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Minor planet

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 It has been suggested that this article or section be merged with Asteroid. (Discuss)
For minor planets out to the orbit of Jupiter, see also the Asteroid article

Minor planets, or asteroids or planetoids, are minor celestial bodies of the Solar system orbiting the Sun (mostly Small solar system bodies) that are smaller than major planets, but larger than meteoroids (commonly defined as being 10 meters across or less[1]), and that are not comets. The distinction is made on visual appearance when discovered: comets must show a perceptible coma, and they get listed in their own catalogs. Minor planets in contrast appear star-like ("asteroid", from Greek αστεροειδής, asteroeidēs = star-like, star-shaped, from ancient Greek Aστήρ, astēr = star); they get a provisional designation by year in the order of discovery, and a designation (a sequential number) and name if their existence is well established and an orbit has been determined. Their physical nature often remains poorly known.

The first named minor planet was Ceres, discovered in 1801 by Giuseppe Piazzi which was originally considered a new planet, and is now classified as a dwarf planet. Sir William Herschel (discoverer of Uranus), coined the term asteroid for the first objects discovered in the 19th century, all of which orbit the sun between Mars and Jupiter, and generally in relatively low-eccentricity (i.e., not very elongated) orbits. But since then, minor planets have been found to cross the orbits of planets, from Mercury to Neptune -- with hundreds of trans-Neptunian objects (TNOs) now known to exist well past Neptune's orbit.

Though the main distinction between a minor planet and a comet lies in the fact that comets show a coma (or atmosphere) and/or a tail, due primarily to sublimation of ices by solar radiation, one can justifiably consider comets to be a subset of the large group known as minor planets. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets eventually are depleted of their volatile ices and then appear as pointlike objects, i.e. asteroids. The outermost regions of the solar system are also believed to contain a cloud of dormant comets, and the closer Trans-neptunian objects that have been discovered may not be fundamentally different from giant proto-comets.

Minor planets are divided into groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered (often the largest). While so-called groups are relatively loose dynamical associations, families are much "tighter" and result usually from the catastrophic breakup of a large parent asteroid sometime in the past. Families have only been recognized within the main asteroid belt. They were first recognised by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor. The term planetule was also coined by the geologist Conybeare to describe minor planets [2]

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Groups out to the orbit of Earth

There are relatively few asteroids that orbit close to the Sun. Several of these groups are hypothetical at this point in time, with no members having yet been discovered; as such, the names they have been given are provisional.

  • Vulcanoid asteroids are hypothetical asteroids with an aphelion less than 0.4 AU, ie, they orbit entirely within the orbit of Mercury. A few searches for Vulcanoids have been conducted but there have been none discovered so far.
  • Apoheles are asteroids whose aphelion is less than 0.983 AU, meaning they orbit entirely within Earth's orbit. "Apohele" is Hawaiian for "orbit". Other proposed names for this group are Inner-Earth Objects (IEOs) and Anons (as in "Anonymous"). As of September 2007 there are only five known Apoheles with an arc of observations greater than 20 days: 2003 CP20, 2004 JG6, 2004 XZ130, 2005 TG45 and 2006 WE4; while there are other three possible candidates, but with a too short arc of observations: 1998 DK36,2006 KZ39 and 2007 EB26.
  • Mercury-crosser asteroids having a perihelion smaller than Mercury's 0.3075 AU.
  • Venus-crosser asteroids having a perihelion smaller than Venus's 0.7184 AU. This group includes the above Mercury-crossers (if their aphelion is greater than Venus's perihelion. All known Mercury crossers satisfy this condition).
  • Earth-crosser asteroids having a perihelion smaller than Earth's 0.9833 AU. This group includes the above Mercury- and Venus-crossers, apart from the Apoheles. They are also divided into the
  • Arjuna asteroids are somewhat vaguely defined as having orbits similar to Earth's; i.e., with an average orbital radius of around 1 AU and with low eccentricity and inclination. Due to the vagueness of this definition some asteroids belonging to the Apohele, Amor, Apollo or Aten groups can also be classified as Arjunas. The term was introduced by Spacewatch and does not refer to an existing asteroid; examples of Arjunas include 1991 VG.
  • Earth Trojans are asteroids located in the Earth-Sun Lagrangian points L4 and L5. Their location in the sky as observed from Earth's surface would be fixed at about 60 degrees east and west of the Sun, and as people tend to search for asteroids at much greater elongations few searches have been done in these locations. No Earth trojans are currently known.
  • Near-Earth asteroids is a catch-all group for asteroids whose orbit closely approaches that of Earth. It includes almost all of the above groups, as well as the Amor asteroids.

Groups out to the orbit of Mars

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