Planet

This article is about the astronomical object. For other uses, see Planet (disambiguation).

Planetary-sized objects to scale:
Top row: Uranus and Neptune;
second row: Earth, white dwarf star Sirius B, Venus;
bottom row (reproduced and enlarged in lower image) – above: Mars and Mercury;
below: the Moon, dwarf planets Pluto and Haumea

A planet (from Greek πλανήτης αστήρ planētēs astēr "wandering star") is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.^[a][1][2] The term planet is ancient, with ties to history, science, mythology, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of the gods. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists.

The planets were thought by Ptolemy to orbit the Earth in deferent and epicycle motions. Though the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. By careful analysis of the observation data, Johannes Kepler found the planets' orbits to be not circular, but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology.

Planets are generally divided into two main types: large, low-density gas giants, and smaller, rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four gas giants, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites. Additionally, the Solar System also contains at least five dwarf planets^[3] and hundreds of thousands of small Solar System bodies.

Since 1992, hundreds of planets around other stars ("extrasolar planets" or "exoplanets") in the Milky Way Galaxy have been discovered. As of November 17, 2011, 702 known extrasolar planets are listed in the Extrasolar Planets Encyclopaedia, ranging from the size of terrestrial planets somewhat larger than Earth to gas giants larger than Jupiter.^[4]

History

Further information: History of astronomy and Definition of planet

See also: Timeline of solar system astronomy

Printed rendition of a geocentric cosmological model from Cosmographia, Antwerp, 1539

The idea of planets has evolved over its history, from the divine wandering stars of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.

The five classical planets, being visible to the naked eye, have been known since ancient times, and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. Ancient Greeks called these lights πλάνητες ἀστέρες (planetes asteres "wandering stars") or simply "πλανήτοι" (planētoi "wanderers"),^[5] from which today's word "planet" was derived.^[6][7] In ancient Greece, China, Babylon and indeed all pre-modern civilisations,^[8][9] it was almost universally believed that Earth was in the center of the Universe and that all the "planets" circled the Earth. The reasons for this perception were that stars and planets appeared to revolve around the Earth each day,^[10] and the apparently common-sense perception that the Earth was solid and stable, and that it was not moving but at rest.

The name for planets in Chinese astronomy had the same motive as the Greek name, 行星 "moving star". In Japanese during the Edo period there were two competing terms, 惑星 "confused star" and 遊星 "wandering star". In modern Japan, terminology was unified in favour of 惑星, but in science fiction the alternative term 遊星 retains some currency.^{[citation needed]}

Babylon

Main article: Babylonian astronomy

The first civilization known to possess a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as the second millennium BC.^[11] The MUL.APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon and planets over the course of the year.^[12] The Babylonian astrologers also laid the foundations of what would eventually become Western astrology.^[13] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,^[14] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.^[15]

The Sumerians, predecessors of the Babylonians who are considered as one of the first civilizations and are credited with the invention of writing, had identified at least Venus by 1500 BC.^[16] Shortly afterwards, the other inner planet Mercury and the outer planets Mars, Jupiter and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times.^[17]

Greco-Roman astronomy

See also: Greek astronomy

Ptolemy's 7 planetary spheres

1 Moon

2 Mercury

3 Venus

4 Sun

5 Mars

6 Jupiter

7 Saturn

Because they were not as interested in divination as the Babylonians, the ancient Greeks initially did not attach as much significance to the planets. The Pythagoreans, in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras or Parmenides are said to have first identified the evening star and morning star (Venus) as one and the same.^[18] In the 3rd century BC, Aristarchus of Samos proposed a heliocentric system, according to which the Earth and planets revolved around the sun. However, the geocentric system would remain dominant until the Scientific Revolution.

By the 1st century BC, during the Hellenistic period, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century CE. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries.^[11][19] To the Greeks and Romans there were seven known planets, each presumed to be circling the Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.^[7][19][20]

India

Main articles: Indian astronomy and Hindu cosmology

In 499 CE, the Indian astronomer Aryabhata propounded a planetary model which explicitly incorporated the Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also believed that the orbit of planets are elliptical.^{[unreliable source?][21]} Aryabhata's followers were particularly strong in South India, where his principles of the diurnal rotation of the earth, among others, were followed and a number of secondary works were based on them.^{[22][page needed]}

In 1500, Nilakantha Somayaji of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhata's model.^[23][24] In his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, he developed a planetary model where Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits the Earth, similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century. Most astronomers of the Kerala school who followed him accepted his planetary model.^[23][24][25]

Medieval Muslim astronomy

Main articles: Astronomy in medieval Islam and Islamic cosmology

In the 11th century, the transit of Venus was observed by Avicenna, who established that Venus was, at least sometimes, below the Sun.^[26] In the 12th century, Ibn Bajjah observed "two planets as black spots on the face of the Sun," which was later identified as a transit of Mercury and Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.^[27] However, Ibn Bajjah could not have observed a transit of Venus, as none occurred in his lifetime.^{[citation needed]}

European Renaissance

Renaissance planets, ca. 1543 to 1781

1 Mercury

2 Venus

3 Earth

4 Mars

5 Jupiter

6 Saturn

See also: Heliocentrism

With the advent of the Scientific Revolution, understanding of the term "planet" changed from something that moved across the sky (in relation to the star field); to a body that orbited the Earth (or that were believed to do so at the time); and in the 16th century to something that directly orbited the Sun when the heliocentric model of Copernicus, Galileo and Kepler gained sway.

Thus the Earth became included in the list of planets,^[28] while the Sun and Moon were excluded. At first, when the first satellites of Jupiter and Saturn were discovered in the 17th century, the terms "planet" and "satellite" were used interchangeably – although the latter would gradually become more prevalent in the following century.^[29] Until the mid-19th century, the number of "planets" rose rapidly since any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community.

19th century

New planets, 1807–1845

1 Mercury

2 Venus

3 Earth

4 Mars

5 Vesta

6 Juno

7 Ceres

8 Pallas

9 Jupiter

10 Saturn

11 Uranus

In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century (such as Ceres, Pallas, and Vesta) were very different from the traditional ones. These bodies shared the same region of space between Mars and Jupiter (the Asteroid belt), and had a much smaller mass; as a result they were reclassified as "asteroids." In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Since there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846, there was no apparent need to have a formal definition.^[30]

20th century

Planets 1854–1930, 2006–present

1 Mercury

2 Venus

3 Earth

4 Mars

5 Jupiter

6 Saturn

7 Uranus

8 Neptune

However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth,^[31] the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Raymond Lyttleton suggested that Pluto may be an escaped satellite of Neptune,^[32] and Fred Whipple suggested in 1964 that Pluto may be a comet.^[33] However, as it was still larger than all known asteroids and seemingly did not exist within a larger population,^[34] it kept its status until 2006.

Planets 1930–2006

1 Mercury

2 Venus

3 Earth

4 Mars

5 Jupiter

6 Saturn

7 Uranus

8 Neptune

9 Pluto

In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12.^[35] This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).^[36]

The discovery of extrasolar planets led to another ambiguity in defining a planet; the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as "brown dwarfs".^[37] Brown dwarfs are generally considered stars due to their ability to fuse deuterium, a heavier isotope of hydrogen. While stars more massive than 75 times that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can fuse deuterium. However, deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.^[38]

21st century

With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There was particular disagreement over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium.

A growing number of astronomers argued for Pluto to be declassified as a planet, since many similar objects approaching its size had been found in the same region of the Solar System (the Kuiper belt) during the 1990s and early 2000s. Pluto was found to be just one small body in a population of thousands.

Some of them including Quaoar, Sedna, and Eris were heralded in the popular press as the tenth planet, failing however to receive widespread scientific recognition. The announcement of Eris in 2005, an object 27% more massive than Pluto, created the necessity and public desire for an official definition of a planet.

Acknowledging the problem, the IAU set about creating the definition of planet, and produced one in August 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).^[39]

Extrasolar planet definition

In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated the following working definition, mostly focused upon the boundary between planets and brown dwarves:^[2]

Artistic comparison of Eris, Pluto, Makemake, Haumea, Sedna, Orcus, 2007 OR₁₀, Quaoar, and Earth (scales are outdated)

1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance as the Sun^[40]) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.
2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located.
3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

This definition has since been widely used by astronomers when publishing discoveries of exoplanets in academic journals.^[41] Although temporary, it remains an effective working definition until a more permanent one is formally adopted. However, it does not address the dispute over the lower mass limit,^[42] and so it steered clear of the controversy regarding objects within the Solar System. This definition also makes no comment on the planetary status of objects orbiting brown dwarfs, such as 2M1207b.

One definition of a sub-brown dwarf is a planet-mass object that formed through cloud-collapse rather than accretion. This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.^[43][44] One reason for the dissent is that oftentimes it may not be possible to determine the formation process: for example an accretion-formed planet around a star may get ejected from the system to become free-floating, and likewise a cloud-collapse-formed sub-brown dwarf formed on its own in a star cluster may get captured into orbit around a star.

Dwarf planets 2006–present

Ceres

Pluto

Makemake

Haumea

Eris

The 13 Jupiter-mass cutoff is a rule of thumb rather than something of precise physical significance. The question arises: what is meant by deuterium burning? This question arises because large objects will burn most of their deuterium and smaller ones will burn only a little, and the 13 MJ value is somewhere in between. The amount of deuterium burnt also depends not only on mass but on the composition of the planet, on the amount of helium and deuterium present.^[45]

Another criterion for separating planets and brown dwarfs, rather than deuterium burning, formation process or location is whether the core pressure is dominated by coulomb pressure or electron degeneracy.^[46][47]

2006 definition

Main article: IAU definition of planet

The matter of the lower limit was addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, the assembly voted to pass a resolution that defined planets within the Solar System as:^[1]

A celestial body that is (a) in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Under this definition, the Solar System is considered to have eight planets. Bodies which fulfill the first two conditions but not the third (such as Pluto, Makemake and Eris) are classified as dwarf planets, provided they are not also natural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion.^[48] After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.^[49]

This definition is based in theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter:^[50]

The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Asteroids and comets, including KBOs [Kuiper belt objects], differ from planets in that they can collide with each other and with planets.

In the aftermath of the IAU's 2006 vote, there has been controversy and debate about the definition,^[51][52] and many astronomers have stated that they will not use it.^[53] Part of the dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition.

Beyond the scientific community, Pluto has held a strong cultural significance for many in the general public considering its planetary status since its discovery in 1930. The discovery of Eris was widely reported in the media as the tenth planet and therefore the reclassification of all three objects as dwarf planets has attracted a lot of media and public attention as well.^[54]

Former classifications

The table below lists Solar System bodies formerly considered to be planets:

Body (current classification)				Notes
Star	Dwarf planet	Asteroid	Moon
Sun			The Moon	Classified as planets in antiquity, in accordance with the definition then used.
			Io, Europa, Ganymede, and Callisto	The four largest moons of Jupiter, known as the Galilean moons after their discoverer Galileo Galilei. He referred to them as the "Medicean Planets" in honor of his patron, the Medici family.
			Titan,[b] Iapetus,[c] Rhea,[c] Tethys,[d] and Dione[d]	Five of Saturn's larger moons, discovered by Christiaan Huygens and Giovanni Domenico Cassini.
	Ceres[e]	Pallas, Juno, and Vesta		The first known asteroids, from their discoveries between 1801 and 1807 until their reclassification as asteroids during the 1850s.[55] Ceres has subsequently been classified as a dwarf planet in 2006.
		Astrea, Hebe, Iris, Flora, Metis, Hygeia, Parthenope, Victoria, Egeria, Irene, Eunomia		More asteroids, discovered between 1845 and 1851. The rapidly expanding list of planets prompted their reclassification as asteroids by astronomers, and this was widely accepted by 1854.[56]
	Pluto[f]			The first known Trans-Neptunian object (i.e. minor planet with a semi-major axis beyond Neptune). In 2006, Pluto was reclassified as a dwarf planet.
	Eris (dwarf planet) (originally nicknamed Xena)			Discovered in 2003, this Trans-Neptunian object (i.e. minor planet with a semi-major axis beyond Neptune) was recognised in 2005, before, like Pluto, in 2006 getting reclassified as a dwarf planet.

Mythology and naming

See also: Weekday names and Naked-eye planet

The gods of Olympus, after whom the Solar System's planets are named

The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece, the two great luminaries the Sun and the Moon were called Helios and Selene; the farthest planet was called Phainon, the shiner; followed by Phaethon, "bright"; the red planet was known as Pyroeis, the "fiery"; the brightest was known as Phosphoros, the light bringer; and the fleeting final planet was called Stilbon, the gleamer. The Greeks also made each planet sacred to one among their pantheon of gods, the Olympians: Helios and Selene were the names of both planets and gods; Phainon was sacred to Cronus, the Titan who fathered the Olympians; Phaethon was sacred to Zeus, Cronus's son who deposed him as king; Pyroeis was given to Ares, son of Zeus and god of war; Phosphoros was ruled by Aphrodite, the goddess of love; and Hermes, messenger of the gods and god of learning and wit, ruled over Stilbon.^[11]

The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros after their goddess of love, Ishtar; Pyroeis after their god of war, Nergal, Stilbon after their god of wisdom Nabu, and Phaethon after their chief god, Marduk.^[57] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.^[11] The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. However, unlike Ares, Nergal was also god of pestilence and the underworld.^[58]

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (or Latin) names rather than the Greek ones. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.^[59] When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Cronus). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained with Neptūnus (Poseidon). Uranus is unique in that it is named by a Greek deity rather than his Roman counterpart.

Some Romans, following a belief possibly originating in Mesopotamia but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet).^[60] Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Since each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar after the Nundinal cycle was rejected – and still preserved in many modern languages.^[61] Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, (Tuesday) Wóden (Wednesday), Thunor (Thursday), and Fríge (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively.

Earth is the only planet whose name in English is not derived from Greco-Roman mythology. Since it was only generally accepted as a planet in the 17th century,^[28] there is no tradition of naming it after a god (the same is true, in English at least, of the Sun and the Moon, though they are no longer considered planets). The name originates from the 8th century Anglo-Saxon word erda, which means ground or soil and was first used in writing as the name of the sphere of the Earth perhaps around 1300.^[62][63] As with its equivalents in the other Germanic languages, it derives ultimately from the Proto-Germanic word ertho, "ground,"^[63] as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde. Many of the Romance languages retain the old Roman word terra (or some variation of it) that was used with the meaning of "dry land" (as opposed to "sea").^[64] However, the non-Romance languages use their own respective native words. The Greeks retain their original name, Γή (Ge or Yi).

Non-European cultures use other planetary naming systems. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Surya for the Sun, Chandra for the Moon, and Budha, Shukra, Mangala, Bṛhaspati and Shani for the traditional planets Mercury, Venus, Mars, Jupiter and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China and the countries of eastern Asia historically subject to Chinese cultural influence (such as Japan, Korea and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn).^[61]

Formation

Main article: Nebular hypothesis

It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets.^[65] After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag.^[66]

An artist's impression of protoplanetary disk

When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting-Robertson drag and other effects.^[67][68] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.^[69] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies.

The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core.^[70] Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets.^[71] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.)

With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity – an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium) – is now believed to determine the likelihood that a star will have planets.^[72] Hence, it is thought that a metal-rich population I star will likely possess a more substantial planetary system than a metal-poor, population II star.

Solar System

Planets and dwarf planets of the Solar System. (Sizes to scale, distances not to scale)

The inner planets. From left to right: Mercury, Venus, Earth and Mars (sizes to scale).

The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune (Sizes to scale, distances not to scale)

Main article: Solar System

See also: List of gravitationally rounded objects of the Solar System

According to the IAU's current definitions, there are eight planets and five dwarf planets in the Solar System. In increasing distance from the Sun, the planets are:

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0.055 Earth masses.

The planets of the Solar System can be divided into categories based on their composition:

• Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth and Mars. At 0.055 Earth masses, Mercury is the smallest terrestrial planet (and smallest planet) in the Solar System, while Earth is the largest terrestrial planet.
• Gas giants (Jovians): Planets largely composed of gaseous material and significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Jupiter, at 318 Earth masses, is the largest planet in the Solar System, while Saturn is one third as big, at 95 Earth masses.
  • Ice giants, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass (only 14 and 17 Earth masses), and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice.
• Dwarf planets: Before the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently five dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto, Haumea, Makemake and Eris. Several other objects in both the Asteroid belt and the Kuiper belt are under consideration, with as many as 50 that could eventually qualify. There may be as many as 200 that could be discovered once the Kuiper belt has been fully explored. Dwarf planets share many of the same characteristics as planets, although notable differences remain – namely that they are not dominant in their orbits. By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto, Haumea, and Makemake are members of the Kuiper belt and Eris is a member of the scattered disc. Scientists such as Mike Brown believe that there are probably over one hundred trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.^[73]

Planetary attributes

	Name	Equatorial diameter[a]	Mass[a]	Orbital radius (AU)	Orbital period (years)[a]	Inclination to Sun's equator (°)	Orbital eccentricity	Rotation period (days)	Confirmed moons[c]	Rings	Atmosphere
Terrestrial planet	Mercury	0.382	0.06	0.39	0.24	3.38	0.206	58.64	0	no	minimal
	Venus	0.949	0.82	0.72	0.62	3.86	0.007	−243.02	0	no	CO2, N2
	Earth[b]	1.00	1.00	1.00	1.00	7.25	0.017	1.00	1	no	N2, O2
	Mars	0.532	0.11	1.52	1.88	5.65	0.093	1.03	2	no	CO2, N2
Gas giant	Jupiter	11.209	317.8	5.20	11.86	6.09	0.048	0.41	64	yes	H2, He
	Saturn	9.449	95.2	9.54	29.46	5.51	0.054	0.43	62	yes	H2, He
	Uranus	4.007	14.6	19.22	84.01	6.48	0.047	−0.72	27	yes	H2, He
	Neptune	3.883	17.2	30.06	164.8	6.43	0.009	0.67	13	yes	H2, He
Dwarf planet	Ceres	0.08	0.000 2	2.5–3.0	4.60	10.59	0.080	0.38	0	no	none
	Pluto	0.18	0.002 2	29.7–49.3	248.09	17.14	0.249	−6.39	4	no	temporary
	Haumea	0.15×0.12	0.000 7	35.2–51.5	282.76	28.19	0.189	0.16	2	?	?
	Makemake	~0.12	0.000 7	38.5–53.1	309.88	28.96	0.159	?	0	?	? [d]
	Eris	0.19	0.002 5	37.8–97.6	~557	44.19	0.442	~0.3	1	?	? [d]

^a Measured relative to the Earth.

^b See Earth article for absolute values.

^c Jupiter has the most secured satellites (64) in the solar system.^[74]

^d Like Pluto, when near perihelion, a temporary atmosphere is suspected.

Extrasolar planets

Main article: Extrasolar planet

Exoplanets, by year of discovery, through 2011-07-10.

The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. Of the 702 extrasolar planets discovered by November 17, 2011,^[4] most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass have been observed.^[4] The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12.^[75]

There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses,^[4] such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436.^[76] These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths).^[77]

Another new category are the so-called "super-Earths", possibly terrestrial planets larger than Earth but smaller than Neptune or Uranus. To date, about twenty possible super-Earths (depending on mass limits) have been found, including OGLE-2005-BLG-390Lb and MOA-2007-BLG-192Lb, frigid icy worlds discovered through gravitational microlensing,^[78][79] Kepler 10b, a planet with a diameter roughly 1.4 times that of Earth, (making it the smallest super-Earth yet measured)^[80] and five of the six planets orbiting the nearby red dwarf Gliese 581. Gliese 581 d is roughly 7.7 times Earth's mass,^[81] while Gliese 581 c is five times Earth's mass and was initially thought to be the first terrestrial planet found within a star's habitable zone.^[82] However, more detailed studies revealed that it was slightly too close to its star to be habitable, and that the farther planet in the system, Gliese 581 d, though it is much colder than Earth, could potentially be habitable if its atmosphere contained sufficient greenhouse gases.^[83]

Size comparison of HR 8799 c (gray) with Jupiter. Most exoplanets discovered thus far are larger than Jupiter, though discoveries of smaller planets are expected in the near future.

It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. There may also exist a class of hot Jupiters, called Chthonian planets, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine Chthonian planets have been discovered.^[84]

More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes. Currently the COROT and Kepler spacecraft are searching for stellar luminosity variations due to transiting planets. Several projects have also been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. These include the proposed NASA's, Terrestrial Planet Finder, and Space Interferometry Mission programs, and the CNES' PEGASE.^[85] The New Worlds Mission is an occulting device that may work in conjunction with the James Webb Space Telescope. However, funding for some of these projects remains uncertain. The first spectra of extrasolar planets were reported in February 2007 (HD 209458 b and HD 189733 b).^[86][87] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.^[88]

Planetary-mass objects

A planetary-mass object, PMO, or planemo is a celestial object with a mass that falls within the range of the definition of a planet – i.e. a mass greater than that of a minor object, yet smaller than that of a nuclear-reactive brown dwarf or star. By definition, all planets are planetary-mass objects, but the purpose of the term is to describe objects which do not conform to typical expectations for a planet. Free-floating planets not orbiting a star may be rogue planets ejected from their system, or objects that have formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs).

Rogue planets

Main article: Rogue planet

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.^[89] Some scientists have argued that such objects found roaming in deep space should be classed as "planets", although others have suggested that they could be low-mass stars.^[90][91]

Sub-brown dwarfs

Main article: Sub-brown dwarf

Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud-collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Sub-brown dwarfs may be free-floating such as Cha 110913-773444, or orbiting a larger object such as 2MASS J04414489+2301513.

For a brief time in 2006, astronomers believed they had found a binary system of such objects, Oph 162225-240515, which the discoverers described as "planemos", or "planetary-mass objects". However, recent analysis of the objects has determined that their masses are probably each greater than 13 Jupiter-masses, making the pair brown dwarfs.^[92][93][94]

Former stars

In close binary star systems one of the stars can lose mass to a heavier companion. See accretion-powered pulsars. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar PSR J1719-1438.^[95]

Satellite planets and belt planets

Some large satellites are of similar size or larger than the planet Mercury, e.g. Jupiter's Galilean moons and Titan. Alan Stern has argued that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet, and proposes the term satellite planet for a planet-sized object orbiting another planet. Likewise, planet-sized objects in the asteroid belt or Kuiper belt should also be planets according to Stern.^[96]

Attributes

Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whilst others are also common to extrasolar planets.

Dynamic characteristics

See also: Kepler's laws of planetary motion

Orbit

The orbit of the planet Neptune compared to that of Pluto. Note the elongation of Pluto's orbit in relation to Neptune's (eccentricity), as well as its large angle to the ecliptic (inclination).

According to current definitions, all planets must revolve around stars; thus, any potential "rogue planets" are excluded. In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates (counter-clockwise as seen from above the Sun's north pole). At least one extrasolar planet, WASP-17b, has been found to orbit in the opposite direction to its star's rotation.^[97] The period of one revolution of a planet's orbit is known as its sidereal period or year.^[98] A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the star's gravity. Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. The closest approach to its star is called its periastron (perihelion in the Solar System), while its farthest separation from the star is called its apastron (aphelion). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory.^[99]

Each planet's orbit is delineated by a set of elements:

• The eccentricity of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, while planets with high eccentricities have more elliptical orbits. The planets in the Solar System have very low eccentricities, and thus nearly circular orbits.^[98] Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.^[100][101]
• 

  Illustration of the semi-major axis
  The semi-major axis is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit (see image). This distance is not the same as its apastron, as no planet's orbit has its star at its exact centre.^[98]
• The inclination of a planet tells how far above or below an established reference plane its orbit lies. In the Solar System, the reference plane is the plane of Earth's orbit, called the ecliptic. For extrasolar planets, the plane, known as the sky plane or plane of the sky, is the plane of the observer's line of sight from Earth.^[102] The eight planets of the Solar System all lie very close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it.^[103] The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes.^[98] The longitude of the ascending node is the angle between the reference plane's 0 longitude and the planet's ascending node. The argument of periapsis (or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.^[98]

Axial tilt

Earth's axial tilt is about 23°.

Planets also have varying degrees of axial tilt; they lie at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses seasons; changes to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.^[104] Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to possess negligible to no axial tilt, as a result of their proximity to their stars.^[105]

Rotation

The planets rotate around invisible axes through their centres. A planet's rotation period is known as a stellar day. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the sun's north pole, the exceptions being Venus^[106] and Uranus^[107] which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise.^[108] However, regardless of which convention is used, Uranus has a retrograde rotation relative to its orbit.

The rotation of a planet can be induced by several factors during formation. A net angular momentum can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the gas giants can also contribute to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter the spin axis of the planet.^[109] There is great variation in the length of day between the planets, with Venus taking 243 Earth days to rotate, and the gas giants only a few hours.^[110] The rotational periods of extrasolar planets are not known; however their proximity to their stars means that hot Jupiters are tidally locked (their orbits are in sync with their rotations). This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night.^[111]

Orbital clearing

The defining dynamic characteristic of a planet is that it has cleared its neighborhood. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the IAU's official definition of a planet in August, 2006. This criterion excludes such planetary bodies as Pluto, Eris and Ceres from full-fledged planethood, making them instead dwarf planets.^[1] Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.^[112]

Physical characteristics

Mass

A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium. This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.^[113]

Mass is also the prime attribute by which planets are distinguished from stars. The upper mass limit for planethood is roughly 13 times Jupiter's mass for objects with solar-type isotopic abundance, beyond which it achieves conditions suitable for nuclear fusion. Other than the Sun, no objects of such mass exist in the Solar System; but there are exoplanets of this size. The 13MJ limit is not universally agreed upon and the Extrasolar Planets Encyclopaedia includes objects up to 20 Jupiter masses,^[114] and the Exoplanet Data Explorer up to 24 Jupiter masses.^[115]

The smallest known planet, excluding dwarf planets and satellites, is PSR B1257+12A, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury.^[4]

Internal differentiation

Illustration of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen

Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated interior consisting of a dense planetary core surrounded by a mantle which either is or was a fluid. The terrestrial planets are sealed within hard crusts,^[116] but in the gas giants the mantle simply dissolves into the upper cloud layers. The terrestrial planets possess cores of magnetic elements such as iron and nickel, and mantles of silicates. Jupiter and Saturn are believed to possess cores of rock and metal surrounded by mantles of metallic hydrogen.^[117] Uranus and Neptune, which are smaller, possess rocky cores surrounded by mantles of water, ammonia, methane and other ices.^[118] The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.^[116]

Atmosphere

See also: Extraterrestrial atmospheres

Earth's atmosphere

All of the Solar System planets have atmospheres as their large masses mean gravity is strong enough to keep gaseous particles close to the surface. The larger gas giants are massive enough to keep large amounts of the light gases hydrogen and helium close by, while the smaller planets lose these gases into space.^[119] The composition of the Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular oxygen.^[120] The only solar planet without a substantial atmosphere is Mercury which had it mostly, although not entirely, blasted away by the solar wind.^[121]

Planetary atmospheres are affected by the varying degrees of energy received from either the Sun or their interiors, leading to the formation of dynamic weather systems such as hurricanes, (on Earth), planet-wide dust storms (on Mars), an Earth-sized anticyclone on Jupiter (called the Great Red Spot), and holes in the atmosphere (on Neptune).^[104] At least one extrasolar planet, HD 189733 b, has been claimed to possess such a weather system, similar to the Great Red Spot but twice as large.^[122]

Hot Jupiters have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.^[123][124] These planets may have vast differences in temperature between their day and night sides which produce supersonic winds,^[125] although the day and night sides of HD 189733 b appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.^[122]

Magnetosphere

Schematic of Earth's magnetosphere

One important characteristic of the planets is their intrinsic magnetic moments which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet.^[126]

Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.^[126] In addition, the moon of Jupiter Ganymede also has one. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System (so strong in fact that it poses a serious health risk to future manned missions to its moons). The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet.^[126]

In 2004, a team of astronomers in Hawaii observed an extrasolar planet around the star HD 179949, which appeared to be creating a sunspot on the surface of its parent star. The team hypothesised that the planet's magnetosphere was transferring energy onto the star's surface, increasing its already high 7,760 °C temperature by an additional 400 °C.^[127]

Secondary characteristics

Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies (this is also common in satellite systems). All except Mercury and Venus have natural satellites, often called "moons". Earth has one, Mars has two, and the gas giants have numerous moons in complex planetary-type systems. Many gas giant moons have similar features to the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa).^{[128][129][130]}

The rings of Saturn

The four gas giants are also orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny 'moonlets' whose gravity shapes and maintains their structure. Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites that fell below their parent planet's Roche limit and were torn apart by tidal forces.^[131][132]

No secondary characteristics have been observed around extrasolar planets. However the sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc.^[90]

Related terms

• Comet
• Double planet
• Dwarf planet
• Exoplanet – celestial body outside the Solar System
• Mesoplanet
• Minor planet – celestial body smaller than a planet
• Planetar (astronomy)
• Planetary mnemonic
• Planetesimal
• Protoplanet
• Rogue planet

See also

	Astronomy portal
	Solar System portal
	Space portal

• Extraterrestrial skies
• List of hypothetical Solar System objects
• Landings on other planets
• Space exploration
• List of planet-satellite systems
• Planetary habitability
• Planetary science
• Exoplanetology
• Theoretical planetology
• Planets in astrology
• Planets in science fiction

Notes

1. ^{^} This definition is drawn from two separate IAU declarations; a formal definition agreed by the Union in 2006, and an informal working definition established by the Union in 2003. The 2006 definition, while official, applies only to the Solar System, while the 2003 definition applies to planets around other stars. The extrasolar planet issue was deemed too complex to resolve at the 2006 IAU conference.
2. ^{^} Referred to by Huygens as a Planetes novus ("new planet") in his Systema Saturnium
3. ^{^} Both labelled nouvelles planètes (new planets) by Cassini in his Découverte de deux nouvelles planetes autour de Saturne
4. ^{^} Both once referred to as "planets" by Cassini in his An Extract of the Journal Des Scavans.... The term "satellite", however, had already begun to be used to distinguish such bodies from those around which they orbited ("primary planets").
5. ^{^} Recently reclassified as a dwarf planet in 2006.
6. ^{^} Regarded as a planet from its discovery in 1930 until redesignated as a trans-Neptunian dwarf planet in August 2006.

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External links

• International Astronomical Union website
• Photojournal NASA
• NASA Planet Quest – Exoplanet Exploration
• Illustration comparing the sizes of the planets with each other, the Sun, and other stars
• "IAU Press Releases since 1999 "The status of Pluto: A Clarification"". Archived from the original on 2007-12-14. http://web.archive.org/web/20071214043704/http://www.iau.org/STATUS_OF_PLUTO.238.0.html. 
• "Regarding the criteria for planethood and proposed planetary classification schemes." article by Stern and Levinson
• Planetary Science Research Discoveries (educational site with illustrated articles)