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Asteroids are
minor planets, especially those of the
inner Solar System. The larger ones have also been called
planetoids. These terms have historically been applied to any astronomical object orbiting the
Sun that did not show the disc of a planet and was not observed to have the characteristics of an active
comet. As
minor planets in the outer Solar System were discovered and found to have volatile-based surfaces that resemble those of comets, they were often distinguished from asteroids of the
asteroid belt.
[1] In this article, the term "asteroid" refers to the minor planets of the inner Solar System including those co-orbital with
Jupiter.
There are millions of asteroids, many thought to be the shattered remnants of
planetesimals, bodies within the young Sun's
solar nebula that never grew large enough to become
planets.
[2] The large majority of known asteroids orbit in the asteroid belt between the orbits of
Mars and Jupiter, or are co-orbital with Jupiter (the
Jupiter trojans). However, other orbital families exist with significant populations, including the
near-Earth objects. Individual asteroids are classified by their characteristic
spectra, with the majority falling into three main groups:
C-type,
M-type, and
S-type. These were named after and are generally identified with
carbon-rich,
metallic, and
silicate (stony) compositions, respectively. The size of asteroids varies greatly; the largest is almost 1,000 km (625 mi) across.