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What is a Planet?

In ancient times the concept of the planet was simple: the planets were the five permanent star-like objects in the sky that moved relative to the fixed stars. This was a purely phenomenological classification. The physical characteristics that distinguish the planets from the fixed stars were unknown and largely unsuspected. The only other bodies known to the ancient astronomers, excepting the Moon and the Sun, were the comets, and these were occasional, temporal guests in the sky.

In modern times, the list of planets expanded as systematic observations of the ecliptic revealed Uranus and Neptune. These two planets fit the ancient description of planet nicely, as they were larger than the four terrestrial planets. But with the discovery of the asteroids, beginning with Ceres in 1801, and the discovery of a ninth planet, Pluto, in 1930, the idea of the planet became arbitrary. The asteroids were considered minor planets, but the difference in mass between the largest asteroid, Ceres, and the smallest planet, Pluto, was relatively small. This gap between planet and minor planet evaporated in the years following Pluto's discovery with the discovery of objects within the Kuiper Belt—an asteroid belt beyond the orbit of Neptune. Pluto was found to have a companion, Charon, that was not much smaller than Pluto. Two other bodies were found beyond the orbit of Neptune that are similar to Pluto in size—Eris, which is larger than Pluto, and 2003 VB12 (Sedna), which is probably smaller than Pluto. With expectations that more large objects would be found in the Kuiper Belt, the astronomy community was in need of a precise definition for the term planet.

The formal definition of a planet was approved on August 24, 2006 by the General Assembly of the IAU:

A “planet” is a celestial body that (a) is 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.

Objects meeting all of these criteria are classical planets. If the third condition is not met, the object is a dwarf planet. An objects that satisfy only the first requirement is a small solar system body.

The physical condition that a body must meet, that it is nearly a sphere, is the condition that the planet's shape is set by pressure being in balance with both gravitational force and centrifugal force. It is the flattening of a planet by centrifugal force that prompted the “nearly round” in the definition of a planet; Saturn and Jupiter in particular are flattened at the poles by centrifugal force. A body need not be large to satisfy this condition. It is expected that a planet with a mass greater than 5×1020 kg (0.008% of Earth's mass) and a diameter greater than 800 km (6% of Earth's diameter) has a gravitational field that can overcome the structural forces within it. Depending on the composition of the body, the mass or diameter can be smaller than these values.

Selected Solar System Bodies

Values Relative to Earth's

Classical Planets
Dwarf Planets
Eris (2003 UB313)0.28%18.8%
Possible Dwarf Planets
2003 VB12 (Sedna)0.12%14.3%

The roundness condition is met by many bodies within the solar system, including many of the moons of the giant planets and the Earth's Moon. For this reason, the first condition specifies that a moon cannot be a planet. Even with this condition, however, many small objects orbiting the Sun would count as planets. This motivates the third criterion, which creates the distinction between classical planets, which are the eight planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, and dwarf planets, which include the asteroid Ceres and the Kuiper Belt objects Pluto and Charon. The distinction between the classical and the dwarf planets is whether the planet is solitary in its orbit or is a member of a collection of similar bodies.

The number of classical planets in our Solar System could rise as astronomers explore the region beyond Neptune. The Kuiper belt currently contains three dwarf planets, Charon, Pluto, and Eris, given in order from the smallest to the largest. The trans-Neptunian object 2003 UV12, which appears to have half the diameter of Pluto, is probably a dwarf planet. Half a dozen more dwarf planets of equivalent size are expected be found within the Kuiper Belt. Any one of these objects could be a classical planet if it is large enough to clear its orbit of other Kuiper Belt objects. Large bodies may also exists beyond the Kuiper Belt, out in the regions populated by the long-period comets. Regardless of whether the list of classical planets lengthens, it is certain that the list of dwarf planets will grow dramatically in the coming decades.

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