Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System they are smaller than planets but larger than meteoroids.
The term asteroid has historically been applied primarily to minor planets of the inner Solar System, as the outer Solar System was poorly known when it came into common usage. The distinction between asteroids and comets is made on visual appearance.
Comets show a perceptible coma while asteroids do not.
Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid.
The term asteroid is somewhat ill defined. It never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union until 2006, when the term small Solar System body was introduced to cover both minor planets and comets.
The 2006 definition of SSSB says that they include most of the Solar System asteroids, most Trans Neptunian Objects TNOs, comets, and other small bodies. Other languages prefer planetoid Greek for planet like, and this term is occasionally used in English for the larger asteroids.
The word planetesimal has a similar meaning, but refers specifically to the small building blocks of the planets that existed at the time the Solar System was forming. The term planetule was coined by the geologist William Daniel Conybeare to describe minor planets, but is not in common use.
When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until small Solar System body was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation.
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 comets are eventually depleted of their surface volatile ices and become asteroids. A further distinction is that comets typically have more eccentric orbits than most asteroids, most asteroids with notably eccentric orbits are dormant or extinct comets.
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