What Are Minor Planets ?

Minor planet is the official term for asteroids and trans-Neptunian objects. They are objects in the solar system that orbit the Sun like planets, but which are smaller than planets and not counted among them. If a minor planet produces coma it is called a comet (though conversely not all comets are classified as minor planets). The largest minor planets are also called planetoids.

Minor planet groups and families

Minor planets are divided into groups and families based on their orbital characteristics. It is customary to name a group of asteroids after the first member of that group to be discovered (generally the largest). Groups are relatively loose dynamical associations, whereas families are much "tighter" and result most probably from the catastrophic breakup of a large parent asteroid sometime in the past. They were first recognised by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honour. The only prominent families are:

Other families have been identified using a variety of techniques, most prominently the Hierarchical Clustering Method (HCM) and the Wavelet Analysis Method (WAM):

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.

Groups out to the orbit of Mars

Groups out to the orbit of Jupiter

A large number of asteroids have orbits between the orbits of Mars and Jupiter, roughly 2 to 4 AU, in a region known as the Main belt. These couldn't form a planet due to the gravitational influence of Jupiter. Jupiter's gravitational influence, through orbital resonance, clears Kirkwood gaps in the asteroid belt, first recognised by Daniel Kirkwood in 1874. As a result of these gaps the asteroids in this region are divided into a large number of groups. They are:

Between the Hildas and the Trojans (roughly 4.05 AU to 5.0 AU), there's a 'forbidden zone'. Aside from 279 Thule and five objects in unstable-looking orbits, Jupiter's gravity has swept everything out of this region.

Groups beyond the orbit of Jupiter

Most of the minor planets beyond the orbit Jupiter are believed to be composed of ices and other volatiles. Many are similar to comets, differing only in that the perihelia of their orbits are too distant from the Sun to produce a significant tail.

Quasi-satellites and "horseshoe objects"

Some asteroids have unusual "horseshoe orbits" that are co-orbital with the Earth or some other planet. Examples are 3753 Cruithne and 2002 AA29. The first instance of this type of orbital arrangement was discovered between Saturn's moons Epimetheus and Janus.

Sometimes these "horseshoe objects" temporarily become quasi-satellites for a few decades or a few hundred years, before returning to their prior status. Both Earth and Venus are known to have quasi-satellites.

Such objects, if associated with Earth or Venus or even hypothetically Mercury are a special class of Aten asteroids. However, such objects could be associated with outer planets as well.