planet

The Solar System now has 8 classical planets and several known dwarf planets, including Pluto, Ceres, and Eris

On Aug. 24, 2006, astronomers at a meeting of the International Astronomical Union (IAU) in Prague agreed upon definitions that would distinguish between classical planets and dwarf planets.

A classical planet is a celestial body that:

Is in orbit around the Sun.
Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
Has cleared the neighborhood around its orbit.

The eight classical planets in the Solar System are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

A dwarf planet is a celestial body that:

Is in orbit around the Sun.
Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
Has not cleared the neighborhood around its orbit.
Is not a satellite.

Pluto is a dwarf planet and also the prototype of a new category called plutonian objects. Other known objects that appear to satisfy the dwarf planet criteria are Ceres and Eris (formerly known as 2003 UB313). Membership of the dwarf planet category is likely to grow rapidly in the near future. The IAU plans to establish a process to assign borderline objects into either dwarf planet or other categories.

All objects orbiting the Sun that are not classical planets or dwarf planets are referred to collectively as "small Solar System bodies". These currently include most of the Solar System asteroids, most trans-Neptunian objects (TNOs), comets, and other small bodies.

Other potential dwarf planets

Among other Kuiper Belt objects with claims to possible dwarf planethood are

2003 EL61 ("Xena")
2005 FY9
Sedna
Orcus
Quaoar
Varuna
2002 TX300
Ixion
2002 AW197

Also, the following asteroids, in addition to Ceres, are possible dwarf planet candidates on the basis that they are roughly round:

Normally any object with a mass greater than 0.6% that of our Moon and a diameter greater than 800 km would be roughly spherical but borderline cases will have to be resolved by more observation.

Other ways of categorizing planets

By composition the classical planets can be divided into terrestrial, or rocky, planets (Mercury, Venus, Earth, and Mars), which are made primarily of rock and metal and have relatively high densities, slow rotation, and solid surfaces, and jovian planets, or gas giants, (Jupiter, Saturn, Uranus, and Neptune), which are composed mostly of hydrogen and helium and generally have low densities, rapid rotation, and deep atmospheres. By size, the planets Mercury, Venus, Earth, and Mars, all with diameters less than 13,000 km, are described as small, while Jupiter, Saturn, Uranus, and Neptune, all with diameters greater than 48,000 km, are said to be giant. By position relative to the Sun, there are the inner planets (Mercury, Venus, Earth, and Mars) and the outer planets (Jupiter, Saturn, Uranus, and Neptune), with the asteroid belt marking the boundary between the inner and outer Solar System. By position relative to Earth, Mercury and Venus are described as inferior planets, whereas all planets beyond and including Mars are said to be superior.

Extrasolar planets

A large number of planets have also been found orbiting stars other than the Sun. The criteria adopted by astronomers in 2006 to distinguish between classical planets, dwarf planets, and smaller objects was not intended to be applied to these extrasolar planets. There is an additional complication in determining whether some massive companion objects are planets or brown dwarfs (the least massive kind of star).

Related categories

• PLANETS AND MOONS
• EXTRASOLAR PLANETS AND SUBSTELLAR OBJECTS

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