Minor planet

Minor planets, or planetoids are minor bodies of the Solar system orbiting the Sun (or of other planetary systems orbiting other stars) that are larger than meteoroids (the largest of which might be taken to be around 10 meters or so across) but smaller than major planets (Mercury having a diameter of about 4880 km). The term minor planet is sometimes used as a synonym for asteroid though this is technically incorrect; asteroids are one group of minor planets, a category which also includes Trans-Neptunian objects and other small bodies in our solar system and elsewhere.

Related Topics:
Solar system - Sun - Planetary systems - Meteoroid - Planet - Mercury - Asteroid - Trans-Neptunian objects

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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 transneptunian 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; indeed, some (perhaps all) comets eventually are depleted of their volatile ices and then appear as "stellar" objects, or minor planets.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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 honor.

Related Topics:
Kiyotsugu Hirayama - 1918

~ ~ ~ ~ ~ ~ ~ ~ ~ ~