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Minor planet

Minor planet is a term used since the 19th century to describe objects in orbit around the Sun that are not major planets or comets.^[1] The term planetoid has also been used.^[2] Historically, the terms asteroid, minor planet, and planetoid have been synonymous,^[3][2] but the issue has been complicated by the discovery of numerous small objects beyond the orbit of Jupiter, which may or may not be classified as asteroids.^[3] Before 2006, the International Astronomical Union had officially used the term minor planet, but during its 2006 meeting, it reclassified minor planets and comets into dwarf planets and small solar system bodies.^[4] Minor planets may now be classified as dwarf planets if their self-gravity is sufficient to achieve hydrostatic equilibrium, that is, an approximately round shape. All objects orbiting the Sun other than the major planets and dwarf planets are to be officially called small solar system bodies^[4] (the IAU states: "the term 'minor planet' may still be used, but generally the term 'small solar system body' will be preferred."^[5]).

In our solar system Mimas is the smallest body that seems to be in hydrostatic equilibrium, while Pallas may be the largest notably out of round. The IAU has so far declared only three bodies to be dwarf planets: Ceres, Pluto, and Eris.

Classifications of minor planets according to a descriptive system are:

• Asteroids; traditionally, most have been bodies in the inner solar system.^[3]

• Main belt asteroids, those following roughly circular orbits between Mars and Jupiter. These are the original, largest, and best known group of asteroids or minor planets.
• Near earth asteroids, those inside the orbit of Mars. Further subclassification of these, based on orbital distance, is traditional.
• Trojan asteroids, those in Jupiter's orbit and gravitationally locked to it.

• Centaurs, bodies in the outer solar system. These have unstable orbits because of the huge influence of the giant planets' gravity.
• Trans-Neptunian objects, bodies out beyond Neptune, the outermost major planet.

• The Kuiper belt, objects outside the orbit of Neptune and within roughly 55 AU of the Sun.

• Classical Kuiper belt objects, those in primordial near-circular orbits beyond Neptune; these appear to all be inside its 1:2 resonance.
• Plutinos, bodies in the 2:3 resonance, like Pluto. This is a rather large group

• Scattered Disk objects, those having aphelia farther than the Kuiper belt objects. These are thought to have been scattered by Neptune, though some having very large perihelia such as Sedna may be primordial.

See also

• Solar System
• Asteroid group
• Small Solar System body

References