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Revision as of 12:21, 1 December 2005 by 152.93.79.20 (talk) (→Definition and classification of planets)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff) 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

External links

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