In astronomy, asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids. Smaller objects are usually called meteoroids.


243 Ida and its moon Dactyl. Dactyl is the first satellite of an asteroid to be discovered. image: wikipedia

These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disk of a planet and was not observed to have the characteristics of an active comet, but as minor planets in the outer Solar System were discovered, their volatile-based surfaces were found to resemble comets more closely and so were often distinguished from traditional asteroids.

Thus the term asteroid has come increasingly to refer specifically to the small bodies of the inner Solar System out to the orbit of Jupiter.

 They are grouped with the outer bodies—centaurs, Neptune trojans, and trans-Neptunian objects—as minor planets, which is the term preferred in astronomical circles.

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.

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). 

Other orbital families exist with significant populations, including the near-Earth asteroids. 


Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups:

  • C-type,  (Carbon rich)
  • S-type, and (Stony)
  • M-type. (metallic)

These were named after and are generally identified with carbon-rich, stony, and metallic compositions, respectively.

Some notably large asteroids 

  • Ceres - the largest asteroid, but is regarded as a dwarf planet  (historically it was also considered a planet). 
    • (965.2 × 961.2 × 891.2) ± 2.0 km

  • Palas (2 Pallas)  also was considered a planet
    • 550±8 × 516±6 × 476±6 km, 512±6 km

  • Vesta (4 Vesta)
    • (572.6 × 557.2 × 446.4) ± 0.2 km,  525.4±0.2 km

Light curve-based 3D-model of 10 Hygiea ( 350–500 kilometres) . image:Astronomical Institute of the Charles University: Josef Ďurech, Vojtěch Sidorin/wikipedia

  • 10 Hygiea 
    • 530×407×370 ±7 km, 431 km

Extra solar or interstellar asteroids

These are asteroids that originate from outside our solar system. This is mostly determined by looking at the orbital characteristics that show it is not bound by the Sun. 

Artist’s impression of the interstellar asteroid `Oumuamua

An artist’s impression shows the first interstellar asteroid: `Oumuamua. image: ESO/M. Kornmesser

The first such asteroid was discovered in October 2017 by the Pan-STARRS 1 telescope in Hawai`i. various telescopes around the world then confirmed it. The object was reclassified from a comet as an interstellar asteroid and named 1I/2017 U1 (`Oumuamua)

The Orbit of ‘Oumuamua

The orbit of the first extrasolar asteroid. image: Credit: ESO/K. Meech et al.

update: It looks like this may be a comet instead according to recent infrared readings. See this report from the European Space Agency 

See also