Minor Planet - Populations

Populations

Hundreds of thousands of minor planets have been discovered within the Solar System, with the 2009 rate of discovery at over 3,000 per month. Of the more than 535,000 registered minor planets, 251,651 have orbits known well enough to be assigned permanent official numbers. Of these, 16,154 have official names. As of September 2010, the lowest-numbered unnamed minor planet is (3708) 1974 FV₁; but there are also some named minor planets above number 240,000.

There are various broad minor-planet populations:

• Asteroids; traditionally, most have been bodies in the inner Solar System.
  • Main-belt asteroids, those following roughly circular orbits between Mars and Jupiter. These are the original and best-known group of asteroids or minor planets.
  • Near-Earth asteroids, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on orbital distance, is used:
    • Aten asteroids, those that have semi-major axes of less than one Earth orbit. Those Aten asteroids that have their aphelion within Earth's orbit are known as Apohele asteroids;
    • Amor asteroids are those near-Earth asteroids that approach the orbit of the Earth from beyond, but do not cross it. Amor asteroids are further subdivided into four subgroups, depending on where their semimajor axis falls between Earth's orbit and the asteroid belt;
    • Apollo asteroids are those asteroids with a semimajor axis greater than Earth's, while having a perihelion distance of 1.017 AU or less. Like Aten asteroids, Apollo asteroids are Earth-crossers.
  • Earth trojans, asteroids sharing Earth's orbit and gravitationally locked to it. As of 2011, the only one known is 2010 TK₇.
  • Mars trojans, asteroids sharing Mars's orbit and gravitationally locked to it. As of 2007, eight such asteroids are known.
  • Jupiter trojans, asteroids sharing Jupiter's orbit and gravitationally locked to it. Numerically they are estimated to equal the main-belt asteroids.
• Distant minor planets; an umbrella term for minor planets in the outer Solar System.
  • Centaurs, bodies in the outer Solar System between Jupiter and Neptune. They have unstable orbits due to the gravitational influence of the giant planets, and therefore must have come from elsewhere, probably outside Neptune.
  • Neptune trojans, bodies sharing Neptune's orbit and gravitationally locked to it. Although only a handful are known, there is evidence that Neptune trojans are more numerous than either the asteroids in the asteroid belt or the Jupiter trojans.
  • Trans-Neptunian objects, bodies at or beyond the orbit of Neptune, the outermost planet.
    • The Kuiper belt, objects inside an apparent population drop-off approximately 55 AU from the Sun.
      • Classical Kuiper belt objects, also known as cubewanos, are in primordial, relatively circular orbits that are not in resonance with Neptune.
      • Plutinos, bodies like Pluto that are in a 2:3 resonance with Neptune, and other resonant trans-Neptunian objects. This is a rather large group.
    • Scattered disc, objects with aphelia outside the Kuiper belt. These are thought to have been scattered by Neptune.
    • Detached objects such as Sedna, with both aphelia and perihelia outside the Kuiper belt.
    • The Oort Cloud, a hypothetical population thought to be the source of long-period comets that may extend out to 50,000 AU from the Sun.