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==Definition and classification of planets==
{{main|Definition of planet}}

Much like "]", "planet" is a word without a precise definition, with ] and ] playing as much of a role as ] and ]. Recent definitions have been vague and imprecise; ''The American Heritage Dictionary'', for instance, formerly defined a planet as:

:''A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.'

However, for some time that definition has been viewed by many as inadequate. The eight largest planets (which are also the eight nearest to the ]) are universally recognised as such, and for this reason are often universally referred to as "major planets", but there is controversy over Pluto and other smaller objects.

===Suggested wide definitions===
Since the discoveries of many of the objects in the ] and around other stars, there has been a concerted push amongst scientists to come up with a precise definition of what constitutes a planet. In ], the IAU set up a working group to develop a scientifically plausible recommendation, but as of August, 2005 they had not reached a conclusion. After the discovery of {{mpl|2003 UB|313}} (informally called "Xena"), a member of the committee, ], has said that the group wanted "to get something done, pronto". He also informed journalists that a "consensus" in the group was moving towards the following definition:

:''A planet is a body that directly orbits a star, is large enough to be round because of self gravity, and is not so large that it triggers nuclear fusion in its interior.''

Note that this definition also covers disputes at the upper end of a planet's size, which provides the extra benefit of forming a barrier between planets and ]. Many consider this definition the best option as it sets up divisions based on physical characteristics rather than an arbitrary size limit. It is also somewhat universal in its application where other definitions have been crafted mainly to sort our own ] into simple categories (such as placing the size limit as just under ], ] or ]). Depending how it is interpreted, objects counted as planets under such a new system would include some or all of the objects listed above, with potentially many more yet to be found.

], head of astronomy at the ], has suggested a similar definition and has also proposed the terms "]" (any object that achieves fusion in its core) and "]" (an object that is round from self-gravity but not a fusor) to help improve the ] nomenclature. Under Basri's definition:

:''A planet is a planemo orbiting a fusor''

These definitions have the advantage of creating a group including larger moons (which share many characteristics with the smaller planets) and also covering ], which some astronomers think should be included in the definition of a planet. Basri has also suggested 'liberal use of adjectives' such as "major", "beltway", "dwarf", "giant", "super" and "historical". Others have suggested categories of planet/planemo based on composition such as "rock" (composed mainly of silicate), "gas" (composed mainly of hydrogen and helium), and "ice" (composed mainly of oxygen and carbon).

===Suggested narrow definitions===
There are alternate suggestions which would instead ''reduce'' the number of planets in the system. Upon his discovery of ], ] of Caltech suggested a definition which would exclude both Sedna and Pluto from being classified as planets, proposing the following:

:''A planet is any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit''

This definition generally plays down the importance of size, but instead focuses on the formation of the proposed planet. Under this definition, no ]s (including Pluto) would be considered planets.

Brown's wish to "demote" Pluto prompted many to criticize him for setting out to create a purely scientific definition for a term which had an existing popular (albeit 'flawed') application. Upon his discovery of {{mp|2003 UB|313}}, Brown indicated he had become a convert to this way of thinking, and proposed that whatever definition of planet be adopted, it should include both Pluto and any ] found to be larger than Pluto.

===Further classification===
Astronomers distinguish between ]s, such as ]s, ]s, and ]s; and major (or true) planets.

Planets within Earth's solar system can be divided into categories according to composition.
*] or rocky: Planets that are similar to ] — with bodies largely composed of ]: Mercury, Venus, Earth, Mars
*] or ]: Those with a composition largely made up of ]eous material: Jupiter, Saturn, Uranus, Neptune. Uranian planets, or ], are a sub-class of gas giants, distinguished from true Jovians by their depletion in hydrogen and helium and a significant composition of rock and ice.
*Icy: Sometimes a third category is added to include bodies like ], whose composition is primarily ]; this category of "icy" bodies also includes many non-planetary bodies such as the icy ]s of the outer planets of our solar system (e.g. ]).

Many consider the Earth and its ] to be a double planet, for several reasons:
*The ], as measured by its diameter, is 1.5 times larger than Pluto.
*The gravitational force of the Sun on the Moon is larger than the gravitational force of the Earth on the Moon (by about a factor of 2.2)

The latter fact is not unique in the solar system, but is unusual for such a large satellite. Other satellites for which the Sun's gravity is actually stronger than the primary's:
*The outermost moon of Jupiter (]; factor of 1.5)
*The outermost moon of Uranus (]; factor of 1.2)
*The two outermost moons of Neptune (] and ]; factor of 2.1)
*Several ]s (e.g. (22) Kalliope I ] by a factor of 1.6; (45) Eugenia I ] by a factor of 2.8; (243) Ida I ] by a factor of 1.3; and, most remarkably, ] by a large factor of 625.)
However, the Earth's moon aside, these objects are much too small to be considered planets.


==See also== ==See also==

Revision as of 12:21, 1 December 2005

For other uses, see Planet (disambiguation).

A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star that is not a star itself. The name comes from the Greek term πλανήτης, planētēs, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relation to the other stars. Based on historical consensus, the International Astronomical Union lists nine planets in our solar system. Since the term planet has no precise scientific definition, however, many astronomers contest that figure. Some say it should be lowered to eight by removing Pluto from the list, whilst others claim it should be raised to fifteen, twenty, or even higher.

Planetary formation

It is not known with certainty how planets are formed. The prevailing theory is that they are formed from those remnants of a nebula that don't condense under gravity to form a protostar. Instead, these remnants become a thin disc of dust and gas revolving around the protostar and begin to condense about local concentrations of mass within the disc. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets. When the protostar has grown such that it ignites to form a star, its solar wind blows away most of the disc's remaining material. 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. Meanwhile, protoplanets that have avoided collisions may become moons of larger planets.

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.

Within our solar system

Main article: Solar system.

All of the accepted planets in the solar system are named after Roman gods, except for Uranus, which is named after a Greek god, and the Earth which was not seen as a planet by the ancients (instead considered the centre of the universe). The designated planetary names are near universal in the Western world, but some non-European languages, such as Chinese, use their own. Moons are also named after gods and characters from classical mythology or (in the case of Uranus) after Shakespearean characters. Asteroids can be named, at the discretion of their discoverers, after anybody or anything (subject to approval by the International Astronomical Union's panel on nomenclature). The process of naming planets and their features is known as planetary nomenclature.

Accepted planets

Planets in approximate scale of size, but not distance. Note a portion of the solar disc shown at the top

According to the authority of the International Astronomical Union, there are nine planets in our solar system (in increasing distance from the Sun):

  1. Mercury (astronomical symbol ☿)
  2. Venus (♀)
  3. Earth (♁) with one confirmed natural satellite, Luna (the Moon)
  4. Mars (♂) with two confirmed natural satellites, Deimos and Phobos
  5. Jupiter (♃) with sixty-three confirmed natural satellites
  6. Saturn (♄) with forty-six confirmed natural satellites
  7. Uranus (Astronomical symbol for Uranus), with twenty-seven confirmed natural satellites
  8. Neptune (♆) with thirteen confirmed natural satellites
  9. Pluto (♇) with three confirmed natural satellites (Charon, S/2005 P 1, S/2005 P 2)

However, there is some pressure for Pluto to be reclassified as a Kuiper belt object, especially in light of the discovery of 2003 UB313. This object, however, has not yet received a definitive classification from the IAU.

Other candidates

When Ceres was found orbiting between Mars and Jupiter in 1801, it was initially touted as a planet, but after many smaller objects were found with a similar orbit, it was classified as an asteroid. However, due to its large size (relative to the other asteroids), and its roughly spherical shape, Ceres would be considered a planet by some astronomers' definitions.

Similarly, since 1992 many objects have been found in the predicted Kuiper Belt that exists beyond Neptune. Several of the largest of these have challenged the planetary status quo, as they are both spherical and larger than the bodies in the Mars-Jupiter asteroid belt, and are similar in size, orbit and composition to Pluto. However, as yet none have been accepted as planets by the IAU. The most significant of these are (in increasing distance from the Sun) Orcus, 2003 EL61,Quaoar, 2005 FY9, 2003 UB313 and Sedna. (It should be noted that Sedna is often considered to be beyond the Kuiper Belt and actually a member of the inner Oort Cloud.)

Like Ceres before it, Sedna was widely touted as a planet when it was discovered in 2003, as it was the largest object found since Pluto. However, mainly due to its size still being smaller than Pluto's, it did not achieve planetary status from the IAU. However, the discovery in 2005 of 2003 UB313, with a size and mass larger than Pluto seems to have forced the issue. As of September 2005 it has not yet been accepted as a planet, but the IAU is expected to announce a definition of a planet by the end of the year, which will either see 2003 UB313 become a planet, or have Pluto stripped of its status.

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See also

References

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