Planet: Encyclopedia II - Planet - Within our solar system

Planet - Within our solar system

Main article: Solar system

The process of naming planets and their features is known as planetary nomenclature. All the currently accepted planets in the solar system are named after Roman gods, except for Uranus (named after a Greek god) and the Earth, which was not seen as a planet by the ancients but rather the centre of the universe. The original number of planets was seven: Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn. These were all seen as regular "wanderers" in the sky. Developments in Astronomy removed the Sun and the Moon, and added the currently accepted planetary members of the Solar System.

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 characters from English literature. Asteroids can be named after anybody or anything at the discretion of their discoverers, subject to approval by the IAU's nomenclature panel.

Planet - Accepted planets

According to the authority of the IAU, there are nine planets in our solar system. In increasing distance from the Sun they are:

1. Mercury (astronomical symbol ☿)
2. Venus (♀)
3. Earth (♁) with one confirmed natural satellite, Luna (the Moon)
4. Mars (♂) with two confirmed natural satellites, Phobos and Deimos
5. Jupiter (♃) with sixty-three confirmed natural satellites
6. Saturn (♄) with forty-six confirmed natural satellites
7. 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.

Planet - 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 order of increasing distance from the Sun) 90482 Orcus, 2003 EL₆₁, 50000 Quaoar, 2005 FY₉, 2003 UB₃₁₃, 28978 Ixion, 20000 Varuna, 19521 Chaos, and 90377 Sedna. (However, it should be noted that Sedna is often considered to be beyond the Kuiper Belt; being either a member of the scattered disc or 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 (nicknamed Xena), 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.

Adapted from the Wikipedia article "Within our solar system", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki