Asteroid

In Astronomy the term asteroid (alternately planetoid) refers to bodies, orbiting a star, too small to be planets.

Ancient Astronomers recognized seven planets -- seven objects with a regular motion against the background of the fixed stars. They counted Sol, our sun, and Luna, our moon, and Mercury Venus, Mars, Jupiter and Saturn.

After Galileo introduced telescopes to star gazing Planet Uranus could be seen to be a planet, in spite of its dimness, due to its distance from Sol.

Telescopes also enabled the discovery of other dim objects orbiting Sol, including Ceres, Asteroid Juno, Vesta -- which all orbited in the gap between Mars and Jupiter. They too were briefly considered planets. However, as a growing number of these dim objects were found Astronomers decided that, for clarity these smaller objects were given the new classification, asteroids.

The current definition of planet includes a requirement that its gravity is strong enough to sweep its orbit of smaller objects -- not true for the objects in the asteroid belt.

Since the discovery of the asteroids in the asteroid belt other small bodies, like asteroid Hermes, which has an orbit within the orbit of Planet Earth have been discovered, but they too are classified as asteroids.