What are often called minor planets?

Minor Planets

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), 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", 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.

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