Asteroid

Pronunciation

• enPR: ăs'tÉ™-roid", IPA: /ˈæstÉ™roɪd/

Origin

Borrowing from grc ἀστεροειδής, from ἀστήρ (astēr, "star") + εἶδος (eidos, "form").

Full definition of asteroid

Noun

1. (astronomy) A naturally occurring solid object, which is smaller than a planet and is not a comet, that orbits a star
2. (astronomy) In the Solar system, such a body that orbits within the orbit of Jupiter
• 2007, , Fundamental Astronomy, The orbital planes of asteroids, minor bodies that circle the Sun mainly between the orbits of Mars and Jupiter, are often more tilted ...
3. (zoology) Any member of the taxonomic class Asteroidea; a starfish

Usage notes

The term "asteroid" has never been precisely defined. It was coined for objects which looked like stars in a telescope but moved like planets. These were known from the asteroid belt between Mars and Jupiter, and were later found co-orbiting with Jupiter (Trojan asteroids) and within the orbit of Mars. They were naturally distinguished from comets, which did not look at all starlike. Starting in the 1970s, small non-cometary bodies were found outside the orbit of Jupiter, and usage became divided as to whether to call these "asteroids" as well. Some astronomers restrict the term "asteroid" to rocky or rocky-icy bodies with orbits up to Jupiter. They may retain the term planetoid for all small bodies, and thus tend to use it for icy or rocky-icy bodies beyond Jupiter, or may use dedicated words such as centaurs, Kuiper belt objects, transneptunian objects, etc. for the latter. Other astronomers use "asteroid" for all non-cometary bodies smaller than a planet, even large ones such as Sedna and (occasionally) Pluto. However, the distinction between asteroid and comet is an artificial one; many outer "asteroids" would become comets if they ventured nearer the Sun. The official terminology since 2006 has been small Solar System body for any body that orbits the Sun directly and whose shape is not dominated by gravity.

Derived terms

Related terms