A planet that orbits a star other than the Sun. The first such planet to be discovered, in 1991, was found orbiting a pulsar, although most of the more than 100 extrasolar planets that have since been identified orbit normal stars. Many of them, known as hot Jupiters, are very large and revolve around their star in extremely close orbits, at less than the distance of Mercury's orbit around the Sun. Other Jupiter-sized and larger planets have been found in highly eccentric orbits. Evidence suggests that extrasolar planets may be relatively common throughout the universe. In 2004, astronomers located the first extrasolar planet with an atmosphere containing oxygen and carbon. The planet, HD 209458b (also called Osiris), orbits a star 150 light-years from Earth. The apparent lack of terrestrial, Earth-sized planets among those that have so far been discovered may simply be the result of the much greater difficulty in identifying smaller, less massive bodies at such great distances. Also called exoplanet
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.