and needs much more in the way of verifiable sourcing. At present, it is a case of undue weight (unless, again, more sources are provided. I'd suggest that the best course is to develop the text here, rather than in the artcile, until agreement is reached as to the best approach. --Ckatz chat spy 10:28, 21 December 2009 (UTC)

Dictionary.com defines Terrestrial as representing the earth as distinct from other planets. So that would (IMHO) include Titan (most Earth-like body), Io (with active volcanos), and Europa (with a probable subsurface ocean). Now the question becomes what to call Ganymede and Callisto with their possible subsurface water, and Triton like bodies that have a thin atmosphere. Dictionary.com defines World (using definition #15) as any heavenly body, of course definition #1 states, " the earth or globe, considered as a planet.". Isn't semantics fun? We could also title the section "Worlds with Earth-like characteristics", but I do believe we are doing an injustice if we do not addreess this issue. -- Kheider (talk) 19:35, 21 December 2009 (UTC) Others have their own definition. :) Ruslik_ Zero 14:36, 22 December 2009 (UTC) "Terrestrial" is a good word IMO, and AFAIK the one that is most frequently used for such things. However, there is no dividing line between 'rocky' and 'icy': Ceres is widely considered terrestrial, yet may be half ice. So is Callisto terrestrial? Titan? By the time you get to Pluto, 'terrestrial' would no longer seem to apply. Perhaps we could word it in such a way as to be clear that the concept is inherently ambiguous. kwami (talk) 20:18, 23 December 2009 (UTC)

Dr. James Schombert has also stated, (at bottom of page) "Large amounts of outgassing have drained the inner moons, Io and Europa of their icy materials making them rich in rocky materials." I agree that we need to make it clear that the concept is inherently ambiguous nor does it have an official definition. When does something go from rocky to icy? How do you define Earth-like? In the pre-voyager era, Earth-like pretty much meant non-gas giant since we knew almost nothing about the large moons of the solar system. -- Kheider (talk) 21:12, 23 December 2009 (UTC)

Exo-water planets could result from inward planetary migration and originate as protoplanets that formed from volatile ice-rich material beyond the snow-line but that never attained masses sufficient to accrete large amounts of H/He nebular gas. Water worlds might be thought of as a bigger and hotter version of Jupiter's Galilean moons. -- Kheider (talk) 01:56, 24 December 2009 (UTC)

### References Edit

1. ^ abcDr. James Schombert (2004). "Primary Atmospheres (Astronomy 221: Lecture 12 Solar System Review)". Department of Physics University of Oregon . Retrieved 2009-12-22 .
2. ^
3. "IAU Snobbery". NASA Watch (not a NASA Website). June 15, 2008 . Retrieved 2008-07-05 .
4. ^ ab
5. Rosaly Lopes and Robert M. Nelson (2009-08-06). "Surface features on Titan form like Earth's, but with a frigid twist". IAU . Retrieved 2009-12-21 .

This article claims the moon lacks an iron core. But if I remember right. http://en.wikipedia.org/wiki/Moon#Internal_structure Yes, even elsewhere on Wikipedia it is noted to have "an iron-rich core". Which is it? Or is there some threshold for "made of iron" that the moon's "iron-rich" core not actually cross? Someone who knows more should correct it. Thanks, NickRinger (talk) 17:25, 4 May 2011 (UTC)

The moon's iron core is much smaller, relative to its own size, than Earth's. I'll adjust the text to clarify this. --Christopher Thomas (talk) 17:49, 4 May 2011 (UTC)

According to that table, Gliese 581c is smaller than 581e. But the latter is much lighter AND closer to the star (roughly implying higher density), so shouldn't 581e be the smaller one of the two? --Roentgenium111 (talk) 23:38, 3 August 2010 (UTC)

This problem has now dissolved, with the finding of a smaller planet. --Roentgenium111 (talk) 20:21, 7 July 2011 (UTC)

What's this table doing in the article? Most of the planets in there have masses comparable to Jupiter and are probably NOT terrestrial planets! —Preceding unsigned comment added by 131.111.8.102 (talk) 11:47, 21 January 2008 (UTC)

I agree this list is very speculative and even false in the case of most habitable.
There are many others with better irradiance figures, which is the primary criterion.

Planet. Perhelion. Average. Aphelion

Mars . 52.45%. 43.11%. 36.06%
HD 160691 b. 103.07%. 78.37%. 61.59%
HD 125612 b. 213.56%. 79.47%. 41.13%
HD 28185 b. 93.69%. 81.03%. 70.77%
HD 190228 b. 262.13%. 85.17%. 41.65%
Gliese 876 c. 162.61%. 86.65%. 53.73%
HD 188015 b. 120.50%. 87.06%. 65.83%
Gl 581 g. 89.13%. 89.13%. 89.13%
HD 16175 b. 548.49%. 92.20%. 36.47%
HD 100777 b. 237.79%. 97.40%. 52.66%
Earth. 103.43%. 100.00%. 96.74%
HD 38083 b. 290.32%. 101.06%. 50.83%
HD 108874 b. 119.47%. 103.33%. 90.25%
HD 155358 c. 155.02%. 105.26%. 76.11%
HD 142415 b. 425.29%. 106.32%. 47.25%
HD 20367 b. 185.73%. 110.12%. 72.79%
HD 82943 b. 182.79%. 111.50%. 75.03%
HD 221287 b. 136.29%. 115.36%. 98.90%
HD 45364 b. 167.83%. 116.07%. 85.02%
HD 92788 b. 221.33%. 117.95%. 73.13%
HD 153950 b. 329.92%. 143.71%. 80.04%
HD 69830 d. 166.74%. 144.22%. 125.96%
Venus. 193.93%. 191.30%. 188.73%

I am for serious amendments or deletion of the section.24.78.172.60 (talk) 18:13, 11 July 2011 (UTC)

The first two entries in the list are certainly not "speculative", since mass and size are objective measures of a planet, though I agree the last line is rather speculative. (The Jupiter-sized planets mentioned by 131.111.8.102 above were already removed from the table some time ago.) The table currently doesn't intend to list the "most habitable" planet - but if you have reliable sources saying that "irradiance is the primary criterion for habitability" and for the list of numbers you give above, we can gladly add a line for "most habitable planet" to the table. But many (all?) planets on your list are gas giants as massive or even more massive than Jupiter, so they're certainly not habitable to any life form that we know of. --Roentgenium111 (talk) 19:29, 11 July 2011 (UTC)

hello world. I am looking for an extrasolar planet thats in its habitable zone. i know there has to be one out there somewhere. most people think that a habitable zone is much smaller than it really is. in truth there are many variables that can decide how large the habitabloe zone is. such as atmospheric composition, planetary comositon, and type of star if the planet has a largly co2 based atmosphere than it will be farther than a planet without as much co2. a planet with an extremely bright star will be farther than a planet with a dim star. Also a planet with a lot of carbon in the suface will absorb more sunlight, and heat, than a planet without it.206.78.212.250 (talk) 19:32, 26 August 2011 (UTC)Robert Moore

There is a category for that. http://en.wikipedia.org/wiki/Category:Extrasolar_planets_in_the_habitable_zone This cat is also mentioned in the article "habitable zone". It doesn't belong in this article though as this is about terrestrial planets. --EvenGreenerFish (talk) 00:01, 22 December 2011 (UTC) Yes the focus of this article appears to be of Terrestrial planets in the habitable zone which irks me. Terrestrial planets can be searing hot and devoid of water. Simply because Earth is Terrestrial, it should not follow the Anthropocentric focus that this article has ended up with. --EvenGreenerFish (talk) 11:24, 11 March 2012 (UTC)

I suggest that the most Earth-like table be split to Earth analog, since that is what the table is about, the most Earth-like analog. 70.24.248.23 (talk) 23:25, 26 November 2011 (UTC)

I see one more issue. Earth analog is about planets which have multiple similarities to earth. terrestrial is about planets with the composition of earth. how about earth-sized planets? since we just found 2, and dont know their composition, and know they are not earth analogs, should we have an article simply on earth sized planets? or is this too detailed? Note that some articles like the new Kepler-20f planet, link to terrrestrial, when we dont know yet that they are terrestrial. or do we?Mercurywoodrose (talk) 05:06, 21 December 2011 (UTC)

The simple solution is to create distinct sub-sections in the Earth Analog article which assesses each critieria. --EvenGreenerFish (talk) 00:03, 22 December 2011 (UTC)

What is the density of 'rock' (as used in astronomy)? --JorisvS (talk) 12:51, 12 September 2012 (UTC)

According to Earth#Internal_structure, the crust and mantle of Earth (which consist primarily of "rocks") have densities in the range 2.2 to ("compressed") 5.6 g/cm 3 . This might be a reasonable range for rocks in general, looking e.g. at the Moon's and Vesta's densities (the two being predominantly rocky bodies). --Roentgenium111 (talk) 17:02, 21 September 2012 (UTC)

What exactly is the purpose of that? I replaced it with the exact same picture, minus the moon and ceres, with a much shorter caption where we dont have to explain that the moon and ceres are in fact not planets. The one minus those two is superior and less confusing. Cadiomals (talk) 00:05, 24 September 2012 (UTC)

Others may agree with you, but I happen to think it's more interesting to have a couple of smaller bodies for contrast, and the caption is perfectly clear. Rothorpe (talk) 00:10, 24 September 2012 (UTC) I don't think it's about what certain people deem to be interesting. One man's trash is another man's treasure. Images in Wikipedia are meant not just to be interesting but to educate and convey facts. Eliminating these two non-planetary bodies would eliminate the need for a long caption explaining that these bodies are in fact not considered planets. If you don't mind I would like to replace that picture with an alternate minus those two, as it is more crystal clear than any caption could be. Cadiomals (talk) 01:00, 24 September 2012 (UTC) Since the moon and Ceres are excellent examples of what small "terrestrial" planet-like-bodies look like, I think it is better with them included. -- Kheider (talk) 11:28, 24 September 2012 (UTC) I tend to agree with Cadiomals the lede image of the article should stick closely to the subject, and Moon and Ceres are no terrestrial planets, not being planets. I think I wouldn't even call Ceres a "terrestrial" object, since it has such a large proportion (

25%) of water ice. (Vesta is presumably more "terrestrial" than Ceres.) --Roentgenium111 (talk) 19:07, 24 September 2012 (UTC) Yes, it's likely that Ceres will turn out to be not very terrestrial, but it's a commonly given example. Both it and Luna are "planets" according to Stern, though that's a small-minority position. — kwami (talk) 09:01, 31 December 2013 (UTC)

Shouldn't there be at least some discussion of CoRoT-7b and Kepler-10b in this section, for which we actually have density measurements that imply they are in fact terrestrial. At present it looks like the focus is on a bunch of RV-detected worlds that may or may not be terrestrial in nature. 46.126.76.193 (talk) 22:51, 8 October 2012 (UTC)

Recently, a few known gas-giant uninhabitable planets orbit double stars. Scientists believe that rocky planets do not do so. Now, scientists believe that, it is possible for rocky planets too. Read more on this website:

Rocky Planet Density Equation :

Density of Rocky Planets = (1+Pi) x 10^-9 * Radius^3 + (1 + sqrt 2) x 10^-1 * Radius + 2900 kg/m^3 The Radius must be in kilometers. The first term is the Tri-Axial coefficient of compression which does not really kick-in until the planet get big. The second term is the Uni-Axial coefficient of compression caused by the planets self gravity. The third term varies with the average composition of the near surface materials. For the Moon and Earth, it is 2900 because of the mixture of granite and basalt in the upper 400 km. For Venus the constant is lower (2657.05 kg/m^3) because Venus is very hot (expanded), and is dominated by low density rocks near the surface. For Mars the constant is higher (2941.05 kg/m^3) because Mars is cold (contracted), and its surface is dominated by (red ) Iron rich basalt. Note that the third term can be changed for various groups of planets. For Planets dominated by Ice, especially very thick ice, the third term might be between 900 and 1100 Kg/m^3, or, for a planet like Mercury that is dominated by Iron, the constant, third term, will be much higher. The third term can actually tell you a lot about a planets composition.

Michael W. Clark Golden, Colorado, USA — Preceding unsigned comment added by 63.225.17.34 (talk) 17:11, 22 December 2015 (UTC)

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## Planet Facts

Our Solar System includes the Sun and the planetary system revolving around it. A “planetary system” is a group of non-stellar objects (planets, dwarf planets, moons, asteroids, meteoroids, comets and cosmic dust) that orbit around a star, the sun is classified as a star.

The Solar System includes four terrestrial planets (composed of rock and metal) and four gas giants (gaseous material). The four terrestrial planets are Mercury, Venus, Earth, and Mars, and the four gas giants are Jupiter, Saturn, Uranus, and Neptune. These names are derived from Roman gods and goddesses. So how many planets have we mentioned?

Eight! Just checking if you are still awake.

Scientists believe that the Solar System was formed billions of years ago through the gravitational collapse of a cluster of gas and dust particles in space. The Sun is said to have been formed out of the mass in the center, and the these materials slowly formed the other non-stellar objects planets, moons, and asteroids. The Solar System is an enigma, with man’s quest to learn more, astronomers and scientist continually learn something new about the vastness of space. As technology improves, so we learn more about our Solar System.

It is said that the Sun is halfway in the cycle that began at its birth and ending with its final expansion and collapse as a red giant (the 10 billionth year mark of its life). An event you don’t want to be around for.

Not Just Planets

The Solar System includes the planets’ satellites (moons), comets, asteroids, and meteoroids. Located between Mars and Jupiter is a small asteroid belt. Even though there are only eight planets, our Solar System actually contains hundreds of star systems.

Some of the moons in our Solar System are larger than the planet Mercury! Two of the moons of Jupiter have interesting characteristics Io has active volcanoes, Europa has a thick liquid water ocean layer, though covered by thick crusts of solid ice. Titan, the largest moon of Saturn, has lakes, rivers, and oceans of liquid Methane!

Beyond Neptune’s orbital path lie trans-Neptunian formations made of ice water, ammonia, and methane. Ceres, Pluto, Haumea, Makemake, and Eris rotate using their own gravity like the other planets, however, for this reason, the scientific community has called these “dwarf planets.”

Atmospheres, Rings, Magnetic fields

Nearly every planet, and some moons, is shrouded with atmospheric gases. Take Venus for example, it has a thick atmosphere of carbon dioxide and other toxic gases. Earth’s atmosphere consists of mostly nitrogen and oxygen. Mars’ atmosphere is also has a thin layer of carbon dioxide. The gas giants contain hydrogen and helium in their atmospheres.

From the 17th century to the 1970s, astronomers believed that only Saturn had an outer ring. With the advent of modern telescopes, scientists have observed rings on the other planets, Jupiter, Uranus, and Neptune. Although these planets’ rings are not as prominent as Saturn’s. What are seen as planetary rings are actually clusters of various particles that include dust, asteroids & boulders, they range in size from tiny specks to some that are big as your house!

Planets and stars in the Solar System have magnetic fields, the charged particles surround most of these celestial bodies. The sun has a magnetic field of its own called the heliosphere, this emanates throughout the Solar System.

The Sun is the star at the center of the Solar System. It consists of hot lava and a strong magnetic field. The energy it produces can be likened to hydrogen nuclear fusion reactions at its core. The Sun mass is estimated at 99.86% of the in Solar System, the rest belongs to Jupiter. The Sun consists of 70% hydrogen, 28% helium, and 2% of the remaining consists of carbon, oxygen, iron, neon, and other elements.

By classification the Sun is a yellow dwarf. It is about the top 10th percentile in mass among all stars. It has a surface temperature of about 5505°C (9941°F).

The corona of the Sun is constantly expanding, thus producing a solar wind (charged particle stream that is emitted from the Sun’s upper atmosphere). The solar wind is responsible for creating the heliosphere, which is a large bubble that expands into the outer area of the Solar System known as the interstellar medium. Phenomena such as geomagnetic storms can disrupt Earth’s power grid, Northern Lights, as well as comet plasma tails which are always aligned away from the Sun.

The Sun is often represented in ancient cultures, more often than not, as a deity or god. The ancient Egyptians called it as the god Ra, the Greeks as Helios, and the Romans as Sol.

The Planets

Mercury
Mercury is the smallest and closest planet to the Sun. Mercury has no moons and has no special features other than impact craters and lobed ridges. Mercury’s thin atmosphere consists of particles blasted off by the solar wind from the Sun. It has a large iron core and a thin mantle layer, possibly constant impacts which prevent its layers from developing over time.

Venus
The second planet is slightly smaller than Earth. It has a dense atmosphere and an iron core. It is the hottest planet with blistering surface temperatures (upwards of 400°C/752°F). Venus’ atmosphere is toxic due to clouds of sulfuric acid. The planet may have had water at one point, but these have evaporated over time due to the extreme heat. Volcanic activity have been observed on Venus’ surfaces, though there have been no signs of lava flow.

Earth
Earth is the largest of the inner planets. It is the only place in the universe where life exists. It has one moon. Earth’s core is very active, and it is the only planet with tectonic plates. Earth’s biosphere has long since altered its atmosphere, creating more oxygen as well as an ozone layer to block harmful radiation from space.

Mars
This is the second largest planet and the fourth from the Sun. Mars has a carbon dioxide atmosphere. It has two moons: Deimos and Phobos. These are said to be captured asteroids. Its reddish color is due to the large amounts of iron-oxide on its surface. Its atmosphere is very thin and its surface is riddled with impact craters, like that of the Moon’s.

Jupiter
Jupiter the size is massive, consider 318 times the Earth size. It is the largest planet in the Solar System with 2.5 times the mass of all the other planets combined. It is composed of mostly hydrogen and helium. Jupiter has 67 known moons or satellites. Jupiter has a rapid rotation which has caused it to bulge slightly around its equator. The atmosphere of Jupiter creates lots of storms on its surface, the prominent result of which is the Great Red Spot, which is actually a continuous storm which has endured since the 17th century (when it was first observed by telescope).

Saturn
This planet is just beyond Jupiter and is known for its ring, which actually consists of 9 bands of rings. Saturn is 60% the volume of Jupiter and has the lowest density of all the planets. It has 62 satellites, including Titan and Enceladus. Saturn’s core consists of iron, nickel, silicon, and oxygen compounds, all surrounded by a thick layer of metallic hydrogen. The planetary magnetic field on Saturn has contributed to an electric current through the metallic hydrogen layer.

Uranus
Uranus is unique in that it orbits the sun on its side because of its axial tilt. Uranus has 27 known moons, including Titania, Oberon, Umbriel, Ariel, and Miranda. While Uranus is similar to Jupiter and Saturn in that its atmosphere contains hydrogen and helium, it also contains copious amounts of ice water, ammonia, and methane. Furthermore, it has the coldest atmosphere in the Solar System at -224°C/-435°F. Uranus and Neptune are also known as “ice giants.”

Neptune
Neptune is smaller than Uranus but is denser. Neptune has 13 known satellites, including Triton. Neptune’s surface gravity is only surpassed by Jupiter, and the two are the only planets with greater surface than Earth’s. Neptune contains ice compounds like those of Saturn’s as well as greater concentrations of volatile elements similar to those found on Jupiter and Saturn.

Pluto
Pluto used to be considered a planet in the sense that Mercury or Jupiter are. However, it is now considered a dwarf planet because it lacks characteristics that define the planets in the Solar System.

## Distant planet may be on its second atmosphere

Scientists using NASA's Hubble Space Telescope have found evidence that a planet orbiting a distant star may have lost its atmosphere but gained a second one through volcanic activity.

The planet, GJ 1132 b, is hypothesized to have begun as a gaseous world with a thick hydrogen blanket of atmosphere. Starting out at several times the diameter of Earth, this so-called "sub-Neptune" is believed to have quickly lost its primordial hydrogen and helium atmosphere due to the intense radiation of the hot, young star it orbits. In a short period of time, such a planet would be stripped down to a bare core about the size of Earth. That's when things got interesting.

To the surprise of astronomers, Hubble observed an atmosphere which, according to their theory, is a "secondary atmosphere" that is present now. Based on a combination of direct observational evidence and inference through computer modeling, the team reports that the atmosphere consists of molecular hydrogen, hydrogen cyanide, methane and also contains an aerosol haze. Modeling suggests the aerosol haze is based on photochemically produced hydrocarbons, similar to smog on Earth.

Scientists interpret the current atmospheric hydrogen in GJ 1132 b as hydrogen from the original atmosphere which was absorbed into the planet's molten magma mantle and is now being slowly released through volcanic processes to form a new atmosphere. The atmosphere we see today is believed to be continually replenished to balance the hydrogen escaping into space.

"It's super exciting because we believe the atmosphere that we see now was regenerated, so it could be a secondary atmosphere," said study co-author Raissa Estrela of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "We first thought that these highly irradiated planets could be pretty boring because we believed that they lost their atmospheres. But we looked at existing observations of this planet with Hubble and said, 'Oh no, there is an atmosphere there.'"

The findings could have implications for other exoplanets, planets beyond our solar system.

"How many terrestrial planets don't begin as terrestrials? Some may start as sub-Neptunes, and they become terrestrials through a mechanism that photo-evaporates the primordial atmosphere. This process works early in a planet's life, when the star is hotter," said lead author Mark Swain of JPL. "Then the star cools down and the planet's just sitting there. So you've got this mechanism where you can cook off the atmosphere in the first 100 million years, and then things settle down. And if you can regenerate the atmosphere, maybe you can keep it."

In some ways GJ 1132 b, located about 41 light-years from Earth, has tantalizing parallels to Earth, but in some ways it is very different. Both have similar densities, similar sizes, and similar ages, being about 4.5 billion years old. Both started with a hydrogen-dominated atmosphere, and both were hot before they cooled down. The team's work even suggests that GJ 1132 b and Earth have similar atmospheric pressure at the surface.

But the planets have profoundly different formation histories. Earth is not believed to be the surviving core of a sub-Neptune. And Earth orbits at a comfortable distance from our Sun. GJ 1132 b is so close to its red dwarf star that it completes an orbit around its host star once every day and a half. This extremely close proximity keeps GJ 1132 b tidally locked, showing the same face to its star at all times -- just as our Moon keeps one hemisphere permanently facing Earth.

"The question is, what is keeping the mantle hot enough to remain liquid and power volcanism?" asked Swain. "This system is special because it has the opportunity for quite a lot of tidal heating."

Tidal heating is a phenomenon that occurs through friction, when energy from a planet's orbit and rotation is dispersed as heat inside the planet. GJ 1132 b is in an elliptical orbit, and the tidal forces acting on it are strongest when it is closest to or farthest from its host star. At least one other planet in the host star's system also gravitationally pulls on the planet.

The consequences are that the planet is squeezed or stretched through this gravitational "pumping." That tidal heating keeps the mantle liquid for a long time. A nearby example in our own solar system is Jupiter's moon Io, which has continuous volcanic activity due to a tidal tug-of-war from Jupiter and the neighboring Jovian moons.

Given GJ 1132 b's hot interior, the team believes the planet's cooler, overlying crust is extremely thin, perhaps only hundreds of feet thick. That's much too feeble to support anything resembling volcanic mountains. Its flat terrain may also be cracked like an eggshell due to tidal flexing. Hydrogen and other gases could be released through such cracks.

NASA's upcoming James Webb Space Telescope has the ability to observe this exoplanet. Webb's infrared vision may allow scientists to see down to the planet's surface. "If there are magma pools or volcanism going on, those areas will be hotter," explained Swain. "That will generate more emission, and so they'll be looking potentially at the actual geologic activity -- which is exciting!"

## Contents

Ideas concerning the origin and fate of the world date from the earliest known writings however, for almost all of that time, there was no attempt to link such theories to the existence of a "Solar System", simply because it was not generally thought that the Solar System, in the sense we now understand it, existed. The first step toward a theory of Solar System formation and evolution was the general acceptance of heliocentrism, which placed the Sun at the centre of the system and the Earth in orbit around it. This concept had developed for millennia (Aristarchus of Samos had suggested it as early as 250 BC), but was not widely accepted until the end of the 17th century. The first recorded use of the term "Solar System" dates from 1704. [4]

The current standard theory for Solar System formation, the nebular hypothesis, has fallen into and out of favour since its formulation by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace in the 18th century. The most significant criticism of the hypothesis was its apparent inability to explain the Sun's relative lack of angular momentum when compared to the planets. [5] However, since the early 1980s studies of young stars have shown them to be surrounded by cool discs of dust and gas, exactly as the nebular hypothesis predicts, which has led to its re-acceptance. [6]

Understanding of how the Sun is expected to continue to evolve required an understanding of the source of its power. Arthur Stanley Eddington's confirmation of Albert Einstein's theory of relativity led to his realisation that the Sun's energy comes from nuclear fusion reactions in its core, fusing hydrogen into helium. [7] In 1935, Eddington went further and suggested that other elements also might form within stars. [8] Fred Hoyle elaborated on this premise by arguing that evolved stars called red giants created many elements heavier than hydrogen and helium in their cores. When a red giant finally casts off its outer layers, these elements would then be recycled to form other star systems. [8]

### Presolar nebula Edit

The nebular hypothesis says that the Solar System formed from the gravitational collapse of a fragment of a giant molecular cloud. [9] The cloud was about 20 parsec (65 light years) across, [9] while the fragments were roughly 1 parsec (three and a quarter light-years) across. [10] The further collapse of the fragments led to the formation of dense cores 0.01–0.1 parsec (2,000–20,000 AU) in size. [a] [9] [11] One of these collapsing fragments (known as the presolar nebula) formed what became the Solar System. [12] The composition of this region with a mass just over that of the Sun ( M ) was about the same as that of the Sun today, with hydrogen, along with helium and trace amounts of lithium produced by Big Bang nucleosynthesis, forming about 98% of its mass. The remaining 2% of the mass consisted of heavier elements that were created by nucleosynthesis in earlier generations of stars. [13] Late in the life of these stars, they ejected heavier elements into the interstellar medium. [14]

The oldest inclusions found in meteorites, thought to trace the first solid material to form in the presolar nebula, are 4568.2 million years old, which is one definition of the age of the Solar System. [1] Studies of ancient meteorites reveal traces of stable daughter nuclei of short-lived isotopes, such as iron-60, that only form in exploding, short-lived stars. This indicates that one or more supernovae occurred nearby. A shock wave from a supernova may have triggered the formation of the Sun by creating relatively dense regions within the cloud, causing these regions to collapse. [15] Because only massive, short-lived stars produce supernovae, the Sun must have formed in a large star-forming region that produced massive stars, possibly similar to the Orion Nebula. [16] [17] Studies of the structure of the Kuiper belt and of anomalous materials within it suggest that the Sun formed within a cluster of between 1,000 and 10,000 stars with a diameter of between 6.5 and 19.5 light years and a collective mass of 3,000 M . This cluster began to break apart between 135 million and 535 million years after formation. [18] [19] Several simulations of our young Sun interacting with close-passing stars over the first 100 million years of its life produce anomalous orbits observed in the outer Solar System, such as detached objects. [20]

Because of the conservation of angular momentum, the nebula spun faster as it collapsed. As the material within the nebula condensed, the atoms within it began to collide with increasing frequency, converting their kinetic energy into heat. The center, where most of the mass collected, became increasingly hotter than the surrounding disc. [10] Over about 100,000 years, [9] the competing forces of gravity, gas pressure, magnetic fields, and rotation caused the contracting nebula to flatten into a spinning protoplanetary disc with a diameter of about 200 AU [10] and form a hot, dense protostar (a star in which hydrogen fusion has not yet begun) at the centre. [21]

At this point in its evolution, the Sun is thought to have been a T Tauri star. [22] Studies of T Tauri stars show that they are often accompanied by discs of pre-planetary matter with masses of 0.001–0.1 M . [23] These discs extend to several hundred AU—the Hubble Space Telescope has observed protoplanetary discs of up to 1000 AU in diameter in star-forming regions such as the Orion Nebula [24] —and are rather cool, reaching a surface temperature of only about 1,000 K (730 °C 1,340 °F) at their hottest. [25] Within 50 million years, the temperature and pressure at the core of the Sun became so great that its hydrogen began to fuse, creating an internal source of energy that countered gravitational contraction until hydrostatic equilibrium was achieved. [26] This marked the Sun's entry into the prime phase of its life, known as the main sequence. Main-sequence stars derive energy from the fusion of hydrogen into helium in their cores. The Sun remains a main-sequence star today. [27] As the early Solar System continued to evolve, it eventually drifted away from its siblings in the stellar nursery, and continued orbiting the Milky Way's center on its own.

### Formation of the planets Edit

The various planets are thought to have formed from the solar nebula, the disc-shaped cloud of gas and dust left over from the Sun's formation. [28] The currently accepted method by which the planets formed is accretion, in which the planets began as dust grains in orbit around the central protostar. Through direct contact and self-organization, these grains formed into clumps up to 200 m (660 ft) in diameter, which in turn collided to form larger bodies (planetesimals) of

10 km (6.2 mi) in size. These gradually increased through further collisions, growing at the rate of centimetres per year over the course of the next few million years. [29]

The inner Solar System, the region of the Solar System inside 4 AU, was too warm for volatile molecules like water and methane to condense, so the planetesimals that formed there could only form from compounds with high melting points, such as metals (like iron, nickel, and aluminium) and rocky silicates. These rocky bodies would become the terrestrial planets (Mercury, Venus, Earth, and Mars). These compounds are quite rare in the Universe, comprising only 0.6% of the mass of the nebula, so the terrestrial planets could not grow very large. [10] The terrestrial embryos grew to about 0.05 Earth masses ( M ) and ceased accumulating matter about 100,000 years after the formation of the Sun subsequent collisions and mergers between these planet-sized bodies allowed terrestrial planets to grow to their present sizes (see Terrestrial planets below). [30]

When the terrestrial planets were forming, they remained immersed in a disk of gas and dust. The gas was partially supported by pressure and so did not orbit the Sun as rapidly as the planets. The resulting drag and, more importantly, gravitational interactions with the surrounding material caused a transfer of angular momentum, and as a result the planets gradually migrated to new orbits. Models show that density and temperature variations in the disk governed this rate of migration, [31] [32] but the net trend was for the inner planets to migrate inward as the disk dissipated, leaving the planets in their current orbits. [33]

The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, which is the point between the orbits of Mars and Jupiter where the material is cool enough for volatile icy compounds to remain solid. The ices that formed the Jovian planets were more abundant than the metals and silicates that formed the terrestrial planets, allowing the giant planets to grow massive enough to capture hydrogen and helium, the lightest and most abundant elements. [10] Planetesimals beyond the frost line accumulated up to 4 M within about 3 million years. [30] Today, the four giant planets comprise just under 99% of all the mass orbiting the Sun. [b] Theorists believe it is no accident that Jupiter lies just beyond the frost line. Because the frost line accumulated large amounts of water via evaporation from infalling icy material, it created a region of lower pressure that increased the speed of orbiting dust particles and halted their motion toward the Sun. In effect, the frost line acted as a barrier that caused material to accumulate rapidly at

5 AU from the Sun. This excess material coalesced into a large embryo (or core) on the order of 10 M , which began to accumulate an envelope via accretion of gas from the surrounding disc at an ever-increasing rate. [34] [35] Once the envelope mass became about equal to the solid core mass, growth proceeded very rapidly, reaching about 150 Earth masses

10 5 years thereafter and finally topping out at 318 M . [36] Saturn may owe its substantially lower mass simply to having formed a few million years after Jupiter, when there was less gas available to consume. [30] [37]

T Tauri stars like the young Sun have far stronger stellar winds than more stable, older stars. Uranus and Neptune are thought to have formed after Jupiter and Saturn did, when the strong solar wind had blown away much of the disc material. As a result, those planets accumulated little hydrogen and helium—not more than 1 M each. Uranus and Neptune are sometimes referred to as failed cores. [38] The main problem with formation theories for these planets is the timescale of their formation. At the current locations it would have taken millions of years for their cores to accrete. [37] This means that Uranus and Neptune may have formed closer to the Sun—near or even between Jupiter and Saturn—and later migrated or were ejected outward (see Planetary migration below). [38] [39] Motion in the planetesimal era was not all inward toward the Sun the Stardust sample return from Comet Wild 2 has suggested that materials from the early formation of the Solar System migrated from the warmer inner Solar System to the region of the Kuiper belt. [40]

After between three and ten million years, [30] the young Sun's solar wind would have cleared away all the gas and dust in the protoplanetary disc, blowing it into interstellar space, thus ending the growth of the planets. [41] [42]

The planets were originally thought to have formed in or near their current orbits. This has been questioned during the last 20 years. Currently, many planetary scientists think that the Solar System might have looked very different after its initial formation: several objects at least as massive as Mercury were present in the inner Solar System, the outer Solar System was much more compact than it is now, and the Kuiper belt was much closer to the Sun. [43]

### Terrestrial planets Edit

At the end of the planetary formation epoch the inner Solar System was populated by 50–100 Moon- to Mars-sized planetary embryos. [44] [45] Further growth was possible only because these bodies collided and merged, which took less than 100 million years. These objects would have gravitationally interacted with one another, tugging at each other's orbits until they collided, growing larger until the four terrestrial planets we know today took shape. [30] One such giant collision is thought to have formed the Moon (see Moons below), while another removed the outer envelope of the young Mercury. [46]

One unresolved issue with this model is that it cannot explain how the initial orbits of the proto-terrestrial planets, which would have needed to be highly eccentric to collide, produced the remarkably stable and nearly circular orbits they have today. [44] One hypothesis for this "eccentricity dumping" is that the terrestrials formed in a disc of gas still not expelled by the Sun. The "gravitational drag" of this residual gas would have eventually lowered the planets' energy, smoothing out their orbits. [45] However, such gas, if it existed, would have prevented the terrestrial planets' orbits from becoming so eccentric in the first place. [30] Another hypothesis is that gravitational drag occurred not between the planets and residual gas but between the planets and the remaining small bodies. As the large bodies moved through the crowd of smaller objects, the smaller objects, attracted by the larger planets' gravity, formed a region of higher density, a "gravitational wake", in the larger objects' path. As they did so, the increased gravity of the wake slowed the larger objects down into more regular orbits. [47]

### Asteroid belt Edit

The outer edge of the terrestrial region, between 2 and 4 AU from the Sun, is called the asteroid belt. The asteroid belt initially contained more than enough matter to form 2–3 Earth-like planets, and, indeed, a large number of planetesimals formed there. As with the terrestrials, planetesimals in this region later coalesced and formed 20–30 Moon- to Mars-sized planetary embryos [48] however, the proximity of Jupiter meant that after this planet formed, 3 million years after the Sun, the region's history changed dramatically. [44] Orbital resonances with Jupiter and Saturn are particularly strong in the asteroid belt, and gravitational interactions with more massive embryos scattered many planetesimals into those resonances. Jupiter's gravity increased the velocity of objects within these resonances, causing them to shatter upon collision with other bodies, rather than accrete. [49]

As Jupiter migrated inward following its formation (see Planetary migration below), resonances would have swept across the asteroid belt, dynamically exciting the region's population and increasing their velocities relative to each other. [50] The cumulative action of the resonances and the embryos either scattered the planetesimals away from the asteroid belt or excited their orbital inclinations and eccentricities. [48] [51] Some of those massive embryos too were ejected by Jupiter, while others may have migrated to the inner Solar System and played a role in the final accretion of the terrestrial planets. [48] [52] [53] During this primary depletion period, the effects of the giant planets and planetary embryos left the asteroid belt with a total mass equivalent to less than 1% that of the Earth, composed mainly of small planetesimals. [51] This is still 10–20 times more than the current mass in the main belt, which is now about 0.0005 M . [54] A secondary depletion period that brought the asteroid belt down close to its present mass is thought to have followed when Jupiter and Saturn entered a temporary 2:1 orbital resonance (see below).

The inner Solar System's period of giant impacts probably played a role in the Earth acquiring its current water content (

6 × 10 21 kg) from the early asteroid belt. Water is too volatile to have been present at Earth's formation and must have been subsequently delivered from outer, colder parts of the Solar System. [55] The water was probably delivered by planetary embryos and small planetesimals thrown out of the asteroid belt by Jupiter. [52] A population of main-belt comets discovered in 2006 has been also suggested as a possible source for Earth's water. [55] [56] In contrast, comets from the Kuiper belt or farther regions delivered not more than about 6% of Earth's water. [2] [57] The panspermia hypothesis holds that life itself may have been deposited on Earth in this way, although this idea is not widely accepted. [58]

### Planetary migration Edit

According to the nebular hypothesis, the outer two planets may be in the "wrong place". Uranus and Neptune (known as the "ice giants") exist in a region where the reduced density of the solar nebula and longer orbital times render their formation there highly implausible. [59] The two are instead thought to have formed in orbits near Jupiter and Saturn (known as the "gas giants"), where more material was available, and to have migrated outward to their current positions over hundreds of millions of years. [38]

The migration of the outer planets is also necessary to account for the existence and properties of the Solar System's outermost regions. [39] Beyond Neptune, the Solar System continues into the Kuiper belt, the scattered disc, and the Oort cloud, three sparse populations of small icy bodies thought to be the points of origin for most observed comets. At their distance from the Sun, accretion was too slow to allow planets to form before the solar nebula dispersed, and thus the initial disc lacked enough mass density to consolidate into a planet. [59] The Kuiper belt lies between 30 and 55 AU from the Sun, while the farther scattered disc extends to over 100 AU, [39] and the distant Oort cloud begins at about 50,000 AU. [60] Originally, however, the Kuiper belt was much denser and closer to the Sun, with an outer edge at approximately 30 AU. Its inner edge would have been just beyond the orbits of Uranus and Neptune, which were in turn far closer to the Sun when they formed (most likely in the range of 15–20 AU), and in 50% of simulations ended up in opposite locations, with Uranus farther from the Sun than Neptune. [61] [2] [39]

According to the Nice model, after the formation of the Solar System, the orbits of all the giant planets continued to change slowly, influenced by their interaction with the large number of remaining planetesimals. After 500–600 million years (about 4 billion years ago) Jupiter and Saturn fell into a 2:1 resonance: Saturn orbited the Sun once for every two Jupiter orbits. [39] This resonance created a gravitational push against the outer planets, possibly causing Neptune to surge past Uranus and plough into the ancient Kuiper belt. [61] The planets scattered the majority of the small icy bodies inwards, while themselves moving outwards. These planetesimals then scattered off the next planet they encountered in a similar manner, moving the planets' orbits outwards while they moved inwards. [39] This process continued until the planetesimals interacted with Jupiter, whose immense gravity sent them into highly elliptical orbits or even ejected them outright from the Solar System. This caused Jupiter to move slightly inward. [c] Those objects scattered by Jupiter into highly elliptical orbits formed the Oort cloud [39] those objects scattered to a lesser degree by the migrating Neptune formed the current Kuiper belt and scattered disc. [39] This scenario explains the Kuiper belt's and scattered disc's present low mass. Some of the scattered objects, including Pluto, became gravitationally tied to Neptune's orbit, forcing them into mean-motion resonances. [62] Eventually, friction within the planetesimal disc made the orbits of Uranus and Neptune circular again. [39] [63]

In contrast to the outer planets, the inner planets are not thought to have migrated significantly over the age of the Solar System, because their orbits have remained stable following the period of giant impacts. [30]

Another question is why Mars came out so small compared with Earth. A study by Southwest Research Institute, San Antonio, Texas, published June 6, 2011 (called the Grand tack hypothesis), proposes that Jupiter had migrated inward to 1.5 AU. After Saturn formed, migrated inward, and established the 2:3 mean motion resonance with Jupiter, the study assumes that both planets migrated back to their present positions. Jupiter thus would have consumed much of the material that would have created a bigger Mars. The same simulations also reproduce the characteristics of the modern asteroid belt, with dry asteroids and water-rich objects similar to comets. [64] [65] However, it is unclear whether conditions in the solar nebula would have allowed Jupiter and Saturn to move back to their current positions, and according to current estimates this possibility appears unlikely. [66] Moreover, alternative explanations for the small mass of Mars exist. [67] [68] [69]

### Late Heavy Bombardment and after Edit

Gravitational disruption from the outer planets' migration would have sent large numbers of asteroids into the inner Solar System, severely depleting the original belt until it reached today's extremely low mass. [51] This event may have triggered the Late Heavy Bombardment that occurred approximately 4 billion years ago, 500–600 million years after the formation of the Solar System. [2] [70] This period of heavy bombardment lasted several hundred million years and is evident in the cratering still visible on geologically dead bodies of the inner Solar System such as the Moon and Mercury. [2] [71] The oldest known evidence for life on Earth dates to 3.8 billion years ago—almost immediately after the end of the Late Heavy Bombardment. [72]

Impacts are thought to be a regular (if currently infrequent) part of the evolution of the Solar System. That they continue to happen is evidenced by the collision of Comet Shoemaker–Levy 9 with Jupiter in 1994, the 2009 Jupiter impact event, the Tunguska event, the Chelyabinsk meteor and the impact that created Meteor Crater in Arizona. The process of accretion, therefore, is not complete, and may still pose a threat to life on Earth. [73] [74]

Over the course of the Solar System's evolution, comets were ejected out of the inner Solar System by the gravity of the giant planets, and sent thousands of AU outward to form the Oort cloud, a spherical outer swarm of cometary nuclei at the farthest extent of the Sun's gravitational pull. Eventually, after about 800 million years, the gravitational disruption caused by galactic tides, passing stars and giant molecular clouds began to deplete the cloud, sending comets into the inner Solar System. [75] The evolution of the outer Solar System also appears to have been influenced by space weathering from the solar wind, micrometeorites, and the neutral components of the interstellar medium. [76]

The evolution of the asteroid belt after Late Heavy Bombardment was mainly governed by collisions. [77] Objects with large mass have enough gravity to retain any material ejected by a violent collision. In the asteroid belt this usually is not the case. As a result, many larger objects have been broken apart, and sometimes newer objects have been forged from the remnants in less violent collisions. [77] Moons around some asteroids currently can only be explained as consolidations of material flung away from the parent object without enough energy to entirely escape its gravity. [78]

Moons have come to exist around most planets and many other Solar System bodies. These natural satellites originated by one of three possible mechanisms:

• Co-formation from a circumplanetary disc (only in the cases of the giant planets)
• Formation from impact debris (given a large enough impact at a shallow angle) and
• Capture of a passing object.

Jupiter and Saturn have several large moons, such as Io, Europa, Ganymede and Titan, which may have originated from discs around each giant planet in much the same way that the planets formed from the disc around the Sun. [79] [80] [81] This origin is indicated by the large sizes of the moons and their proximity to the planet. These attributes are impossible to achieve via capture, while the gaseous nature of the primaries also make formation from collision debris unlikely. The outer moons of the giant planets tend to be small and have eccentric orbits with arbitrary inclinations. These are the characteristics expected of captured bodies. [82] [83] Most such moons orbit in the direction opposite the rotation of their primary. The largest irregular moon is Neptune's moon Triton, which is thought to be a captured Kuiper belt object. [74]

Moons of solid Solar System bodies have been created by both collisions and capture. Mars's two small moons, Deimos and Phobos, are thought to be captured asteroids. [84] The Earth's Moon is thought to have formed as a result of a single, large head-on collision. [85] [86] The impacting object probably had a mass comparable to that of Mars, and the impact probably occurred near the end of the period of giant impacts. The collision kicked into orbit some of the impactor's mantle, which then coalesced into the Moon. [85] The impact was probably the last in the series of mergers that formed the Earth. It has been further hypothesized that the Mars-sized object may have formed at one of the stable Earth–Sun Lagrangian points (either L4 or L5) and drifted from its position. [87] The moons of trans-Neptunian objects Pluto (Charon) and Orcus (Vanth) may also have formed by means of a large collision: the Pluto–Charon, Orcus–Vanth and Earth–Moon systems are unusual in the Solar System in that the satellite's mass is at least 1% that of the larger body. [88] [89]

Astronomers estimate that the current state of the Solar System will not change drastically until the Sun has fused almost all the hydrogen fuel in its core into helium, beginning its evolution from the main sequence of the Hertzsprung–Russell diagram and into its red-giant phase. The Solar System will continue to evolve until then.

### Long-term stability Edit

The Solar System is chaotic over million- and billion-year timescales, [90] with the orbits of the planets open to long-term variations. One notable example of this chaos is the Neptune–Pluto system, which lies in a 3:2 orbital resonance. Although the resonance itself will remain stable, it becomes impossible to predict the position of Pluto with any degree of accuracy more than 10–20 million years (the Lyapunov time) into the future. [91] Another example is Earth's axial tilt, which, due to friction raised within Earth's mantle by tidal interactions with the Moon (see below), is incomputable from some point between 1.5 and 4.5 billion years from now. [92]

The outer planets' orbits are chaotic over longer timescales, with a Lyapunov time in the range of 2–230 million years. [93] In all cases this means that the position of a planet along its orbit ultimately becomes impossible to predict with any certainty (so, for example, the timing of winter and summer become uncertain), but in some cases the orbits themselves may change dramatically. Such chaos manifests most strongly as changes in eccentricity, with some planets' orbits becoming significantly more—or less—elliptical. [94]

Ultimately, the Solar System is stable in that none of the planets are likely to collide with each other or be ejected from the system in the next few billion years. [93] Beyond this, within five billion years or so Mars's eccentricity may grow to around 0.2, such that it lies on an Earth-crossing orbit, leading to a potential collision. In the same timescale, Mercury's eccentricity may grow even further, and a close encounter with Venus could theoretically eject it from the Solar System altogether [90] or send it on a collision course with Venus or Earth. [95] This could happen within a billion years, according to numerical simulations in which Mercury's orbit is perturbed. [96]

### Moon–ring systems Edit

The evolution of moon systems is driven by tidal forces. A moon will raise a tidal bulge in the object it orbits (the primary) due to the differential gravitational force across diameter of the primary. If a moon is revolving in the same direction as the planet's rotation and the planet is rotating faster than the orbital period of the moon, the bulge will constantly be pulled ahead of the moon. In this situation, angular momentum is transferred from the rotation of the primary to the revolution of the satellite. The moon gains energy and gradually spirals outward, while the primary rotates more slowly over time.

The Earth and its Moon are one example of this configuration. Today, the Moon is tidally locked to the Earth one of its revolutions around the Earth (currently about 29 days) is equal to one of its rotations about its axis, so it always shows one face to the Earth. The Moon will continue to recede from Earth, and Earth's spin will continue to slow gradually. Other examples are the Galilean moons of Jupiter (as well as many of Jupiter's smaller moons) [97] and most of the larger moons of Saturn. [98]

A different scenario occurs when the moon is either revolving around the primary faster than the primary rotates, or is revolving in the direction opposite the planet's rotation. In these cases, the tidal bulge lags behind the moon in its orbit. In the former case, the direction of angular momentum transfer is reversed, so the rotation of the primary speeds up while the satellite's orbit shrinks. In the latter case, the angular momentum of the rotation and revolution have opposite signs, so transfer leads to decreases in the magnitude of each (that cancel each other out). [d] In both cases, tidal deceleration causes the moon to spiral in towards the primary until it either is torn apart by tidal stresses, potentially creating a planetary ring system, or crashes into the planet's surface or atmosphere. Such a fate awaits the moons Phobos of Mars (within 30 to 50 million years), [99] Triton of Neptune (in 3.6 billion years), [100] and at least 16 small satellites of Uranus and Neptune. Uranus's Desdemona may even collide with one of its neighboring moons. [101]

A third possibility is where the primary and moon are tidally locked to each other. In that case, the tidal bulge stays directly under the moon, there is no transfer of angular momentum, and the orbital period will not change. Pluto and Charon are an example of this type of configuration. [102]

There is no consensus as to the mechanism of formation of the rings of Saturn. Although theoretical models indicated that the rings were likely to have formed early in the Solar System's history, [103] data from the Cassini–Huygens spacecraft suggests they formed relatively late. [104]

### The Sun and planetary environments Edit

In the long term, the greatest changes in the Solar System will come from changes in the Sun itself as it ages. As the Sun burns through its supply of hydrogen fuel, it gets hotter and burns the remaining fuel even faster. As a result, the Sun is growing brighter at a rate of ten percent every 1.1 billion years. [105] In about 600 million years, the Sun's brightness will have disrupted the Earth's carbon cycle to the point where trees and forests (C3 photosynthetic plant life) will no longer be able to survive and in around 800 million years, the Sun will have killed all complex life on the Earth's surface and in the oceans. In 1.1 billion years' time, the Sun's increased radiation output will cause its circumstellar habitable zone to move outwards, making the Earth's surface too hot for liquid water to exist there naturally. At this point, all life will be reduced to single-celled organisms. [106] Evaporation of water, a potent greenhouse gas, from the oceans' surface could accelerate temperature increase, potentially ending all life on Earth even sooner. [107] During this time, it is possible that as Mars's surface temperature gradually rises, carbon dioxide and water currently frozen under the surface regolith will release into the atmosphere, creating a greenhouse effect that will heat the planet until it achieves conditions parallel to Earth today, providing a potential future abode for life. [108] By 3.5 billion years from now, Earth's surface conditions will be similar to those of Venus today. [105]

Around 5.4 billion years from now, the core of the Sun will become hot enough to trigger hydrogen fusion in its surrounding shell. [106] This will cause the outer layers of the star to expand greatly, and the star will enter a phase of its life in which it is called a red giant. [109] [110] Within 7.5 billion years, the Sun will have expanded to a radius of 1.2 AU—256 times its current size. At the tip of the red-giant branch, as a result of the vastly increased surface area, the Sun's surface will be much cooler (about 2600 K) than now and its luminosity much higher—up to 2,700 current solar luminosities. For part of its red-giant life, the Sun will have a strong stellar wind that will carry away around 33% of its mass. [106] [111] [112] During these times, it is possible that Saturn's moon Titan could achieve surface temperatures necessary to support life. [113] [114]

As the Sun expands, it will swallow the planets Mercury and Venus. [115] Earth's fate is less clear although the Sun will envelop Earth's current orbit, the star's loss of mass (and thus weaker gravity) will cause the planets' orbits to move farther out. [106] If it were only for this, Venus and Earth would probably escape incineration, [111] but a 2008 study suggests that Earth will likely be swallowed up as a result of tidal interactions with the Sun's weakly bound outer envelope. [106]

Gradually, the hydrogen burning in the shell around the solar core will increase the mass of the core until it reaches about 45% of the present solar mass. At this point the density and temperature will become so high that the fusion of helium into carbon will begin, leading to a helium flash the Sun will shrink from around 250 to 11 times its present (main-sequence) radius. Consequently, its luminosity will decrease from around 3,000 to 54 times its current level, and its surface temperature will increase to about 4770 K. The Sun will become a horizontal giant, burning helium in its core in a stable fashion much like it burns hydrogen today. The helium-fusing stage will last only 100 million years. Eventually, it will have to again resort to the reserves of hydrogen and helium in its outer layers and will expand a second time, turning into what is known as an asymptotic giant. Here the luminosity of the Sun will increase again, reaching about 2,090 present luminosities, and it will cool to about 3500 K. [106] This phase lasts about 30 million years, after which, over the course of a further 100,000 years, the Sun's remaining outer layers will fall away, ejecting a vast stream of matter into space and forming a halo known (misleadingly) as a planetary nebula. The ejected material will contain the helium and carbon produced by the Sun's nuclear reactions, continuing the enrichment of the interstellar medium with heavy elements for future generations of stars. [116]

This is a relatively peaceful event, nothing akin to a supernova, which the Sun is too small to undergo as part of its evolution. Any observer present to witness this occurrence would see a massive increase in the speed of the solar wind, but not enough to destroy a planet completely. However, the star's loss of mass could send the orbits of the surviving planets into chaos, causing some to collide, others to be ejected from the Solar System, and still others to be torn apart by tidal interactions. [117] Afterwards, all that will remain of the Sun is a white dwarf, an extraordinarily dense object, 54% its original mass but only the size of the Earth. Initially, this white dwarf may be 100 times as luminous as the Sun is now. It will consist entirely of degenerate carbon and oxygen, but will never reach temperatures hot enough to fuse these elements. Thus the white dwarf Sun will gradually cool, growing dimmer and dimmer. [118]

As the Sun dies, its gravitational pull on the orbiting bodies such as planets, comets and asteroids will weaken due to its mass loss. All remaining planets' orbits will expand if Venus, Earth, and Mars still exist, their orbits will lie roughly at 1.4 AU (210,000,000 km), 1.9 AU (280,000,000 km), and 2.8 AU (420,000,000 km). They and the other remaining planets will become dark, frigid hulks, completely devoid of any form of life. [111] They will continue to orbit their star, their speed slowed due to their increased distance from the Sun and the Sun's reduced gravity. Two billion years later, when the Sun has cooled to the 6000–8000K range, the carbon and oxygen in the Sun's core will freeze, with over 90% of its remaining mass assuming a crystalline structure. [119] Eventually, after roughly 1 quadrillion years, the Sun will finally cease to shine altogether, becoming a black dwarf. [120]

The Solar System travels alone through the Milky Way in a circular orbit approximately 30,000 light years from the Galactic Centre. Its speed is about 220 km/s. The period required for the Solar System to complete one revolution around the Galactic Centre, the galactic year, is in the range of 220–250 million years. Since its formation, the Solar System has completed at least 20 such revolutions. [121]

Various scientists have speculated that the Solar System's path through the galaxy is a factor in the periodicity of mass extinctions observed in the Earth's fossil record. One hypothesis supposes that vertical oscillations made by the Sun as it orbits the Galactic Centre cause it to regularly pass through the galactic plane. When the Sun's orbit takes it outside the galactic disc, the influence of the galactic tide is weaker as it re-enters the galactic disc, as it does every 20–25 million years, it comes under the influence of the far stronger "disc tides", which, according to mathematical models, increase the flux of Oort cloud comets into the Solar System by a factor of 4, leading to a massive increase in the likelihood of a devastating impact. [122]

However, others argue that the Sun is currently close to the galactic plane, and yet the last great extinction event was 15 million years ago. Therefore, the Sun's vertical position cannot alone explain such periodic extinctions, and that extinctions instead occur when the Sun passes through the galaxy's spiral arms. Spiral arms are home not only to larger numbers of molecular clouds, whose gravity may distort the Oort cloud, but also to higher concentrations of bright blue giants, which live for relatively short periods and then explode violently as supernovae. [123]

### Galactic collision and planetary disruption Edit

Although the vast majority of galaxies in the Universe are moving away from the Milky Way, the Andromeda Galaxy, the largest member of the Local Group of galaxies, is heading toward it at about 120 km/s. [124] In 4 billion years, Andromeda and the Milky Way will collide, causing both to deform as tidal forces distort their outer arms into vast tidal tails. If this initial disruption occurs, astronomers calculate a 12% chance that the Solar System will be pulled outward into the Milky Way's tidal tail and a 3% chance that it will become gravitationally bound to Andromeda and thus a part of that galaxy. [124] After a further series of glancing blows, during which the likelihood of the Solar System's ejection rises to 30%, [125] the galaxies' supermassive black holes will merge. Eventually, in roughly 6 billion years, the Milky Way and Andromeda will complete their merger into a giant elliptical galaxy. During the merger, if there is enough gas, the increased gravity will force the gas to the centre of the forming elliptical galaxy. This may lead to a short period of intensive star formation called a starburst. [124] In addition, the infalling gas will feed the newly formed black hole, transforming it into an active galactic nucleus. The force of these interactions will likely push the Solar System into the new galaxy's outer halo, leaving it relatively unscathed by the radiation from these collisions. [124] [125]

It is a common misconception that this collision will disrupt the orbits of the planets in the Solar System. Although it is true that the gravity of passing stars can detach planets into interstellar space, distances between stars are so great that the likelihood of the Milky Way–Andromeda collision causing such disruption to any individual star system is negligible. Although the Solar System as a whole could be affected by these events, the Sun and planets are not expected to be disturbed. [126]

However, over time, the cumulative probability of a chance encounter with a star increases, and disruption of the planets becomes all but inevitable. Assuming that the Big Crunch or Big Rip scenarios for the end of the Universe do not occur, calculations suggest that the gravity of passing stars will have completely stripped the dead Sun of its remaining planets within 1 quadrillion (10 15 ) years. This point marks the end of the Solar System. Although the Sun and planets may survive, the Solar System, in any meaningful sense, will cease to exist. [3]

The time frame of the Solar System's formation has been determined using radiometric dating. Scientists estimate that the Solar System is 4.6 billion years old. The oldest known mineral grains on Earth are approximately 4.4 billion years old. [127] Rocks this old are rare, as Earth's surface is constantly being reshaped by erosion, volcanism, and plate tectonics. To estimate the age of the Solar System, scientists use meteorites, which were formed during the early condensation of the solar nebula. Almost all meteorites (see the Canyon Diablo meteorite) are found to have an age of 4.6 billion years, suggesting that the Solar System must be at least this old. [128]

Studies of discs around other stars have also done much to establish a time frame for Solar System formation. Stars between one and three million years old have discs rich in gas, whereas discs around stars more than 10 million years old have little to no gas, suggesting that giant planets within them have ceased forming. [30]

### Timeline of Solar System evolution Edit

Note: All dates and times in this chronology are approximate and should be taken as an order of magnitude indicator only.

– The accumulation of particles into a massive object by gravitationally attracting more matter – Cosmological model – History and future of the universe – Accumulation of matter around a planet – the scientific study of the origin, evolution, and eventual fate of the universe – Long term extrapolated geological and biological changes – from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time – The development of planet Earth from its formation to the present day – Idea that long-term presence of human presence in the universe requires a spacefaring civilization – Changes to a star over its lifespan – Formation of galaxies, galaxy clusters and larger structures from small early density fluctuations – Situation in which an astronomical object's orbital period matches its rotational period – Scientific projections regarding the far future
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2. ^ The combined mass of Jupiter, Saturn, Uranus and Neptune is 445.6 Earth masses. The mass of remaining material is
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## Is there a consensus as to where terrrestrial planet atmospheres in our solar system came from? - Astronomy

For some of the planetary atmospheres, it is easy to understand their origin. In the case of the Jovian planets, the last stage of their formation involved the gravitational accumulation of huge amounts of hydroen and other gases, and their atmospheres naturally arose directly from this accumulated material. For Pluto, there is no atmosphere, in the ordinary sense, most of the time, but when it aproaches the Sun, some of the more volatile ices evaporate (or, more accurately, sublime), forming a thin temporary atmosphere of nitrogen and methane. But what about the Terrestrial planets? They presumably formed too slowly to accumulate any gases, taking longer to form than it took the Sun to somehow rid the Solar Nebula of the gases which had originally made up most of its bulk and mass. How did they accumulate gases, if they formed in a region too hot for gases to exist as solids (ices), and formed too slowly to gravitationally attract gases, later on?

The Theory of Cometary Impacts
One possible answer involves cometary impact. This theory is attractive for several reasons. First, comets are made mostly of volatile materials, albeit in a frozen form: water, carbon dioxide, methane, ammonia, and other compounds of carbon, oxygen and nitrogen with each other, and with the hydrogen which made up the bulk of the Solar Nebula. As a result, a sufficiently large cometary bombardment could presumably deliver large amounts of such icy materias to the planetary surfaces, and depending upon how many comets ran into the Terrestrial planets, during the latter stages of their formation, this might explain most, if not all, of their volatile component (atmospheres, and hydrospheres). However, it is difficult to know whether enough comets might have run into the Terrestrial planets during the last stages of their bombardment to explain their atmospheres. The materials running around in the inner Solar System would have been, because of the high temperatures which existed at that time, exclusively rocky and metallic bodies. Even in the asteroid belt, where current temperatures are well below zero degrees Fahrenheit, temperatures could have ranged as high as six to eight hundred Fahrenheit degrees, during the time that microscoic bits of carbon compounds, ice and rocks would have been colliding with each other, and building up the outer planets.
Now, this doesn't mean that comets couldn't have run into the Terrestrial planets at all. Particularly toward the end of their formation, the Jovian planets would have had substantial gravity, and near misses between them and the cometary bodies left over from their formation certainly threw billions of dirty snowballs into the outer reaches of the Solar System, forming the Kuiper Disk, so there is no reason why large numbers of cometary bodies might not have also been thrown into the inner Solar System, adding to the volatile component of the Terrestiral bodies. It is just hard to know, or even estimate, what fraction of the impact features which we see on the ancient surfaces of objects like Mercury and the Moon might have been due to rocky bodies, and what fraction would have been due to icy bodies. In addition, although the detailed composition of some comets appears somewhat similar to the composition of the Terrestrial planets' atmospheres, most comets appear to have differences in the details of their composition (particularly deuterium) which would have led to slightly different compositions for the Terrestrial planet' atmospheres, compared to the ones they actually had. As a result, it is not at all clear whether cometary impacts accounted for a major portion of the original volatile materials inside the forming Terrestrial planets, or just a very minor portion of those materials.
Despite this uncertainty, the cometary impact theory is moderately popular, at the moment, for several reasons. One is the fact that the basic idea is fairly simple, and obvious. Another is the fact that, in 1994, Comet Shoemaker-Levy 9 happened to run into Jupiter. This was a spectacular event, caused by a passage of the comet by Jupiter, a near miss in which the planet's gravity tore the comet into nearly two dozen pieces, and, as they passed the planet, and tried to move into interplanetary space, slowed them down sufficiently that they were forced to fall back into Jupiter, hitting its atmosphere with speeds of the order of 130,000 miles per hour, and producing spectacular explosions in its upper atmosphere.

Hubble Space Telescope composite image of the pieces of Shoemaker-Levy 9
(H. Weaver (JHU), T. Smith (STScI), NASA, apod990814)

Dark spots on right are atmospheric debris produced by the impacts

Time-lapse sequence of photographs showing the gradual fading of the comet's effects.
(H. Hammel (SSI), WFPC2, HST, NASA, apod001105)

These explosions were actually visible from the Earth, and the sooty debris left over after the comets were vaporized and blown to bits covered areas of the upper atmosphere as large as the entire Earth for several weeks afterwards. Such impacts are very rare, even for Jupiter (perhaps once every 10,000 years or so), and the Earth is a much smaller target (both physically, and gravitationally), so they are even rarer here (perhaps once every few millions of years or so), but it did reinforce the idea, in the popular media, that comets could provide the volatile materials which make up our atmosphere and hydrosphere, especially since astronomers interviewed on various late-night talk shows at the time were not at all averse to such striking demonstrations as holding up a glass of water and stating that the contents of the glass were the remains of comets which had struck the Earth at some time in the past.
Of course, although the newsworthiness of this impact doesn't remove the uncertainty as to whether cometary impacts made a significant contribution to the formation of the Earth's atmosphere, more than 4 billion years ago, but it was a striking event (in every sense of the word), and makes the idea more compelling than it might otherwise have been.
The other reason why the cometary impact theory has been in the news recently has to do with some rather odd events in the atmosphere of the Earth. The upper atmosphere of the Earth is continually bombarded by high-energy particles from space (solar wind particles, cosmic rays, and particles ejected from the van Allen radiation belts), and as a result, there is a very faint atmospheric glow, which is easily visible from space, and has been photographed extensively since the dawn of the space age.
Very early on, it was noticed that at various times, small dark spots appear in the photographs, where the glow seems to disappear for periods of ten or twenty minutes, then gradually reappear. When this was noticed, the discoverer of these spots suggested that small cometary objects, with a mass comparable to an ordinary snowball, but a size many tens of meters across (basically a sort of mist of cometary frost filling a region the size of a small building) might be running into the atmosphere, temporarily changing its local composition, and damping out the normal atmospheric glow. Over a period of time, as the water vapor in the impacting objects mixed with the surrounding gases and became appropriately energized by further impacts, the glow returned.
This idea was met with considerable skepticism, for several reasons. One is that there was no guarantee that the dark spots were real. The equipment used to take the photographs was known to have occasional "glitches" in its operation, and single-pixel errors, permanent and temporary, were not unusual in photographs, so it was quite possible that the dark spots were "artifacts", rather than real deviations in the atmospheric behavior. The other reason had to do with the nature of the supposed snowballs. A compact snowball might be able to hold together, during its passage through the inner Solar System, and in fact we run into dozens of such objects every single day. But a haze of frost with the mass of a snowball, and the size of a building, should completely vaporize long before it reached the orbit of the Earth, so it was hard to see how such a thing could even exist near the orbit of the Earth. Worse yet, the number of dark spots per unit area of the atmosphere, multiplied by the size of the Earth, implied that vast numbers of such things were running through the inner Solar System. To the originator of this concept, that was an attractive idea, because it meant that huge amounts of fresh water and other volatile materials could be delivered to the Earth every single day, and over long periods of time, that could easily explain the atmosphere and hydrosphere of the Earth. However, to those who were somewhat dubious, it seemed quite unbelievable that such large numbers of such strange objects could possibly exist, without being detected, in one way or another. As an example, during a meteor shower, such as the Leonid meteor shower, small specks of material not much bigger than a sand grain can be observed hitting the Moon, by producing small flashes of light on its dark side. Even though the proposed snowballs were supposed to be spread out a bit, impacts by objects of that mass, regardless of how they were spread out, would produce very easily visible flashes on the surface of the Moon, flashes which are not observed, implying that the objects do not actually exist.
As a result of these problems, for several decades, this theory was pretty much ignored. However, not long after the impact of Comet Shoemaker-Levy 9 with Jupiter, it was shown, using more modern and, presumably more accurate, satellite observations, that the dark spots are actually real, and this theory was temporarily revived. However, although we still do not understand what is causing the spots, the arguments that strange snow clouds could produce them are still faced with the insuperable arguments against them, already discussed, and virtually no one believes that there is any validity to this theory.
However, despite the uncertainty or even near impossibility associated with these two ideas of cometary impact, their recent prominence in news reports keeps the idea of cometary impacts, as a source of Terrestrial planetary atmospheres, quite alive, and there may be at least some validity to these ideas, particularly for the variation involving impacts by large cometary bodies early in the history of the Solar System, as discussed at the end of this section.

The Theory of Volcanic Outgassing
The alternative theory of atmospheric formation is that gases were somehow trapped within the planets while they were forming, and then released, later on, through volcanic activity. Even nowadays, when fresh magma is brought to the surface of the Earth, from partial melting of upper mantle material, as in the volcanoes on the Hawaiian Islands, and in Iceland, the lavas contain huge amounts of dissolved gases, which provide enough pressure to blow fountains of lava several hundred feet above the surface. Early in the history of the Earth, there must have been even more gases trapped inside it, and when it melted and differentiated, most of its surface must have been molten at some point, and immense amounts of volcanic gases would have been released from its interior.
These gases consist primarily of compounds of hydrogen, which is the most abundant material in the Universe, and in the Solar System, and carbon, nitrogen and oxygen, which are the next most abundant reactive elements (helium is also fairly abundant, but is chemically inert, and doesn't form compounds), and sulfur. Water vapor (H2O) and carbon dioxide (CO2) are by far the most abundant volcanic gases, but sulfur dioxide (SO2), carbon monoxide (CO), hydrogen (H2), hydrogen sulfide (H2S), sulfuric (H2SO4) and hydrochloric acid (HCl) are also fairly abundant in modern-day volcanic gases, and helium (He) and hydrofluoric acid (HF) are not uncommon. In the early stages of Earth's history, nitrogen (N2), methane (CH4) and ammonia (NH3) are also thought to have been released in large amounts. The total amounts of these gases could well have been enormous -- quite possibly several hundred Earth atmospheres -- but the atmosphere wouldn't have remained like that for very long, because ultraviolet radiation from the Sun would have broken down molecules in the upper atmosphere, just as it now does, in the exosphere and upper mesosphere.
As hydrogen was split from its compounds (which represents most of the chemicals mentioned above), the light atoms would have gradually floated to the top of the atmosphere, and been gradually lost. Helium atoms would have also tended to disperse into space, although at a slower rate. Eventually, the major remaining component of the atmosphere would be carbon dioxide, with minor amounts of nitrogen. There would also, at some point, have been various amounts of oxygen and sulfur, but those chemicals are relatively reactive, and would have combined with the surface rocks in various ways, and been removed from the atmosphere. As a result, after a while, the atmospheres of the Terrestrial planets (at least the three that have enough gravity to hold onto gases) would have been very much like the atmosphere of Venus -- primarily carbon dioxide, with trace amounts of nitrogen and other very minor gases. In addition to being the same composition as Venus, they would have also been similarly thick, at least for the Earth, which probably had an atmosphere, at that point, about a hundred times thicker than now, made almost entirely of carbon dioxide. Mars, on the other hand, because of its smaller size, and because it may not have melted and differentiated quite as quickly and thoroughly, might have only had an atmosphere a few times thicker than our present atmosphere, and could have had an atmosphere not thicker than our present atmosphere.

The Evolution of the Terrestrial Planets' Atmospheres
Now, Venus still has an atmosphere just like the one just discussed, but neither the Earth nor Mars have that type of atmosphere. So, if that is the way our atmosphere was formed, why do we have the atmosphere we now have? In a nutshell, because of the presence of liquid water at the surface of the planet.
Because Venus is closer to the Sun than we are, it is considerably hotter (especially so, because of the runaway greenhouse effect which its atmosphere causes), so hot, in fact, that even when the Solar System had just finished forming, and the Sun was a bit fainter than now, the surface temperatures would have been sufficient to boil away any oceans which it might have developed (or, more accurately, to keep water vapor from ever condensing to form an ocean). With all of the water on Venus existing as vapors in its atmosphere, ultraviolet radiation from the Sun would have "soon" broken it down into hydrogen, which would have escaped into space, and oxygen, which would have combined with the surface rocks, leaving only a very minor amount of water (approximately one-quarter of the mass of the clouds, which are made of fuming sulfuric acid).
The Earth, however, is somewhat further, and when the Sun was a little fainter than now, even with the powerful greenhouse effect produced by a thick carbon dioxide atmosphere, would have been too cool to have a runaway greenhouse effect. The surface temperatures would have certainly been higher than the "standard" boiling point of water (212 Fahrenheit degrees, or 100 Celsius degrees), but that is the boiling temperature of water only when you have one Earth atmosphere of pressure. With an atmosphere a hundred times thicker than now, the boiling temperature of water would have been several hundred degrees, and even with a powerful greenhouse effect, water vapor could have, and would have, condensed to form the primitive oceans on the Earth.
With water oceans, the Earth's atmosphere developed very differently, because carbon dioxide can dissolve in water, forming a weak acid, carbonic acid (if you ever took a high school physics or chemistry class, your teacher may have tried to put you off drinking soda pop by leaving teeth or nails in a glass of soda pop overnight, to show how corrosive the material is, despite being a very weak acid). Meanwhile, weathering and erosion of the primitive continents would have been washing various metal oxides (primarily of iron, magnesium and calcium) into the oceans. These metal oxides are technically bases, and when combined with acids, such as carbonic acids, they form materials known as salts. Some such salts are soluble in water, but the salts formed by combining metal oxides with carbonic acid, which are called carbonates, are insoluble, and their solids would have been deposited on the sea floor as iron carbonate (siderite), magnesium carbonate (dolomite), and calcium carbonate (limestone). As the process of continental drift pushed the sea floors and continents together, immense slabs of these carbonate rocks piled up on the continents, until eventually, tens of thousands of cubic miles of what was once carbon dioxide gas was locked up in the surface rocks. As a result, over time, the carbon dioxide atmosphere of the Earth gradually disappeared, leaving only a relatively thin atmosphere of nitrogen and less common gases. (Note: If we wanted to, we could return the atmosphere to its original composition by digging up all those carbonate rocks, heating them up, and driving off the carbon dioxide. It would require a tremendous effort, and a tremendous expense, but it could be done, and if it were done, we would have the same sort of atmosphere we once had, an atmosphere just like that of Venus, and surface temperatures on the Earth would be raised by the greenhouse effects of those gases to several hundred degrees above zero.)

A similar thing might have happened on Mars, as well. Nowadays, Mars cannot have liquid water on the surface, except under very rare circumstances, because the atmospheric pressure is so low that at any temperature much above the freezing point, it would simply boil away into the atmosphere. But if Mars once had a carbon dioxide atmosphere as thick as, or even thicker than, our atmosphere, not only would there have been plenty of pressure to keep water in a liquid state, but temperatures would have been increased, by the greenhouse effect of the carbon dioxide, from their present-day values of almost a hundred degrees below zero, to well above the freezing temperature of water. In fact, conditions might have been very similar to present-day Earth-like conditions. It is not possible that Mars can have had such conditions recently, because its surface has large numbers of relatively ancient craters, which indicate that hardly any weathering and erosion, and relatively little geological activity, has taken place in Mars for the best part of four billion years, but it is quite possible that, around or before four billion years ago, Mars might have had very different conditions from today, including oceans and lakes, clouds and rain and rivers, and there is a large minority of astronomers and lay people who would dearly love to believe that this was so, because then the chance that life might have developed on Mars, early in its history, would be substantially enhanced.
Of course, if Mars did once have oceans, they would have been just as capable of dissolving its carbon dioxide atmosphere as the Earth's oceans were, at dissolving and precipitating out the carbon dioxide atmosphere of the Earth, as carbonate rocks. Over time, the atmosphere would have become thinner and thinner, but whereas, in the case of the Earth, the carbon dioxide would have been almost completely removed, because, being closer to the Sun, temperatures could have remained reasonable even as the atmosphere disappeared, at Mars' greater distance from the Sun, the loss of the carbon dioxide would have removed the greenhouse effect that kept it warm, and at some point, the oceans would have frozen, and the removal of carbon dioxide would have come to an end, leaving at least the thin atmosphere that we currently see.
Whether this is actually true is not known. If it is true, then Mars should have large amounts of frozen water either near, or somewhere below, its surface, perhaps even large amounts of frozen water mixed with carbon dioxide, and substantial amounts of carbonate rock, as well. As of now, however, we don't know if Mars contains any carbonate rock, and although there is a substantial suspicion that there are large amounts of permafrost buried beneath the Martian surface, there is no definite evidence to that effect. The river-like features which are found in some areas of Mars are most easily explained as flash flooding caused by a mixture of soil and water rushing downhill after volcanic heating melted part of a permafrost layer, but there are alternative theories of the formation of those features which involve a fluidized mixture of carbon dioxide and rock, instead. It will take further exploration of Mars, and a detailed study of its surface and interior, to establish, beyond any doubt, exactly what has caused the features that we see, and whether Mars once had a much thicker atmosphere than it now does, or substantial amounts of surface water.

Further Evolution of the Atmosphere of the Earth
This is not quite the end of the story, either, for either Mars or the Earth, as the atmosphere of Mars is much thinner than we would expect, even if the above discussion is correct, and the atmosphere of the Earth contains not only large amounts of nitrogen, but also a substantial minority of oxygen.
In the case of the Earth, we of course know the answer to this additional detail. Early in the history of the planet, in fact, well over 3 billion years ago, life had established an extensive foothold in the oceans of the planet. At first, lifeforms on Earth probably used hydrogen sulfide as their primary energy source, in the same way that nowadays, creatures which live near volcanic vents on the ocean floor do. Fairly early on, however, blue-green algae developed the ability to use photosynthesis -- absorbing sunlight, and using its energy to drive chemical reactions which turned carbon dioxide and water vapor into various sugars, leaving, as a waste product, substantial amounts of oxygen. At first, the amount of oxygen would have been relatively minor, which was good for the life that existed on the Earth at that time, because oxygen is a very active and corrosive material, and is toxic to many types of life (even animals, which depend upon oxygen for their energy, suffer irreparable harm to their lungs if the amount of oxygen in the atmosphere is too high). Over time, however, the amount of oxygen gradually grew, and those lifeforms which were most sensitive to its deleterious effects were driven into niches, such as at the bottom of the sea, where the oxygen abundance was close to zero. Meanwhile, other lifeforms developed, such as primitive animals, which were able to tolerate and even take advantage of the energy released when oxygen combines with other materials.
For a long time, the abundance of free oxygen in the atmosphere was well under 10% of the total, and various materials dissolved in the oceans, such as iron compounds, were free to remain in solution. Around two billion years ago, however, the oxygen content rose to such a high amount that oceanic iron began to oxidize, and rusty clays began to precipitate out of the oceans, all over the Earth, producing a characteristic series of reddish sediments referred to as red beds. During the time prior to the formation of these red beds, iron ores often contained substantial amounts of free iron, but since then, iron ores have mostly contained various oxides, such as hematite and limonite.
Even after that, the oxygen content continued to grow. Nowadays, it is over 20% of the total atmosphere, but that is not necessarily the greatest percentage that it has ever had. Prior to a few hundred million years ago, all lifeforms on Earth lived in the oceans, and the land surfaces were essentially bare. At that time, there was no reason why oxygen content couldn't reach values as high as 30%, and in fact, in the Carboniferous era, when life first began to move into swampy areas on the periphery of the continents, immense insects proliferated, including dragonflies with wingspans of as much as six feet. Nowadays, such large insects cannot exist, because insects do not have lungs. They breathe by absorbing oxygen through microscopic passages, or tubules, which allow air to diffuse into their bodies. In the current, 21% oxygen, atmosphere of the Earth, only insects a foot or less in size can get enough air into their deep interiors to maintain proper bodily functions. But if the oxygen content in the Carboniferous had been closer to 30%, insects could have been considerably larger, and still maintained adequate ventilation in their interiors. This does not, of course, prove that the oxygen content was that high at that time, but it does suggest that it might have been.
Soon afterwards, however, plants began to spread across the land surface of the Earth, and a problem developed which would have guaranteed that the oxygen content could no longer approach such high levels -- namely, the possibility of extensive fires. Because oxygen is so chemically reactive, a land surface covered by plants, in the presence of oxygen abundances well in excess of 25%, would have been extremely susceptible to furious firestorms, unlike anything we can now imagine. As a result, since the development of extensive plant life on the continents, oxygen abundance in the atmosphere has probably normally been well below 25%, as it now is.
One thing which might be noted is that some people like to imagine that life in some ways controls the atmosphere of the Earth, in a sort of almost intelligent feedback mechanism, so that the atmosphere, at least when unaffected by human activity, remains more of less stable, and "just right" for life to flourish. This is rubbish. There are substantial interactions between the atmosphere of the Earth, and the lifeforms which inhabit the Earth, and as one of these changes, it affects the other one, as well. But it is just as likely for changes in the atmosphere to produce sudden and drastic changes in the lifeforms which can inhabit the surface of the Earth, as it is for the atmosphere to somehow coddle and nurture the present, completely temporary variety of life. The history of the Earth in particular, and planets in general, involves long periods of time where things tend to remain more or less the same, but there are frequent catastrophic changes of one sort or another, driven by astronomical, atmospheric, geologic and biological changes, and the only thing that is true about the Earth is that change, and dramatic change at that, is as normal as stability.

Further Evolution of the Atmosphere of Mars
As a not quite final note, we return to the atmosphere of Mars. At one time, it was almost certainly a thick combination of various gases, primarily hydrogen compounds, and carbon dioxide. Later, after the hydrogen molecules had been photodissociated (broken down into hydrogen and other gases by the absorption of ultraviolet light), and the hydrogen had been lost, the atmosphere consisted primarily of carbon dioxide, as it does now, but presumably far more carbon dioxide than it currently has. Presuming that, at that time, the greenhouse effect of the carbon dioxide allowed the planet to have much warmer temperatures than now, it might have had extensive oceans, lakes, rain, rivers and erosion, for a short period of time. But the presence of large numbers of craters, many of which must date back the best part of four billion years, implies that this early episode of clement conditions, if it ever existed, must have soon ended. We might well ask, why?
One possibility is that, during this time, the Solar System was still full of large numbers of relatively large objects, bits of rubble left over from the formation of the planets, which were still running into (or being swept up by) them. Perhaps one such object hit Mars a sufficiently violent blow to eject most of its atmosphere into space. Or, as previously stated, perhaps a good part of the carbon dioxide dissolved in the ancient oceans, and as the greenhouse effect of the atmosphere disappeared, the oceans froze, and that process ceased. If so, however, there should still have been more atmosphere left, than there now is. Why is the current atmosphere so thin?
Partly, this is because Mars is so small that it is only barely able to hold onto gases. As long as the gases in its atmosphere remain as whole molecules, which are relatively heavy, they can be held onto for many billions of years. But there is a continual breakdown of the molecules into individual atoms, as a result of the absorption of ultraviolet light, and the individual atoms are a bit lighter than the molecules that were broken off of. As a result, although the process is very slow, it is possible for otherwise permanently stable portions of the atmosphere to gradually escape into space. It would take billions of years to lose significant amounts of gas in this way, but the Solar System is 4.5 billion years old, so Mars could have lost as much as 90% of the atmosphere that was left over after any initial changes, during that time. Whether this is enough of a loss to explain the current, very thin atmosphere, is not at all certain, and most astronomers and planetary geologists believe that there must still be substantial amounts of gases somehow trapped inside the planet, either never having been outgassed to the surface (as a result of incomplete differentiation and volcanic activity), or combination with the rocks (as in the case of carbonates, if they exist), or in some other way. But, as already mentioned, only time will tell which, if any, of these theories are correct.

## How to make an atmosphere

For gas giants like Jupiter and Saturn, lasting atmospheres come easy. They just grab whatever’s hanging around—usually hydrogen and helium—with their massive gravitational pull.

Rocky, terrestrial planets make their own air. This kind of world takes shape from a swirling cloud of mega-asteroids tens to hundreds of miles across. The coalescing space boulders smash together and heat up to thousands of degrees Fahrenheit. The inferno boils away frozen materials in the rock, and these gases become the planet’s starter atmosphere.

Researchers have long realized that asteroids, as samples of the stuff that Earth formed from, are the key to understanding our planet’s origins. But they’re far away and hard to get to, so instead planetary scientists study the remnants of asteroids that have made their way into the inner solar system and fallen to Earth: meteorites.

“They really are like leftover Legos of planets in our solar system,” Thompson says. “It’s really lucky that they come to us.”

## Formation of the Solar System

The location of a planet at this moment could be deceiving the spot it is currently in might not be where it always has been, or where it is going to end up. Planetary Migration is the movement of planets in a solar system. There are many different theories and hypotheses on how our solar system came to be although, planetary migration is one of the most widely-accepted by scientists. There are multiple different types of migration that help to explain the different ways in which this movement is described. The Nice model is currently the best understanding of how the planets have moved throughout time. This model was proposed by an international collaboration of scientists in 2005 and is used to explain the evolution of the solar system. [12] The Nice model suggests that “at the inner edge of the icy disk, some 35 AU from the Sun, the outermost planet began interacting with icy planetesimals, influencing the second sort of migration to occur: gravitational scattering.” [12] Planetary migration is a very large topic with many subtopics. We have explored the main topics and subtopics in depth and will be explaining them below.

### Nebular Hypothesis and the Sun

Figure 2: http://www.whillyard.com/science-pages/our-solar-system/images/solar-system-formation.jpg

There are many hypotheses and models that exist today of solar system formation. Focused on will be the most popular and accepted model which is called the nebular hypothesis. It was first developed in the 18 th century and has underwent changes and refinement over the years. The concept starts by explaining that the Solar System formed from the gravitational collapse of a fragment of a giant molecular cloud of gas and dust called a nebula. [1] This collapse triggered a spinning momentum, and as it condensed it spun faster atoms collide more frequently creating heat and creating a protostar.[2] Over millions of years the pressure and heat in the star became so high that hydrogen in it started to fuse leading to the star entering the “main sequence” or main phase of its’ life, which it is still in today.[1]

### Planetary Creation

The planets that exist today are thought to have began as small grains of dust leftover from the nebula collapse that orbited around the protostar. These grains of dust would collide and form together, becoming larger and larger, growing only centimetres per year over the course of millions of years.[3] The inner Solar System was too warm for molecules like water and methane to condense, so the planets such as Earth, Mercury, Venus, and Mars could only form from metal compounds which are quite rare in the universe, limiting their possible size. These metal/rocky clumps would become terrestrial planets. The planets that formed farther out, where it was possible for abundant icy compounds to remain frozen grew massive. Giant planets such as Neptune, Uranus, Saturn, and Jupiter.[4] Proof of these conclusions can be found by observing our solar system, firstly “All the planets orbit the Sun in the same direction. Most of their moons also orbit in that direction… (and the Sun) rotate in the same direction. This would be expected if they all formed from a disk of debris around the proto-Sun.”[5] and secondly “The planets also have the right characteristics to have formed from a disk of mainly hydrogen around a young, hot Sun. Those planets near the Sun have very little hydrogen in them as the disk would have been too hot for it to condense when they formed. Planets further out are mostly hydrogen, (since that was what was mostly in the disk), and are much more massive because there was so much more material they could be made from.”.[5] From continuing to observe the planets and materials in our own and other solar systems we can gain better understanding of how they are formed.

### Three Types of Planetary Migration

As stated before, planetary migration “is the decrease or increase in the orbital radius of a planet embedded in a protoplanetary disk due to interactions with the surrounding gas and/or solid material.” [11] Planetary migration is known to occur when there is a change in orbital momentum, such as a planet losing or gaining orbital angular momentum. This can be caused either by friction or by an “imbalance of momentum transfer” [11] between the planet and disk material.

The first type of planetary migration describes a process where a planetary disk pulls or pushes a planet to a new position. This type occurs when a planet is “too small to clear a gap in the protoplanetary disk”. [12]

The second type of migration occurs as a result of gravitational interactions between close-by planets or bodies. Type two only occurs when a large object is able to shatter a smaller one and create an equal force that bounces back on itself and therefor results in planetary movement.

The third type of planetary migration is also due to a gravitational effect, called tidal forces. This type occurs only between the planet and star and almost always results more circular orbits. The third type of planetary migration takes place over a long timescale of billions of years due to the fact that it “occurs through tidal interactions between different celestial bodies”. [12] This type of planetary migration acts over a much longer period of time than any of the other types.

Planetary migration is believed to be responsible for the position of giant extrasolar planets that have been discovered “orbiting at very small orbital radii” [12] and actually may be an important part in the evolution of protoplanetary bodies. It is also thought to have influenced the architecture of the Solar System.

### Results of Planetary Migration

Planetary migration is responsible for many phenomena that we observe in our night sky. It can be attributed to most of what we study in Astronomy, including the formation of planets and the elliptical orbits that our planets take. The solar system is under constant transformation which is a direct cause of planetary migration. The formation of planet cores is unique to planetary migration the debris within the disks are what eventually clumps together amassing for the core of a developing planet the core then moves inward or outward in the disk and this subsequently propagates what the planet will be made of typically of a hard rocky outer crust or else enveloped in gaseous materials.

Core accretion is the first theory on how planets came to be, but does not seem to be a great enough explanation for the development of the gas giants although it seems convincing enough for the creation of terrestrial planets. Disk instability is a relatively new discovery that helps explains what core accretion cannot in terms of an overall explanation of how the planets came to be and provides an explanation as to why there is Jovian planets and Terrestrial planets. The core accretion model states that the rocky cores of the planets were formed first, then gathering the lighter elements to form the planets outer layers this being an explanation for the terrestrial planets. [9] To back up the theory of core accretion is the observation of one such exoplanet gave credibility to the theory. A giant planet orbiting a star like sun called HD 149026 was observed and confirmed. Core accretion sees that a core needs to accumulate critical mass before it can accrete gas, thus, giving credit to the third type of gas-driven migration which uses a vortex of winds to accumulate gases.[9] It is important to note that the core is dependant upon planetesimal accretion, which is the gathering of planetesimal debris to help expand the mass of the core. For the gas giants, this [G3] [G4] [G5] seems untimely the ring of gas that will orbit the sun will only last 4-5 million years. It is either gathered by the planets or it simply evaporates.

The Disk instability theory suggests that over time, clumps will compact into a planet. These planets can form as quickly as a thousand years, trapping the disappearing gases inside. It is also suggested that these masses quickly stabilize themselves so they do not orbit into the sun. With the lighter materials being trapped inside, the heavier denser materials eventually sink to the core. Planetary migration is of course not just exclusively responsible for the formation of the planets, but the eventual orbit pattern that is observed today, along with any changes that have happened over time. [7] Planetary migration interacts with disks that are made of either gas or planetesimals, typically resulting in the alteration of the orbital parameters. The grand tack hypothesis can conceptualize this by suggesting that when Jupiter formed at 3.5 AU (Astronomical Units) from the sun (AU being the measurement of how far the coinciding planet is from the sun.) [8] It is thought that Jupiter migrated closer to about 1.5 AU away from the sun until it reversed its orbit to move outwards during the event of acquiring Saturn in its orbital resonance. This stopped when it reached its current distance at 5.2 AU. [8]

Migrating inward is due to loss of angular momentum. When a planet migrates outward, it is because of a gain in angular momentum until a planet equilibrates and stabilizes. This is also key in explaining the orbital parameters because the planets all have specific movement, which is also the ecliptic pattern that Kepler proved in his first law when researching the orbit of Mars. Of course, this attributed to the different size of the orbit, but all move relative to the ellipse pattern. When a planet orbits, it reaches the parhelion (which means the planet is the closest it will be to the sun on the opposite end of the spectrum). It will be at a point where the planet will be furthest from the sun, called the aphelion (which means that planets orbit slower when at the aphelion opposed to the increased speed in orbit when at their parhelion which attributes to the length of our years). Planetary migration has contributed to all of these theories and hypothesis and has allowed us to gain a better understanding of the processes that formed the planets and their orbital paths.

### Jupiter’s Role in our Solar System’s Evolution

Fig 6: https://thumbs-prod.si-cdn.com/pbcdLmyXhzcl0pofEVkJk3Z9hTM=/800𴨰/filters:no_upscale()/https://public-media.

The Solar System has gone through billions of years of planetary migration and evolution to achieve the current positions of the planets in our Solar System. It is common for other Solar Systems to be tightly packed with planets ranging in sizes around that of Earths, most having circular orbits. Another common trait of planetary development found in solar systems across space is that they contain short orbital periods, usually as short as a few days to months. Planets in our solar system have unique traits that differ from those of common solar systems found throughout space. The role Jupiter played started in the early stages of development when the formation of the solar system was in its infancy. Astronomers believe our solar system’s planet migration has been affected by the early evolution of Jupiter’s inward and outward migration.

When the solar system was in its early formative period, Jupiter migrated inward from 5 AU to a 1.5 astronomical units (AU) before moving back to its position of 5.2 AU where it now resides.[6] This inward migration shows us why the overall mass in many of the Solar Systems terrestrial planets to be low compared to those in other Solar Systems. It is also believed that during the early stages, Jupiter in its inner orbit around the Sun destroyed other planets that eventually formed together into small moons. [10] The debris from the destruction of the planets caused the asteroid belt that resides roughly between Mars and Jupiter. Overall Jupiter caused massive changes to the solar systems structure and can be attributed to Earth’s existence.

### Conclusion

In conclusion, most of what is outlined is backed by scientists around the world and are considered the basis of the beginning of our solar system. Just as anything with due diligence and the ability to create and learn on top of original ideas, comes a well rounded and well-balanced understanding that will allow for a clear solar system identity moving forward. The details of these models are most likely subject to change, but the consensus accommodates more models and is only an expansion of each other to consolidate the ideas into a well working undeniable model that everyone can agree upon. Just like the application of the disk instability model to facilitate the creation of gas giants this model needed to be adopted based on the improbability of the gas giant creation process stated in the core accretion model. The core accretion model gives a great explanation for the formation of terrestrial planets but an inept explanation for the formation of the Jovian planets. Furthermore, the Nebular Hypothesis which is recognized as the process by which our Solar System began, is joined and is the aggregate of all localized planetesimal phenomenon and their beginnings which is all in synchronous to the models that explain how planets found their orbital parameters. Planetary migration is arguably one of the most significant events to ever happen due to the fact that without it, Earth most likely would seize to exist, consciousness would be nothing more than a mere enigma, and the planet, with all of its functions, would only be a paradox. That is how important planetary migration is.

### Sources:

[1] Thierry Montmerle, Jean-Charles Augereau, Marc Chaussidon, Solar System Formation and Early Evolution: The First 100 Million Years . (Springer,2006)

[2] Jane S. Greaves, Disks Around Stars and the Growth of Planetary Systems. ( Science ,2005)

[3] P. Goldreich, W. R. Ward, The Formation of Planetesimals , (Astrophysical Journal, 1973)

[4] Ann Zabludoff, The Nebular Theory of the origin of the Solar System , (University of Arizona, 2003)

[5] Karen Masters, What is the evidence supporting the nebula theory of Solar System formation? (Ask an Astronomer ,2015)

[6] Konstantin Batygin Greg Laughlin, Jupiter’s decisive role in the inner Solar System’s early evolution , (Proceedings of the National Academy of Sciences)

## 7.4 Origin of the Solar System

Much of astronomy is motivated by a desire to understand the origin of things: to find at least partial answers to age-old questions of where the universe, the Sun, Earth, and we ourselves came from. Each planet and moon is a fascinating place that may stimulate our imagination as we try to picture what it would be like to visit. Taken together, the members of the solar system preserve patterns that can tell us about the formation of the entire system. As we begin our exploration of the planets, we want to introduce our modern picture of how the solar system formed.

The recent discovery of hundreds of planets in orbit around other stars has shown astronomers that many exoplanetary systems can be quite different from our own solar system. For example, it is common for these systems to include planets intermediate in size between our terrestrial and giant planets. These are often called superearths. Some exoplanet systems even have giant planets close to the star, reversing the order we see in our system. In The Birth of Stars and the Discovery of Planets outside the Solar System, we will look at these exoplanet systems. But for now, let us focus on theories of how our own particular system has formed and evolved.

### Looking for Patterns

One way to approach our question of origin is to look for regularities among the planets. We found, for example, that all the planets lie in nearly the same plane and revolve in the same direction around the Sun. The Sun also spins in the same direction about its own axis. Astronomers interpret this pattern as evidence that the Sun and planets formed together from a spinning cloud of gas and dust that we call the solar nebula (Figure 7.17).

The composition of the planets gives another clue about origins. Spectroscopic analysis allows us to determine which elements are present in the Sun and the planets. The Sun has the same hydrogen-dominated composition as Jupiter and Saturn, and therefore appears to have been formed from the same reservoir of material. In comparison, the terrestrial planets and our Moon are relatively deficient in the light gases and the various ices that form from the common elements oxygen, carbon, and nitrogen. Instead, on Earth and its neighbors, we see mostly the rarer heavy elements such as iron and silicon. This pattern suggests that the processes that led to planet formation in the inner solar system must somehow have excluded much of the lighter materials that are common elsewhere. These lighter materials must have escaped, leaving a residue of heavy stuff.

The reason for this is not hard to guess, bearing in mind the heat of the Sun. The inner planets and most of the asteroids are made of rock and metal, which can survive heat, but they contain very little ice or gas, which evaporate when temperatures are high. (To see what we mean, just compare how long a rock and an ice cube survive when they are placed in the sunlight.) In the outer solar system, where it has always been cooler, the planets and their moons, as well as icy dwarf planets and comets, are composed mostly of ice and gas.

### The Evidence from Far Away

A second approach to understanding the origins of the solar system is to look outward for evidence that other systems of planets are forming elsewhere. We cannot look back in time to the formation of our own system, but many stars in space are much younger than the Sun. In these systems, the processes of planet formation might still be accessible to direct observation. We observe that there are many other “solar nebulas” or circumstellar disks—flattened, spinning clouds of gas and dust surrounding young stars. These disks resemble our own solar system’s initial stages of formation billions of years ago (Figure 7.18).

### Building Planets

Circumstellar disks are a common occurrence around very young stars, suggesting that disks and stars form together. Astronomers can use theoretical calculations to see how solid bodies might form from the gas and dust in these disks as they cool. These models show that material begins to coalesce first by forming smaller objects, precursors of the planets, which we call planetesimals .

Today’s fast computers can simulate the way millions of planetesimals, probably no larger than 100 kilometers in diameter, might gather together under their mutual gravity to form the planets we see today. We are beginning to understand that this process was a violent one, with planetesimals crashing into each other and sometimes even disrupting the growing planets themselves. As a consequence of those violent impacts (and the heat from radioactive elements in them), all the planets were heated until they were liquid and gas, and therefore differentiated, which helps explain their present internal structures.

The process of impacts and collisions in the early solar system was complex and, apparently, often random. The solar nebula model can explain many of the regularities we find in the solar system, but the random collisions of massive collections of planetesimals could be the reason for some exceptions to the “rules” of solar system behavior. For example, why do the planets Uranus and Pluto spin on their sides? Why does Venus spin slowly and in the opposite direction from the other planets? Why does the composition of the Moon resemble Earth in many ways and yet exhibit substantial differences? The answers to such questions probably lie in enormous collisions that took place in the solar system long before life on Earth began.

Today, some 4.5 billion years after its origin, the solar system is—thank goodness—a much less violent place. As we will see, however, some planetesimals have continued to interact and collide, and their fragments move about the solar system as roving “transients” that can make trouble for the established members of the Sun’s family, such as our own Earth. (We discuss this “troublemaking” in Comets and Asteroids: Debris of the Solar System.)