Astronomy

As the Sun dies, does the Earth's orbit change?

As the Sun dies, does the Earth's orbit change?



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As the Sun nears the end of its life and expands, would the Earths orbit get bigger as it moves out or would it stay in the same orbital plane and not move out? Therefore in the far off future, we'd have 512 days instead of 365 days?

As the Moon is moving out at a distance of 3.8 cms a year (ref: http://curious.astro.cornell.edu/about-us/37-our-solar-system/the-moon/the-moon-and-the-earth/111-is-the-moon-moving-away-from-the-earth-when-was-this-discovered-intermediate), are we moving out or in towards the Sun or is this not possible to work out because there is nothing to measure against?


The answer is yes. As the Sun ages, it will become a red giant and the mass loss rate from its surface will increase. This effect will increase (dramatically) further when the Sun enters the asymptotic giant branch phase, where thermal pulsations drives a cool wind that may carry away a millionth of a solar mass per year, eventually leaving a burned-out core in the form of a white dwarf with about half a solar mass.

At any point in this evolution we can model the evolution of the Earth's orbit using some simple approximations - that the wind from the Sun escapes to infinity, that a negligible proportion is actually accreted by the Earth and nor does it exert a torque, that the mass loss takes place on a timescale much longer than the Earth's orbit and that the mass of the Earth $m$ is always much less than the time-dependent mass of the Sun $M(t)$.

In which case we consider the orbital angular momentum of the Earth: $$ m a omega^2 simeq Gfrac{M m}{a^2},$$ where $a$ is the semi major axis. So the angular momentum $J = m a^2 omega$ is given by $$ J^2 = m^2 a^4 frac{G M m}{m a^3} propto M a$$

As the angular momentum of the Earth's orbit is conserved, the $M(t) a(t)$ is constant and as the Sun loses mass, the semi major axis increases by the same factor.

Coming to the specifics - when the Sun is a half solar mass white dwarf, the semi-major axis will be 2 au (assuming the giant Sun did not quite engulf it - it will be a close-run thing) and Kepler's third law $(P^2 propto a^3/M)$ can be used to estimate an orbital period of 4 years.

The tidal effects of the Sun on the Earth's orbit are quite negligible compared with these mass loss effects.


Rob Jeffries has already answered the question, but has left out a detail that should be included.

The mass that the sun loses to solar wind will cause the Earth to migrate outward. Increased drag from moving through a denser solar wind will slow the Earth and partly counteract the effect. Initially not by much, but as the sun expands the Earth will find itself moving through an increasingly dense medium. According to this paper, drag in the lower chromosphere will eventually be high enough for the planet to fall into the sun.


At the bottom of Earth's orbit

[Update: My apologies: due to a cut-and-paste error, I had mistakenly listed the perihelion distance as the average distance of the Earth to the Sun (147 versus 149 million km). To avoid confusion, I simply replaced the error with the correct value. The rest of the post is correct since this wasn't a math error but a typographical one, and I used the right value when doing my calculations below.] Since last July, the Earth has been falling ever closer to the Sun. Every moment since then, our planet has edged closer to the nearest star in the Universe, approaching it at over 1100 kilometers per hour, 27,500 km/day, 800,000 km every month. But don't panic! We do this every year. And that part of it ends today anyway. The Earth's orbit around the Sun is not a perfect circle. It's actually an ellipse, so sometimes we're closer to the Sun, and sometimes farther away. Various factors change the exact date and time every year -- you can get the numbers at the Naval Observatory site -- but aphelion (when we're farthest from the Sun) happens in July, and perihelion (when we're closest) in January. And we're at perihelion now! Today, January 3, 2011, around 19:00 GMT (2:00 p.m. Eastern US time), the Earth reaches perihelion. At that time, we'll be about 147,099,587 kilometers (91,245,873 miles) from the Sun. To give you an idea of how far that is, a jet traveling at a cruising speed of 800 km/hr would take over 20 years to reach the Sun. Of course, since today is when we're closest to the Sun this year, every day for the next six months after we'll be a bit farther away. That reaches its peak when we're at aphelion this year on July 4th, when we'll be 152,096,155 km (94,507,988 miles) from the Sun. Not that youɽ notice without a telescope, but that means the Sun is slightly bigger in the sky today than it is in July. The difference is only about 3%, which would take a telescope to notice. Frequent BA Blog astrophotograph contributor Anthony Ayiomamitis took these images of the Sun at perihelion and aphelion in 2005:

This may seem a bit odd if you're not used to the physics of orbital motion, but you can think of the Earth as moving around the Sun with two velocities: one sideways as it sweeps around its orbit, the other (much smaller) toward and away from the Sun over the course of a year. The two add together to give us our elliptical orbit. The sideways (what astronomers call tangential ) velocity is about 30 kilometers (18 miles) per second, which is incredibly fast. But then, we do travel an orbit that's nearly a billion kilometers in circumference every year! The velocity toward and away from the Sun (what we call the radial velocity because its direction is along the orbital radius) is much smaller only about 0.3 km/sec (which translates into the numbers I used in the first paragraph above). That's an average over the course of the year which I estimated very simply by taking the difference between our aphelion and perihelion distances -- almost exactly 5 million km (3 million miles) -- and dividing by the time it takes the Earth to move between them: half a year, or about 182 days. The exact speed changes, because at perihelion, we're closer to the Sun and feel its gravity a bit more strongly, so our speed around the Sun is a bit faster than at aphelion. Together, the tangential and radial velocities add up to gives us our overall orbital velocity, which changes with distance from the Sun. In fact, at perihelion today we'll be moving around the Sun at 30.1 km/sec, and at aphelion in July that will have slowed to about 29.6 km/sec. That's a change of about 1.7% enough to measure if you have the right equipment, but not anything youɽ notice in your daily life. This does bring up another interesting point: when we're closer to the Sun we receive more light -- and therefore energy and heat -- from it than when we're farther away. We can calculate that as well. The amount of energy you receive from an object gets smaller with the square of your distance: double your distance and you only get 1/4 the amount of light from it. Go 10 times farther away and that drops to 1/100 or 1%. At aphelion, we're 1.033 times farther from the Sun, so we get (1.033) ^2 or about 1.07 times less light and energy from it. You can flip that around to say that today we are receiving about 7% more sunlight than on aphelion in July! That may seem weird to folks living north of the equator, but seasons are a whole 'nuther issue . Oh, and hey, one more thing. Every now and again I'll hear from a kid or parent who tells me that they had a teacher or friend claim that if the Earth were even a few thousand miles closer or farther from the Sun weɽ burn up or freeze. That's clearly silly, since over the course of six months the Earth's distance to the Sun changes by 3 million miles! Not only that, but the Earth is 8000 miles (13,000 km) across and spins once a day. That means at noon you're 8000 miles closer to the Sun than you are at midnight, and I don't general see people bursting into flame and then freezing in a block of ice every 12 hours. So if you ever hear that particular bit of silliness, refer ɾm here. So there you go. You may not notice the Sun looking slightly bigger, or being warmer, or moving faster * than usual today, but it is. So if you're having a tough day, remember this: it's all uphill from here. Until July.

^* Relative to the stars it's moving faster, that is, since if you were to measure its speed across the sky as it rises and sets, the Sun would actually appear to be moving more slowly, because as you stand on the Earth its spin moves the Sun left to right relative to you (if you're in the northern hemisphere facing south, or standing on your head in Australia facing north) making a single circuit across the sky once per day, while the Earth's orbital motion moves the Sun right to left relative to the stars making a circuit once per year, with that motion fastest at perihelion, therefore subtracting from or slowing the diurnal (daily motion) of the Sun left to right, so the Sun appears to move in the sky most slowly at perihelion. Got it?


Minor Changes in the Orbit

If Earth were to move closer to the sun, it would mean far more intense heat on the planet. Glaciers all over the world would melt rapidly, causing a rise in sea levels and global chaos. Basically, the planet would be flooded. If Earth moved farther from the sun, however, all the bodies of water on the planet would freeze, basically freezing the entire planet with them. Also, the duration of each year would be longer.

Suffice to say, we&rsquore happy with the orbit on which we currently ride, so Earth had better not get any ideas about wandering away from its path!


How does a planet's orbit change as one of the suns in a binary-sun solar system starts do ɽie'?

Given a solar system with a binary sun, where one of the suns is noticeably smaller than the other given that such a system has only one planet with its companion moon.

The smaller sun starts to die. Will the orbit of the lone planet expand or contract? Will that planet's year get longer or shorter as the sun dies off?

Stars give away energy as they radiate (thus reducing their mass) but are also acquiring new mass from from surrounding sources, so stellar masses fluctuate over time, and yes typically they net lose mass over time. However, why do you associate the act of a star "dying" specifically with some noticeable change in its mass?

White dwarfs, neutron stars, and other stellar remnants generally have masses that are fractions of the original star's mass. White dwarfs only exist after a mid-sized star has shed its outer layers, and that shedding process during the asymptotic giant phase causes the star to lose a substantial amount of material. The same can be said of neutron stars, where the remnant is the product of a type II supernova, in which a big chunk of the outer envelopes of the star is blown outwards.

If it's a binary system, a lot of the evolution depends on the size, separation and type of the central stars.

If the stars are close enough, they can begin to interact as the bigger star swells to become a red giant. In this case, mass can transfer onto the smaller star, increasing its brightness, and causing it to grow and evolve faster. For two Sun-like stars, this transfer can't lead to a supernova, although once the initially larger star gets small enough, it'll create a planetary nebula and its core will become a white dwarf. Eventually (read: 'tens of millions of years later'), the secondary star will swell, possibly creating the reverse situation - resulting in a Type Ia supernova as gas accretes onto the white dwarf.

If the central stars are more separated, they will only interact via the stellar wind of the larger star. The smaller star will accrete a bit of this wind, but usually not a significant amount. The changing mass of the stars will cause the planet's orbit to grow and become subsequently slower. If the planet is widely separated from the stars, it is unlikely it would interact much with the wind


2 Answers 2

According to E. V. Petjeva (2011), the measured rate of change of the Earth-Sun distance (astronomical unit) is (1.2 +/- 3.2) cm/yr, with the uncertainty value representing 3 standard deviations. In other words, any change is within the uncertainty of the measurement. She specifically addresses the Krasinsky and Brumberg value. E. M. Standish has also addressed this issue.

The measurements are by radar echos off other planets and radio signals from space craft. See the below references for details.

(Note that "astronomical unit" is not technically the same as Earth-Sun distance, see Standish reference for details, Krasinsky and the others are all measuring the "astronomical unit" also "astronomical unit" was redefined to be a constant in 2012, see Nature reference for details)

This is inspired by . an article by G. A. Krasinsky and V. A. Brumberg, "Secular Increase of Astronomical Unit from Analysis of the Major Planet Motions, and its Interpretation."

Pitjeva and Pitjev (1) provide a simple explanation of the very large secular change in the astronomical unit found by Krasinsky and Brumberg:

"In the paper by Krasinsky and Brumberg the au change was determined simultaneously with all other parameters, specifically, with the orbital elements of planets and the value of the au astronomical unit itself. However, at present it is impossible to determine simultaneously two parameters: the value of the astronomical unit, and its change. In this case, the correlation between au and its change $dot$ reaches 98.1 %, and leads to incorrect values of both of these parameters".

What is the rate at which the Earth-Sun distance is changing?

The first article cited in DavePhD's answer by E.V. Pitjeva is based on the refereed article by Pitjeva and Pitjev (1). Both articles provide rates of change of the Earth-Sun distance (specifically, the Earth-Sun semimajor axis length $a$) and of the 1976 definition of the astronomical unit $au$ as $egin frac a &= (1.35pm0.32)cdot10^<-14>/ ext frac> &= (8pm21)cdot10^<-12>/ ext end$ (Note: the latter is based on the published value of $dot = 1.2pm 3.2$ cm/yr.)

The reason for the nearly three order of magnitude difference between these two figures is that the astronomical unit is not the distance between the Sun and the Earth. While that is how the astronomical unit was originally defined, the two concepts have been effectively divorced from one another since the end of the 19th century, when Simon Newcomb published his Tables of the Motion of the Earth on its Axis and Around the Sun. The divorce was made official in 1976 when the International Astronomical Unit redefined the astronomical unit to be the unit of length that made the Gaussian gravitational constant k have a numerical value of 0.017202098950000 when expressed in the astronomical system of units (the unit of length is one astronomical unit, the unit of mass is one solar mass, and the unit of time is 86400 seconds (one day)).

Who has done this analysis, and what do they find? Also, what methods are used?

There are three key groups:

  • The Institute of Applied Astronomy of the Russian Academy of Sciences, which produces the Ephemerides of Planets and the Moon series (EPMxxxx) of ephemerides (1)
  • The Jet Propulsion Laboratory of NASA, which produces the Development Ephemeris series (DExxx) of ephemerides (2), and also ephemerides for small solar system bodies and
  • The Institute of Celestial Mechanics and of the Calculation of Ephemerides of the Paris Observatory (L'institut de mécanique céleste et de calcul des éphémérides, IMCCE), which produces the Integration Numerique Planetaire de l’Observatoire de Paris series (INPOPxx) of ephemerides (3).

All three numerically solve the equations of motion for the solar system using a first order post Newtonian expansion given a set of states at some epoch time. This integration of course will not match the several hundred thousands of observations that have been collected over time. All three use highly-specialized regression techniques to update the epoch states so as to somehow minimize the errors between estimates and observations. All three carefully address highly correlated state elements, something that Krasinsky and Brumberg did not do. All three share observational data, sometimes cooperate (joint papers, IAU committees, . ), and sometimes compete ("our technique is better than yours (at least for now)").

For example is radar really that precise?

Regarding radar, the distance to the Sun was never measured directly via radar. Unless massively protected with filters, pointing a telescope of any sort directly at the Sun is generally a bad idea. If massively protected with filters, a radio antenna would not see the weak radar return. Those 1960s radar measurements were of Mercury, Venus, and Mars. There's no compelling reason to ping those planets now that humanity has sent artificial satellites in orbit about those planets. Sending an artificial satellite into orbit about a planet (as opposed to flying by it) provides significantly higher quality measurements than do radar pings.


Climate change: How Earth's changing orbit influenced climate and migration from Africa

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World Meteorological Organization reveals state of 2019 climate

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The gradually shifting orbit meant summers were more intense with heavier rainfall at some points in Earth&rsquos history and drier at others. The climate change process falls under the so-called Milankovitch cycles, which explain how the planet&rsquos orbital movements affect the climate.

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Researchers at the University of Wisconsin-Madison have now analysed the changing climate patterns in the last 140,000 years to show how they helped people migrate out of Africa.

Approximately 125,000 years ago, northern Africa and the Arabian Peninsula witnessed much more intense monsoons in the summertime.

With the increased summer rain, the Saharan and Arabian deserts were smaller and bordered by large grasslands.

More rain also meant more vegetation and resources were available for Africa&rsquos humans to thrive.

Climate change: Ancient climate patterns forced humans to migrate from Africa (Image: GETTY)

Climate change: Earth's orbit around the Sun shifts over many thousands of years (Image: NASA)

READ MORE

At the same time, the Mediterranean and the Levant &ndash Syria, Israel, Lebanon, Jordan and Palestine &ndash underwent increased rainfall in the wintertime.

These unusual climate patterns were the result of Earth&rsquos position relative to the Sun.

The planet&rsquos Northern Hemisphere was tilted towards the Sun much closer in the summer and much farther in the winter.

Professor emeritus John Kutzbach from the University of Wisconsin&ndashMadison said: &ldquoIt&rsquos like two hands meeting. There were stronger summer rains in the Sahara and stronger winter rains in the Mediterranean.&rdquo

As a result of the Milankovitch cycles, these parts of the world should be positioned in the same way about 21,000 years.

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But the cycles also mean the opposite was true about every 10,000 years.

There were stronger summer rains in the Sahara and stronger winter rains in the Mediterranean

Professor emeritus John Kutzbach, University of Wisconsin&ndashMadison

So if Africa experienced more vegetation and rainfall 125,000, 105,000 and 83,000 years ago, summers would have been drier and less green 105,000, 95,000 and 73,000 years ago, respectively.

The study also found the planet was in the grips of a glacial period between 70,000 and 15,000 years ago.

Climate models used in the study found there was a reduced amount of greenhouses gases in the wintertime as a result.

Climate change: Northern Africa was subject to heavier rain in the summer and more vegetation (Image: GOOGLE MAPS)

Climate change: Global temperatures are currently on the rise (Image: NASA)

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The reduced greenhouse gases intensified the winter storms in the Mediterranean but also led to a cooling of the climate around the equator.

The cooling led to an overall drier climate with less forest coverage.

The researchers have proposed the climate change patterns affected the amount of available vegetation, forcing humans living in Africa to settle new areas with more water and plants.

For the purpose of the study, the researchers modelled 140,000 years pf Earth&rsquos climate and orbital movements using the Community Climate System Model version 3 from the US National Center for Atmospheric Research.

The study came after Professor Kutzbach first studied in the 1970s and 1980s how changes in Earth&rsquos orbit can affect the climate.

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He said: &ldquoMy early work prepared me to think about this.&rdquo

However, the researcher noted there are some gaps in the model that do not, for instance, get cold enough in southern Europe during the glacial period.

Professor Kutzbach said: &ldquoThis is by no means the last word.

&ldquoThe results should be looked at again with an even higher-resolution model.&rdquo


Does Earth have a second moon?

Many planets in our solar system have more than one moon. Mars has two moons, Jupiter has 67, Saturn 62, Uranus 27, Neptune 14. Those numbers keep changing, and you can see a relatively current count of solar system moons here from NASA’s Jet Propulsion Laboratory. It makes sense that the outer worlds, with their stronger gravity, would have more moons. Meanwhile, our planet Earth has just one moon. Doesn’t it?

Moons are defined as Earth’s natural satellites. They orbit around the Earth. And, in fact, although Earth sometimes has more than one moon, some objects you might have heard called Earth’s second moon aren’t, really. Let’s talk about some non-moons first.

3753 Cruithne in 2001. Astronomer Duncan Waldron discovered this faint asteroid on October 10, 1986, on a photographic plate taken with the UK Schmidt Telescope at Siding Spring Observatory in Australia. Image via Sonia Keys via Wikimedia Commons. The orbits around the sun of Cruithne and Earth over the course of a year (from September 2007 to August 2008). More information about this animation here.

Quasi-satellites are not second moons for Earth. A quasi-satellite is an object in a co-orbital configuration with Earth (or another planet). Scientists would say there is a 1:1 orbital resonance between Earth and this object. In other words, a quasi-satellite is orbiting the sun, just as Earth is. Its orbit around the sun takes exactly the same time as Earth’s orbit, but the shape of the orbit is slightly different.

The most famous quasi-satellite in our time – and an object you might have heard called a second moon for Earth – is 3753 Cruithne. This object is five kilometers – about three miles – wide. Notice it has an asteroid name. That’s because it is an asteroid orbiting our sun, one of several thousand asteroids whose orbits cross Earth’s orbit. Astronomers discovered Cruithne in 1986, but it wasn’t until 1997 that they figured out its complex orbit. It’s not a second moon for Earth it doesn’t orbit Earth. But Cruithne is co-orbiting the sun with Earth. Like all quasi-satellites, Cruithne orbits the sun once for every orbit of Earth.

As seen from Earth Cruithne has what is known as a horseshoe orbit. In other words, viewed from Earth, it appears to orbit a point beside Earth. More information about horseshoe orbits here.

Earth’s gravity affects Cruithne, in such a way that Earth and this asteroid return every year to nearly the same place in orbit relative to each other. However, Cruithne won’t collide with Earth, because its orbit is very inclined with respect to ours. It moves in and out of the plane of the ecliptic, or plane of Earth’s orbit around the sun.

Orbits like that of Cruithne aren’t stable. Computer models indicate that Cruithne will spend only another 5,000 years or so in its current orbit. That’s a blink on the long timescale of our solar system. The asteroid might then move into true orbit around Earth for a time, at which time it would be a second moon – but not for long. Astronomers estimate that, after 3,000 years orbiting Earth, Cruithne would escape back into orbit around the sun.

By the way, Cruithne isn’t the only quasi-satellite in a 1:1 resonance orbit with Earth. The objects 2010 SO16 and (277810) 2006 FV35, among others, are also considered quasi-satellites to Earth.

These objects are not second moons for Earth, although sometimes you might hear people mistakenly say they are. Does Earth ever have more than one moon? Surprisingly (or not), the answer is yes.

Asteroids that are captured temporarily by Earth's gravity have crazy orbits around us, because they're pulled from all sides by the Earth, sun and moon. Image Credit: K. Teramuru, UH Ifa

Earth does sometimes have temporary moons. In March of 2012, astronomers at Cornell University published the result of a computer study, suggesting that asteroids orbiting the sun might temporarily become natural satellites of Earth. In fact, they said, Earth usually has more than one temporary moon, which they called minimoons. These astronomers said the minimoons would follow complicated paths around Earth for a time, as depicted in the images above and below. Eventually, they would break free of Earth’s gravity – only to be immediately recaptured into orbit around the sun, becoming an asteroid once more. The little moons envisioned by these astronomers might typically be only a few feet across and might orbit our planet for less than a year before going back to orbit the sun as asteroids.

Diagram of the orbit for 2006 RH120 during a period of time that it is orbiting the Earth during a temporary satellite capture event. Image via Wikimedia Commons.

Have astronomers detected any of these minimoons? Yes. Writing in the magazine Astronomy in December 2010, Donald Yeomans (Manager of NASA’s Near-Earth Object Program Office at NASA’s Jet Propulsion Laboratory) described an object discovered in 2006 that appears to fit that description. The object – now designated 2006 RH120 – is estimated to be 5 meters (about 15 feet) in diameter. Yeomans said that, when this object was discovered in a near-polar orbit around Earth, it was thought at first to be a third stage Saturn S-IVB booster from Apollo 12, but later determined to be an asteroid. 2006 RH120 began orbiting the sun again 13 months after its discovery, but it’s expected to sweep near Earth and be re-captured as a minimoon by Earth’s gravity later in this century.

Bottom line: That asteroid called 3753 Cruithne is not a second moon for Earth, but its orbit around the sun is so strange that you still sometimes hear people say it is. Meanwhile, astronomers have suggested that Earth does frequently capture asteroids, which might orbit our world for about a year before breaking free of Earth’s gravity and orbiting the sun once more.


What We’ve Learned About Pluto

One year after NASA’s New Horizons spacecraft had its quick, close-up look at Pluto, scientists are reaping the scientific rewards.

And while the planet has cooled over the last 900,000 or so years, it will reach another super season in another 900,000 years when the planet’s Southern Hemisphere tips toward the sun at the exact point it swings closest.

Earth’s orbit, will also change one day. While our planet’s closest approach to the sun currently takes place during the northern winter, it has slowly shifted over time. In about 10,000 years, its closest approach will occur six months later during the northern summer.

But given Earth’s relatively circular orbit, it will never have super seasons like those on Pluto or the extreme, asymmetric seasons of Mars. Instead, our planet will stay relatively stable — a characteristic that just might have given rise to life.