Classification of Planets?

Classification of Planets?

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The data from Kepler has taught us that there is a much larger number of "Neptune-class" planets out there than we previously thought. I wonder, however, if this "dominance" of Neptunes is because of the wide range of radii we use to define this class. If I'm not mistaken, we classify a planet as "Earth-like" when it is in the range 0.75 to 1.25 R(Earth), "Super-earth" from about 1.25 to 2.0 R(Earth) and "Neptune class" from 2.0 to 6.0 R(Earth) (a much larger range which would stand out even more if we knew masses). Are we maintaining the nomenclature to classify planets of different characteristics or are we just "solar system biased"?

Classification of planets

…Jupiter to Neptune are called giant planets or Jovian planets. Between these two main groups is a belt of numerous small bodies called asteroids. After Ceres and other larger asteroids were discovered in the early 19th century, the bodies in this class were also referred to as minor planets or…

…also called the Jovian, or giant, planets—Jupiter, Saturn, Uranus, and Neptune—are large objects with densities less than 2 grams per cubic cm they are composed primarily of hydrogen and helium (Jupiter and Saturn) or of ice, rock, hydrogen, and helium (Uranus and Neptune). The dwarf planet Pluto is unique—an icy,…

Extrasolar planets

…upper atmospheres of some close-in giant planets. The first detected transiting planet was HD 209458b in 1999. Both radial velocity and transit techniques are most sensitive to large planets orbiting close to their stars.

The free-floating giant planets had a different history in that they were probably formed in circumstellar disks but were ejected from their solar systems through gravitational interactions.

…first known “hot Jupiter,” a gas giant planet orbiting very close to its star. Such planets upended then-current ideas of planetary system formation, which were based on the solar system, in which gas giants orbit far from the Sun. The hot Jupiters likely formed far from their stars and migrated…

Extraterrestrial life

The atmosphere of Jupiter is composed of hydrogen, helium, methane, ammonia, some neon, and water vapour. These are exactly the gases used in experiments that simulate the early Earth. Laboratory and computer experiments have been performed on the application of


…systems, many of which contain giant planets orbiting very close to their stars. (See below Studies of other solar systems.) Another has been the unexpected finding from the Galileo spacecraft mission that Jupiter’s atmosphere is enriched with volatile substances such as argon and molecular nitrogen (see Jupiter:

…many discoveries were systems comprising giant planets the size of several Jupiters orbiting their stars at distances closer than that of the planet Mercury to the Sun. Totally different from Earth’s solar system, they appeared to violate a basic tenet of the formation process discussed above—that giant planets must form…

The four outer giant gas planets are roughly similar in size and chemical composition, but each has a set of moons that differ widely in their characteristics, and in some ways they and their satellites resemble miniature solar systems. The four rocky inner planets had a common origin…


by Steven J. Dick

Table of contents:
1. Introduction to the Three Kingdom System
2. Defining astronomy&rsquos 82 classes
3. Classification principles in the Three Kingdom system
4. Uses of the system and future development Acknowledgments

Although classification has been an important aspect of astronomy since stellar spectroscopy in the late 19th century, to date no comprehensive classification system has existed for all classes of objects in the universe. Here we present such a system, and lay out its foundational definitions and principles. The system consists of the three kingdoms of planets, stars and galaxies, 18 families, and 82 classes of objects. Gravitation is the defining organizing principle for the families and classes, and the physical nature of the objects is the defining characteristic of the classes. The system should prove useful for both scientific and pedagogical purposes.

1. Introduction to the Three Kingdom System

This article introduces a &rarr classification system of celestial objects developed by the author. In contrast to biology, physics and chemistry, and despite a long and distinguished history of classifying specific objects such as stars and galaxies, astronomy lacks a comprehensive classification system for what has become a veritable celestial zoo. What would such a system look like, and based on what principles? Here we present a system devised for pedagogic use over the last several decades (Figure 1), but that will also be useful for scientific purposes. This so-called Three Kingdom System begins with the three kingdoms of planets, stars and galaxies, stipulates six families for each kingdom, and distinguishes 82 distinct classes of astronomical objects. Like biology, it is hierarchical, extending from kingdom to family to class, with the possible extension to further categories lower in the hierarchy such as type and subtype. As in biological classification it occasionally adds an intermediate subfamily level wherever useful. With the benefit of hindsight, and with utility in mind, the system incorporates some classes as they have historically been defined, and adds others as they might be defined in a more coherent and consistent system.

In constructing such a system one immediately runs into the problem of how to define the categories of kingdom, family and class. The three kingdoms adopted here (planets, stars, galaxies) are the three canonical divisions adopted in astronomy textbooks for almost a century, since it became clear that galaxies were indeed a separate realm from our Milky Way Galaxy, as determined by the American astronomer Edwin Hubble in the early 1920s. For each kingdom six astronomical families are delineated, based on the object&rsquos origin (proto-), location (circum- and inter-), subsidiary status (sub-) and tendency to form systems (systems), in addition to the &ldquocentral&rdquo family (planet, star or galaxy) with respect to which the other families are defined. These considerations give rise to astronomy&rsquos 18 families, and the symmetry of the six families of each kingdom reflects their physical basis in gravity&rsquos action in all three kingdoms.

For a more general introduction to astronomical classification and its issues see Buta, Corwin et al. (2007), DeVorkin (1981), Dick (2013 2018), Feigelson (2012), Gray and Corbally (2009), Morgan (1937 1988), Morgan and Keenan (1973), and Sandage (2005).

2. Defining astronomy&rsquos 82 classes

The Three Kingdom System contains 82 classes of objects, as delineated in Figure 1.

But this begs the question: How does one define a class of astronomical objects? More specifically, how does one recognize a new class of objects? We have tackled these questions in previous books, including Discovery and Classification in Astronomy: Controversy and Consensus (Dick 2013), and Classifying the Cosmos: How We Can Make Sense of the Celestial Landscape (Dick 2019), in which the Three Kingdom System is laid out in full and the history and science of each class is described.

One way of approaching the question of the definition of class is by looking at history, where (exceptions like stars and galaxies notwithstanding) classification has often been ad hoc, haphazard and historically contingent on circumstance. If astronomical history demonstrates anything, it is that the classification of astronomical objects has been based on many characteristics, depending on the state of knowledge and the needs of a particular community at the time. For example planets could be divided according to their physical nature (terrestrial, gas giant and ice giants) or as the recent discovery of planetary systems has taught us, by orbital characteristics (highly elliptical or circular), proximity to their parent star (&ldquohot Jupiters&rdquo) and so on. Historically, binary stars have often been classified by the method of observation as visual, spectroscopic, eclipsing and astrometric, or (after more information became known) by the configuration or contents of the system, such as a white dwarf binary, or by the dominant wavelength of its electromagnetic radiation, as in an X-ray binary. While these overlapping systems have served astronomers well and illustrate how the same object may be classified in many ways, such designations are the source of much confusion among students, not to mention indecipherable to the public.

History also demonstrates that at the time of discovery, by the very nature of the problem, it is sometimes difficult to decide if a new class of object has been discovered. Perhaps by analogy with the Earth&rsquos Moon Galileo decided relatively quickly that the four objects he first saw circling Jupiter in 1610 were satellites, proof that the Moon was not unique, but a member of a class of circumplanetary objects (even if he did not speak in terms of &ldquoclass&rdquo). But the object he first saw surrounding Saturn was not at all obviously a ring, and awaited the interpretation of Christiaan Huygens more than 40 years later. Even in the late 20th century it was not immediately evident that pulsars were neutron stars, or that quasars were active galactic nuclei, both qualifying in the end for new class status.

Inconsistency notwithstanding, the criterion that astronomers have most often used in the astronomical literature for determining class status &mdash and the one we adopt for the Three Kingdom System &mdash is the physical nature of the object. In the planetary kingdom, for example, rather than orbital characteristics the definition of planetary classes in our own solar system has been based on their physical characteristics as rocky, gaseous, or icy in composition pulsar planets have also been distinguished by being inferred as physically very different again due to the extreme nature of their environment and probable different origin. As we have noted, new classes of planets will undoubtedly be uncovered as observations of extrasolar planets progress, but thus far not enough is known about their physical nature to do so. Many of the extrasolar planets discovered so far are believed to be gas giants many are close to their stars and thus called &ldquohot Jupiters.&rdquo The first terrestrial extrasolar planets have also been claimed, in the form of &ldquoSuper-Earths&rdquo and the first rocky transiting system, known as CoRoT-7b.

This history indicates that a comprehensive classification system for astronomy can perhaps do no better than to use the typological definition of &lsquoclass&rsquo largely discarded by biologists: &ldquomembership in a class is determined strictly on the basis of similarity, that is, on the possession of certain characteristics shared by all and only members of that class. In order to be included in a given class, items must share certain features which are the criteria of membership or, as they are usually called, the defining properties. Members of a class can have more in common than the defining properties, but they need not. These other properties may be variable &ndash an important point in connection with the problem of whether or not classes may have a history&rdquo (Mayr 1988, 337).

But what is the unit of classification for astronomy? For physics it is elementary particles. For chemistry it is the elements, defined by atomic number in the Periodic Table. For biology it is species at the macro level, giving rise to biology&rsquos &ldquoFive Kingdoms,&rdquo still favored by some macrobiologists, and genetic sequences of 16S ribosomal RNA at the molecular level, giving rise to Carl Woese&rsquos &ldquoThree Domains&rdquo of Archaea, Bacteria and Eucarya &mdash favored by most molecular biologists [1]. For astronomy, the unit of classification adopted here is the astronomical object itself, and with some theoretical justification. For as strong and weak forces are dominant in particle physics, and as the electromagnetic force is dominant in chemistry (except for nuclear chemistry), so in astronomy is it the weakest but most far-reaching force of gravity that predominantly acts on and shapes these astronomical objects. Though other considerations such as hydrostatics and gas and radiation pressure come into play, gravity is the determining factor for the structure and organization of planets, stars and galaxies, their families and classes of objects. To put it another way, the strong interaction holds protons and neutrons together and allows atoms to exist the electromagnetic interaction holds atoms and molecules together and allows the Earth to exist and the gravitational interaction holds astronomical bodies together and allows the solar system, stellar systems and galactic systems to exist [2]. Gravity is thus a prime candidate &mdash the one adopted here &mdash to serve as the chief organizing principle for a comprehensive classification system for all astronomical objects.

Where does such a definition of class lead in the construction of a classification system? In the kingdom of the stars stellar spectra were first classified on what turned out to be a temperature sequence, a system devised at Harvard in the late 19th century with its familiar O, B, A, F, G, K, and M stars and so on. Spectra were later classified on a luminosity scale, devised at Yerkes Observatory in the 1940s, the so-called MKK (Morgan-Keenan-Kellman) system with its dwarfs, giants and supergiants [3]. Which to choose to delineate &ldquoclasses&rdquo for stars in a more comprehensive system for astronomical objects? We have adopted the Yerkes/MKK system (now known as the MK system) as a more evolved two-dimensional system based on spectral lines sensitive not only to temperature, but also to surface gravity (g) and luminosity. As astronomers Richard Gray and Christopher Corbally recently put it in their magisterial volume Stellar Spectral Classification in connection with the luminosity classes, &ldquoStars readily wanted to be grouped according to gravity as well as according to temperature, and this grouping could be done by criteria in their spectra&rdquo (Gray and Corbally 2009, 10). The resulting luminosity classes (main sequence, subgiant, giant, bright giant, and supergiant labeled from Roman numeral V to I respectively), together with the stellar endpoint classes (supernova, white dwarf, neutron star and black hole) not only have significance in the evolutionary sequence, but also have a real history of discovery that can be uncovered. W. W. Morgan delineated these luminosity classes to begin with because he realized each grouping of stars formed a sequence of near constant log g (surface gravity) (Gray and Corbally 2009, 9-10 and Morgan 1937, 380 ff.). Thus gravity as a sculpting force for stars was recognized already by the founders of the MKK system as the dominating force for the luminosity classes.

The choice of luminosity for stellar classes does not subordinate the Harvard system of spectral types. To the contrary, Harvard spectral types are still an integral part of the system. As the originators of the Yerkes/MKK system argued, it is simply the case that their system contains more information and better represents the physical nature of stars, as astronomers gradually separated them (over the 30 years from 1910 to 1940) into supergiants, bright giants, giants and subgiants. In other words, since 1943 with the Yerkes/MKK system, modern astronomy has a formal two-dimensional temperature-luminosity system with distinct classes, building on the Hertzsprung-Russell diagram, which was literally a two-dimensional plot of temperatures versus luminosities when it was first constructed around 1914. Both the Harvard and the Yerkes systems are represented in the full designation of a star, as in Sirius (A1V) as a main sequence star with Harvard spectral type A1.

Thus, choices for class status become more clear-cut once there is a guiding principle such as physical meaning, which goes to the heart of Morgan&rsquos quest for &ldquothe thing itself&rdquo. Again in the stellar kingdom, for the interstellar medium instead of &ldquodiffuse nebulae&rdquo (a morphological classification), classes in the Three Kingdom System are distinguished according to physical constitution of the nebulae: gas (cool atomic neutral hydrogen, hot ionized hydrogen and molecular), and dust (reflection nebulae). These categories are used in astronomy and subsume classifications based on morphology that are historically contingent. In the galactic kingdom galaxy morphologies (elliptical, lenticular, spiral, barred spiral and irregular) laid out by Edwin Hubble in the 1920s also reflect compositional differences (as Morgan&rsquos galaxy classification system showed), so the principle of physical meaningfulness still holds.

3. Classification principles in the Three Kingdom System

As we have stipulated, by definition Kingdoms are delineated by the three central prototypes of objects in the universe &ndash planets, stars, and galaxies, as enshrined in canonical textbooks since the 1950s. Families are delineated by the various manifestations of the gravitational force acting on astronomical objects, e.g. protoplanetary, planetary, circumplanetary, subplanetary, interplanetary, and systems. As in any classification system, there will be ambiguities of placement in lower taxon levels. These can be mitigated by a system of classification principles. For the Three Kingdom System these include the following when it comes to the determination of classes and the placement of objects in classes:

  1. Classes are delineated based on the physical nature of the object, defined as physical composition wherever possible.
  2. An object should always be placed in its most specific class.
  3. To the extent possible, classes already in use are retained, as in the luminosity classes of the MK system and the Hubble classes for galaxies, supplemented by new knowledge.
  4. The recommendations of the International Astronomical Union are followed e.g. a dwarf planet is not a class of planet.
  5. Potential, but unverified, classes are not included.

Figure 1 is the result of applying these principles to astronomical objects. For those who do not recognize their favorite objects, it is likely because they exist at a taxonomic level below that of &ldquoclass&rdquo. The plethora of variable stars, for example, are not classes of objects in this system, on the same level as giant and dwarf stars, and so on. Rather, they are types of these stars that could be elaborated in a more complete system.

It is important to emphasize that classification in astronomy has similarities and differences with classification in biology, chemistry and physics. The most obvious difference between the classes (species) in biology and the classes in astronomy, at least as depicted in our Three Kingdom System, is the sheer number of species. E. O. Wilson, the Harvard naturalist who is one of the chroniclers of the diversity of life, has estimated that by 2009, 150 years after Darwin&rsquos Origin of Species, some 1.8 million species had been discovered and described, out of perhaps tens of millions that now exist. And this does not include what Wilson (in a rare astronomical analogy employed in the domain of biology) calls the &ldquodark matter&rdquo of the microscopic universe, which could be tens or hundreds of millions of species of sub-visible organisms [4].

The number of &lsquospecies&rsquo or classes in astronomy is obviously put to shame by the effusive and creative diversity of biology, no matter how one defines class or what classification system one uses. In terms of number, astronomy&rsquos classes, at least as defined in the Three Kingdom System, are more comparable to elements in chemistry (93 natural and 15 artificial), or to the phyla (32) and classes (90) in just one of Lynn Margulis&rsquos Five Kingdoms (Animalia) of biology, which contains almost a million species by itself. Any such comparison depends not only on how one defines a class of astronomical objects, but also whether the classes as defined here in the Three Kingdom System are really analogous to species in the biological hierarchy of classification, or to elements in the linear classification. That is also a matter of definition, and in part a subjective matter based on relation to higher and lower categories in the system. One can argue whether a giant star of luminosity class III in the MK system should be called a class or a type, but one cannot argue that a particular member of the class, a type of giant star such as an RR Lyrae, for example, should be placed at a higher level in the system than the class of which it is a member.

This classification exercise also illustrates a problem that astronomical taxonomy has in common with biological taxonomy: classification characteristics do not necessarily conform to evolutionary relationships. The class of giants as defined by the MK system definition was not precisely the same as the class of giants that Henry Norris Russell declared about 1910, nor is it entirely coextensive with the evolutionary states of the giant stars as known today. Russell&rsquos definition (and the Mt. Wilson system) was based on size and luminosity, as determined by their distances and apparent magnitudes, which could be converted to luminosity. The MKK definition was based on spectroscopy, in particular &lsquoline ratios&rsquo defined by standard stars. If an unclassified star matched the standard in a spectroscopic sense, it became a member of that class, such as a giant, without regard to its internal structure or evolutionary status. While luminosities and MK definitions are still used, today astrophysicists often think of giant stars and other stellar classes in terms of their evolutionary state, which for a giant is normally undergoing core helium fusion, but varies depending on the star&rsquos mass and where it stands in the spectral temperature sequence. Moreover a particular class may be adjusted based on new data in the early 1990s the Hipparcos satellite determined distances ten times more accurate than ground-based parallaxes, and correspondingly more accurate luminosities. The data showed that many of the luminosities were in error, and in the post-Hipparcos, and now the Gaia spacecraft era, the modern concept of a giant star (core helium fusion with shell hydrogen burning via the CNO cycle) is by no means co-extensive with MK class III defined by spectral line ratios. Nevertheless, the general classes of stars remain, but with a broader definition than determined by the MK system.

In short, astronomical classes have evolved in a way analogous to biology, where &ldquothe way it looks&rdquo (the phenotype) was primary in the five kingdom classification embraced by zoologists, as opposed to the deeper structure based on &rarr genetic makeup (the genotype). But whereas in biology Woese&rsquos three domain system caused an uproar in biology with its finding of a completely new domain of life and different relationships for parts of the classification system, the classification of stars by how they physically operate rather than by how they appear has thus far led to broader thinking with only minor adjustments [5].

4. Uses of the system and future development

A good classification system must not only be useful, but should also lead to deeper understanding and advance its subject. The uses of the Three Kingdom System are at least threefold, all of which may potentially lead to deeper understanding for different audiences.

First, for scientific purposes, as a comprehensive system for all astronomical objects based on consistent physical principles, the Three Kingdom System brings a consistent set of classification principles to discussions such as the status of Pluto as a planet. It suggests that the definition of a planet should not be based primarily on hydrostatic equilibrium, or roundness, or dynamical considerations, but on physical constitution &ndash just as stellar classification was based on consistent physical principles as determined by spectroscopy. Other criteria may indeed enter any classification decision, but they should be secondary. The Three Kingdom System thus brings consistency to astronomical classification, and more clarity in making classification decisions. In the process it might also, over the longer term, bring consistency to astronomical nomenclature as far as taxa such as class and type are concerned.

Secondly, again for scientific purposes, the symmetric structure of the Three Kingdom System facilitates comparisons at three different scales. In the comparison of families across kingdoms, one can ask, for example, how the interplanetary, interstellar and intergalactic media compare, and analyze what this tells us about the nature of the cosmos. Similarly for protoplanetary, protostellar and protogalactic processes, and so on. Such comparison are sometimes already made, but the Three Kingdom System cries out for such comparison in a systematic way. Comparisons of classes across kingdoms may also prove enlightening. Planetary rings, stellar rings and galactic rings in the form of stellar streams have much in common as broken up remains, but at vastly different scales and energies. Similarly for planetary, stellar, and galactic jets, or subgalactic, substellar and subplanetary objects. However, since the bedrock definition of a class is that at least one representative object must have been observed, we have not included a class of planetary jets, even though the discovery of brown dwarf jets in 2007 led to speculation that planetary jets might exist during the accretion phase of gas giants. Based on symmetry among families in the Three Kingdoms, we might also predict the existence of such jets, as well as other objects. While some might argue that volcanic eruptions or water spouts from Europa or Enceladus might qualify as jets, this does not seem to me quite analogous to stellar and galactic jets formed by energetic processes. But one could argue.

Thirdly, there is an educational advantage for the teaching of astronomy. The Three Kingdom System allows students to perceive immediately where an object fits in the scheme of astronomical objects. In assessing a new discovery, for example, whether the object is a type, class, family or kingdom should help a student to see its relative importance in the astronomical zoo. Thus definitive proof of a new Kingdom in astronomy would be vastly more important than, say, a new type of subgiant star. Moreover, the decision as to whether a particular class should be placed in a particular family can lead to fruitful discussion among students, and maybe even scientists. For example, the question of whether a globular cluster is circumgalactic or not will lead students to realize that these objects are not found just surrounding the galaxy, but also within the galaxy, and so on.

Finally, as new discoveries are made in astronomy the Three Kingdom System may well be elaborated. For the most part the additions and revisions will be made at the class and type level, for example, as new classes of planets are discovered, or new classes of baryonic dark matter objects are revealed, or newly detected objects are analyzed such as the mysterious &ldquoG objects&rdquo at the center of our galaxy that look like gas clouds but behave like stars (W. M. Keck Observatory 2018). It is not out of the question that a new family could be added, though this seems unlikely given our definition of family. At the kingdom level, surprisingly, one can already glimpse a possible new entry: the universe itself may be one of a class of objects in what has been called the multiverse. Because this is a kingdom that so far we have not seen, but only inferred from concepts like the anthropic principle, it has not been included in the Three Kingdom System at present. Only time will tell. More fundamentally we must always remember we are classifying baryonic objects composed of protons, electrons and neutrons, and that baryonic matter constitutes only 4.6 % of the matter and energy content of the universe. Non-baryonic dark matter is 23%, and dark energy (believed to be responsible for the accelerating universe) is 72%. But we have no idea what that dark matter and dark energy may be. Classification of the objects that we know notwithstanding, plenty of work remains for future astronomers based on what we do not yet know.

Finally, it is essential to emphasize that because all classes and classification systems are socially constructed, the Three Kingdom System for astronomy is not the only system that could be proposed. But in the end, like the other classification systems, its raison d&rsquoêtre and its staying power are dependent on its accuracy, simplicity and utility, both in scientific and pedagogical terms. Such features are an asset for astronomical classes and classification systems in general.


1. On the three domain versus five kingdom controversy in biology see especially Sapp (2009). On classification in physics and chemistry see Gordin (2004), Pickering (1984) and Gell-Mann (1994).

2. Davies (2007), especially chapter 4. Isaac Asimov has made the same point in his popular books for example, Asimov (1992, 263).

3. For more on these classification systems for stars see Dick (2013, Chapter 4). A recent popular account of the development of the Harvard system is Sobel (2016).

4. Wilson (2010, xi). In 2011 a group of biologists using a novel analysis estimated 8.7 million eukaryotic species exist, give or take a million. Eukaryotic species contain a nucleus, in contrast to prokaryotes (Strain 2011).

5. Taxonomy has also evolved, see Mayr (1982, 145), for stages in classification, and microtaxonomy vs macrotaxonomy.


Asimov, Isaac. 1992. Atom: Journey Across the Subatomic Cosmos. New York: Penguin.

Buta, Ronald J., Harold G. Corwin, Jr., and Stephen C. Odewahn. 2007. The de Vaucouleurs Atlas of Galaxies. Cambridge: Cambridge University Press.

Davies, Paul. 2007. Cosmic Jackpot: Why Our Universe is Just Right for Life. Boston and New York: Houghton-Mifflin.

DeVorkin, David H. 1981. &ldquoCommunity and Spectral Classification in Astrophysics: The Acceptance of E. C. Pickering&rsquos System in 1910&rdquo. Isis, 72, 29-49

Dick, Steven J. 2013. Discovery and Classification in Astronomy: Controversy and Consensus. Cambridge: Cambridge University Press.

Dick, Steven J. 2019. Classifying the Cosmos: How We Can Make Sense of the Celestial Landscape. New York: Springer.

Feigelson, Eric. 2012. &ldquoClassification in Astronomy: Past and Present.&rdquo In Advances in Machine Learning and Data Mining for Astronomy. London: CRC Press, Taylor & Francis Group, eds.: Michael J. Way, Jeffrey D. Scargle et al., 3-10.

Gell-Mann, Murray. 1994. The Quark and the Jaguar: Adventures in the Simple and the Complex. New York: W. H. Freeman.

Gordin, Michael D. 2004. A Well-Ordered Thing: Dmitrii Mendeleev and the Shadow of the Periodic Table. New York: Basic Books.

Gray, Richard O. and Christopher J. Corbally. 2009. Stellar Spectral Classification. Princeton and Oxford: Princeton University Press.

Mayr, Ernst. 1982. The Growth of Biological Thought: Diversity, Evolution and Inheritance. Cambridge, MA: Harvard University Press.

Mayr, Ernst. 1988. Toward a New Philosophy of Biology. Cambridge, MA: Harvard University Press.

Morgan, William Wilson 1937. &ldquoOn the Spectral Classification of the Stars of Types A to K&rdquo. Astrophysical Journal 85, no. 5: 380-97.

Morgan, William Wilson and P. C. Keenan. 1973. &ldquoSpectral Classification&rdquo. Annual Reviews of Astronomy and Astrophysics, 11, 29-50

Morgan, William Wilson. 1988. &ldquoA Morphological Life&rdquo. Annual Reviews of Astronomy and Astrophysics, 26, 1-9.

Pickering, Andrew. 1984. Constructing Quarks: A Sociological History of Particle Physics. Edinburgh: Edinburgh University Press.

Sandage, Alan. 2005. &ldquoThe Classification of Galaxies: Early History and Ongoing Developments&rdquo. Annual Reviews of Astronomy and Astrophysics, 43, 581-624.

Sapp, Jan. 2009. The New Foundations of Evolution. Oxford: Oxford University Press.

Sobel, Dava. 2016. The Glass Universe: How the Ladies of the Harvard Observatory Took the Measure of the Stars. New York: Viking.

Strain, Daniel. 2011. &ldquo8.7 Million: A New Estimate for All the Complex Species on Earth&rdquo. Science 333, no. 6046: 1083.

W. M. Keck Observatory. 2018, &ldquoMore Mystery Objects Detected Near Milky Way&rsquos Supermassive Black Hole&rdquo. News Release, June 6 7, 2018,

Wilson, Edward Osborne. 2010. &ldquoForeword&rdquo. In Kingdoms and Domains: An Illustrated Guide to the Phyla on Earth by Lynn Margulis and Michael Chapman. Amsterdam: Elsevier, lxi-lxii.

Version 1.0, published 2019-05-08, last edited 2019-05-16
Article category: KOS, specific (domain specific)

Color Classification of Extrasolar Giant Planets: Prospects and Cautions

Atmospheric characterization of directly imaged planets has thus far been limited to ground-based observations of young, self-luminous, Jovian planets.

Near-term space- and ground- based facilities like WFIRST and ELTs will be able to directly image mature Jovian planets in reflected light, a critical step in support of future facilities that aim to directly image terrestrial planets in reflected light (e.g. HabEx, LUVOIR). These future facilities are considering the use of photometry to classify planets. Here, we investigate the intricacies of using colors to classify gas-giant planets by analyzing a grid of 9,120 theoretical reflected light spectra spread across different metallicities, pressure-temperature profiles, cloud properties, and phase angles.

We determine how correlated these planet parameters are with the colors in the WFIRST photometric bins and other photometric bins proposed in the literature. Then we outline under what conditions giant planet populations can be classified using several supervised multivariate classification algorithms. We find that giant planets imaged in reflected light can be classified by metallicity with an accuracy of >90% if they are a priori known to not have significant cloud coverage in the visible part of the atmosphere, and at least 3 filter observations are available.

If the presence of clouds is not known a priori, directly imaged planets can be more accurately classified by their cloud properties, as oppposed to metallicity or temperature. Furthermore, we are able to distinguish between cloudy and cloud-free populations with >90% accuracy with 3 filter observations. Our statistical pipeline is available on GitHub and can be extended to optimize science yield of future mission concepts.

Natasha E. Batalha, Adam J. R. W. Smith, Nikole K. Lewis, Mark S. Marley, Jonathan J. Fortney, Bruce Macintosh
(Submitted on 23 Jul 2018)

New class of planets - Super-Puffs!

Kepler 51’s trio, which was first described in 2014, take planetary puffiness to new levels. Their discovery was “straight-up contrary to what we teach in undergraduate classrooms,” Berta-Thompson said.

The researchers developed new estimates for the trio’s masses and densities. And, sure enough, all three planets had a density less than 0.1 grams per cubic centimeter of volume—almost identical to the sweet pink treats you can buy at any fairground, Libby-Roberts said.

Both Kepler 51 planets seemed to be shedding gas at a rapid pace. The innermost of the three worlds, for example, dumps an estimated tens of billions of tons of material into space every second. The group calculated that if that trend continued, the planets could shrink considerably over the next billion years, losing their cotton candy-like puffiness.

In the end, they might wind up looking more like a common class of exoplanets called “mini-Neptunes.”

Edited by BillP, 20 December 2019 - 12:15 AM.

#2 Cali

Sure, why not? I prefer Raspberry but the Little Lady likes Cherry. “mini-Neptunes” seems like a Great Treat! Anyone have a recipe? (Cheese-Balls come to mind. )

Edited by Cali, 20 December 2019 - 04:37 AM.

#3 sg6

Some of these exoplanet descriptions leave me puzzled.

I have read of "second earths" that have twice the gravity of earth and a surface temperature of 400C, hardly a second earth.

Now we have exoplanets that are "Super puffs" or candy floss.

I assume they have a department for "Notable names and quotes" and have asked them to come up with some title.

Suppose one has to question if they are planets at all. Reads more of a what was a potentially condensing gass ball that is going to be stripped away and possibly to leave very little. Probably to form a gas planet one would need to shrink and exist at the lower temperature orbit from the central star.

Lets dig into it: It is losing tens of billions of tons of material every second and if it continues it could shrink considerably over the next billion years.

What do they expect it to do? Lose tens of billions of tons of material every second and over a billion years and get bigger?

Not much of a calculation really.

I see one of the co-authors are Netflix.

#4 BillP

What do they expect it to do? Lose tens of billions of tons of material every second and over a billion years and get bigger?

Not much of a calculation really.

I see one of the co-authors are Netflix.

Don't confuse the media article language which I linked with the scientific research paper that the article is based on. The article provides a link to it. As far as Netflix, well you do have to credit those who provided funding, otherwise no research would have been conducted. That in no way diminishes the credentials of the others. Although, Netflix has a substantial research department and they are particularly savvy in algorithm development for machine learning as well as analytics, and may have collaborated with the team on exoplanet selection algorithms. FWIW, some of the exoplanet search algorithms in use by researchers were based on how Netflix approaches machine learning for their subscribers. So instead of providing funding to this team, they may have very well contributed to the research effort directly with their machine learning and analytics expertise. So much of the science breakthroughs we have are not just from universities, but from the science research departments within big businesses! E.g., https://www.inverse. -for-exoplanets

1 Department of Astrophysical and Planetary Sciences, University of Colorado

2 Anton Pannekoek Institute for Astronomy, University of Amsterdam

3 Department of Astrophysical Sciences, Princeton University

5 The University of Texas at Austin, Austin, TX

6 NASA Goddard Space Flight Center

7 GSFC Sellers Exoplanet Environments Collaboration, NASA GSFC

8 Division of Geological and Planetary Sciences, California Institute of Technology

Classification of Planets? - Astronomy

Perhaps even harder to distinguish are the false claims by those who say they are scientific from the claims arrived at using proper research methods. Much of the television and internet media are awash in pseudoscience claims that are wrapped in a technical/scientific-looking veneer in an attempt to give them some legitimacy. Pseudoscience (fake or bogus science) says it is scientific but does not follow the rigorous error-correction process of true science, particularly peer review. A couple of places in astronomy where pseudoscience claims are commonly made are astrology and UFOs as alien spacecraft. Astrology is dealt with here in this section and UFOs are dealt with in the Pseudoscience vs. science article. I also wrote a short critique about the "Flat Earth" idea that is gaining adherents since 2018. The critique is for my local newspaper column that has a strict word limit.

Many astronomy students take the class believing they are going to ``learn about the stars and planets.'' You will learn about these things! However, quite often when I probe a little more what people mean by that phrase ``learn about the stars and planets'', I find out that many people are thinking about astrology---a belief system in which the positions of the planets among the stars are thought to hold the key to understanding what you can expect from life. I find that even many of those who have a four-year college degree (including some college professors!) are thinking this when I tell them that I teach astroNOMY. Astronomy is a science, astrology is NOT. Today the two subjects are very different from one another, but hundreds of years ago astronomy and astrology were very similar to one another.

History of Astrology

While most astrologers were developing ways to predict the future of human events by careful observations of the sky, early astronomers were developing ways to predict the motions of the planets, Sun, and Moon. Most early astronomers were motivated by the idea that if they could accurately predict the motions of the planets then they would be able to accurately predict the future of persons. Astronomy broke away from astrology and became a science when astronomers became more interested in explaining what made the planets move the way they do and not in divining the future and interactions of individuals.

A new class of planet

Three examples of a new family of planets, which orbit a pair of stars rather than a single one, have been discovered. The Milky Way may contain millions of these circumbinary planets. See Letter p.475

Although more than 700 extrasolar planets have been detected, none was known to orbit more than one star until the recent discovery 1 of a circumbinary planet, which orbits a pair of stars. This concept was previously confined to theory — and to science fiction, for example the planet Tatooine in Star Wars. On page 475 of this issue, Welsh et al. 2 describe the discovery of two more such planets and provide insight into their frequency of occurrence. The previously discovered planet 1 and the new ones, each of which orbits its own system of two stars, were found using NASA's Kepler space telescope. Footnote 1

For hundreds of years, scientists assumed that the Solar System is a typical example of a planetary system. That assumption was challenged in 1995 by the discovery 3 of 51 Pegasi b, the first planet to orbit a normal star other than the Sun. Although this planet is probably a gas giant (the lower limit on its mass is 0.47 Jupiter masses), it orbits at only 0.052 astronomical units ( AU ) from its star (1 AU is the average distance between Earth and the Sun). This means that 51 Pegasi b is 100 times closer to its star than Jupiter is to the Sun.

Planet 51 Pegasi b was discovered through precise measurements of the velocity of its parent star, which revealed the motion induced in the star by the presence of the orbiting planet. This method has proved very successful for spotting planets, and can be credited with the discovery of roughly 400 so far. As observational programmes continue, they become sensitive to planets on wider orbits (longer orbital periods). The dominant population of extrasolar planets currently consists of objects that are more massive than Jupiter and are separated from their host stars by several astronomical units many of these are in multi-planet systems.

The other very successful method for discovering planets is to look for those that periodically transit (eclipse) their parent star. These transiting planets are a gold mine of information: they are the only ones whose size can be obtained, by measuring the amount of starlight blocked during transit. This means that their surface gravities and mean densities can be calculated, ultimately allowing the investigation of their internal structure and formation process.

The transit method has led to the discovery of more than 200 planets, predominantly by teams that operate small wide-field robotic survey telescopes at observatories spread around the world, such as HATNet 4 and SuperWASP 5 . These surveys are heavily biased towards large planets with small orbits. As a result, they are unparalleled sources of oddballs such as WASP-17 (ref. 6), the biggest and most rarefied planet known (up to twice the radius of Jupiter and only 6% as dense), and WASP-18 (ref. 7), which is ten times the mass of Jupiter and whirls around its host star every 23 hours (Jupiter's orbital period is 11.9 years).

The overriding aim of planetary research is to find one that might support life. Habitability most probably requires a rocky surface with liquid water, which, in turn, demands a planet no bigger than two Earth radii on an orbit with a period of hundreds of days. The transits of such a body will not only be infrequent but will cause the light from the star to drop by a puny 0.01%. Such a signal is much too meagre to pick up with ground-based telescopes, which suffer from the blurring effect of Earth's atmosphere, as well as inevitable interruptions due to daylight and bad weather.

Finding a habitable planet requires a larger — and much more expensive — telescope outside Earth's atmosphere. Enter the Kepler spacecraft, the primary aim of which is to use the transit method to discover Earth-like planets. It monitors 150,000 stars in the constellations Cygnus and Lyra, and has already found more than 2,000 candidate transiting planets. Three of these have been confirmed to be circumbinary planets: Kepler-34 b (Fig. 1) and Kepler-35 b, which Welsh et al. describe in their study 2 , and Kepler-16 b (ref. 1). Not only does each of these three planets transit both of its parent stars, but the stars themselves eclipse each other.

The outer ellipse represents the orbital motion of the circumbinary planet Kepler-34 b, labelled b, around its host binary star system, which is composed of stars A and B in orbit around one another (as indicated by arrows). The plus sign shows the system's centre of mass. Spheres denote the orbital positions of the three bodies. One astronomical unit ( AU ) is the average distance between Earth and the Sun. Gravitational effects between the three bodies mean that this orbital configuration is gradually changing, so the bodies follow different paths on successive orbits. This is why the orbit of the planet shows a discontinuity in the upper part of the figure. Kepler-34 b is one of two circumbinary planets discovered by Welsh and colleagues 2 . (Modified from ref. 2.)

Although the discovery 1 of Kepler-16 b revealed that it was possible for such an object to exist, Welsh and colleagues' identification of two more circumbinary planets not only shows that such a planet is no freak object, but also allows an estimate of their prevalence to be made. The authors 2 find that, for short-period binary star systems, the frequency of occurrence of circumbinary planets is at least 1%. Taking into account the fraction of stars that are short-period binaries, this result implies that there are millions of such planets distributed throughout the Galaxy. This analysis does not account for longer-period binary star systems, which are similarly plentiful in the Galaxy.

Some circumbinary planets may even be habitable, although the three known ones are not. Kepler-16 b is slightly too cold, and Kepler-34 b and Kepler-35 b are too hot. They also have extreme seasons because the light received from their parent stars changes not only during the stars' orbital periods (tens of days) and the planetary orbital period (hundreds of days), but also on much longer timescales through precession of the orbits due to three-body effects.

What common characteristics do these three planets have? The central binary systems have orbital separations of between 0.18 and 0.22 AU , and the planets orbit their hosts at distances of between 0.6 and 1.1 AU . They are thus all close to the smallest possible stable orbits, but the fact that such planets were the first to be found is at least partly an effect of the detection method. As Kepler continues to observe, it will become sensitive to planets on longer periods: these three systems may represent only the tip of the iceberg.

Answer to Problem AM

Explanation of Solution

The necessary conditions for a celestial body to be a planet are as follows,

(1) It must be in an orbit about the Sun.

(2) It must have enough mass for self-gravity so that it forms an approximate round shape.

(3) It must be the dominant body within its orbit.

The celestial bodies which do not qualify all of these conditions are called dwarf planets. Examples of dwarf planets are Pluto, Ceres, Eris, Haumea and makemake.

So, dwarf planet is a new class of planets including Pluto.

Therefore, the word dwarf planet can be picked from the list.

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Astronomy 1101 - From Planets to the Cosmos

Astronomy 1101, From Planets to the Cosmos, is an overview of astronomy from our solar system to the universe as a whole. It is a General Education (GE) Physical Science course in the Natural Science category. The goals of courses in this category are for students to understand the principles, theories, and methods of modern science, the relationship between science and technology, the implications of scientific discoveries, and the potential of science and technology to address problems of the contemporary world.

Course Objectives

By the end of this course, students should successfully be able to:

  • Understand the basic facts, principles, theories, and methods of modern science.
  • Understand key events in the development of science and recognize that science is an evolving body of knowledge.
  • Describe the interdependence of scientific and technological developments.
  • Recognize the social and philosophical implications of scientific discoveries and understand the potential of science and technology to address problems of the contemporary world.

Astronomy 1101 will meet these expected outcomes by covering three interconnected themes:

  • The Long Copernican Revolution
    The nature of our solar system planetary systems around other stars the physics of gravity.
  • The Lives of Stars
    The nature and evolution of stars the origin of the elements the physics of light.
  • The Cosmos
    The nature and evolution of galaxies evidence for the Big Bang the structure of the universe on its largest scales.

This course attempts to convey a number of the facts that astronomers and astrophysicists have learned about these topics, to describe the outstanding scientific problems that are the focus of current research, to illustrate ways in which physical principles are used to understand the universe, and to show how scientific theories are developed and tested against observations.

Among the questions that you should be able to answer by the end of the course are the following:

  • What is the architecture of our solar system, and how do we find other planetary systems?
  • What is a star? How do stars form and evolve?
  • What is a galaxy? How do galaxies form and evolve?
  • What is the evidence for dark matter and dark energy?
  • What is the Big Bang?
  • What evidence supports or challenges our explanations for the physical nature of stars, galaxies, and the cosmos?

Course Organization

This is a 4 credit hour course each week, there will be 3 hours of lecture and one two-hour laboratory session. For Arts and Sciences students in a Bachelor of Arts program, this course meets the Arts and Sciences GE requirement of a natural sciences course that includes a laboratory component.

Course Catalog Description

Overview of the Copernican revolution, the discovery of the nature of our solar system, light, gravity, and planets around other stars the nature and evolution of stars and origin of the chemical elements the history of galaxies and the expanding universe. Weekly laboratory. Not recommended for students who plan to major in astronomy or physics.

Prerequisites: Math 1050 (075) or 102, or an ACT math subscore of 22 or higher that is less than two years old, or Math Placement R or higher or permission of instructor. Not open to students with credit for 1140, 1144, 1161H (H161), 1162H (H162), 2161H, 2162H, 2291 (291), or 2292 (292).

This 4-unit course has a required 1-unit lab section that all students must enroll in.


The University of Michigan Department of Astronomy may have established one of the first research observatories in the Midwest more than 150 years ago, but more than anything, it’s a young and vibrant department. That’s because we’ve spent the last decade hiring exceptional faculty, investing in high-profile facilities, and restructuring our curriculum — all to propel our graduates into the top ranks of the field. We invite you to explore our website or download our graduate viewbook, "The Full Spectrum" to learn more.

We strive to support our students and faculty on the front lines of learning and research and to steward our planet, our community, our campus. To do this, the Department of Astronomy needs your support.


The Michigan Institute for Research in Astrophysics (MIRA) was launched in 2014 to leverage astrophysics expertise from across the university and advance the field through themed conferences, collaborative research and related efforts. Learn more.