A planet orbiting a star other than the Sun; also known as an exoplanet. Planets of other stars are mostly too faint to be seen directly with existing instruments, so indirect methods of detection are necessary. The first extrasolar planet, found in 1992, orbits a pulsar, PSR 1257+12. A cyclic change in the timing of the pulsar’s radio emissions was interpreted as motion of the pulsar about its centre of mass with an orbiting planet. A similar approach, known as the radial velocity method, has been used to find other extrasolar planets. The technique looks for a cyclical Doppler shift in light from a star that would occur as it orbited its common centre of mass with one or more planets. Using this technique, astronomers in 1995 detected the first known extrasolar planet around an ordinary star, 51 Pegasi, 50 l.y. away. The planet has a mass about half that of Jupiter and an orbital period of 4.2 days. The first multi-planet system, also discovered by the radial velocity method, was found in 1999 around Upsilon Andromedae. An alternative approach, the transit method, is to look for dips in the brightness of a star as an orbiting planet moves in front of it. Although this method is restricted to planets whose orbital plane lies close to our line of sight, it has so far been the most productive, accounting for nearly 80% of all discoveries. The first transit of an extrasolar planet across the face of a star, HDE 209458 in Pegasus, was observed in 1999. A third technique, which has also borne fruit, looks for a spike on the light curve of a gravitational microlensing event that would be caused by a planet accompanying the lensing star.

Planet-finding spacecraft such as CoRoT and Kepler have used the transit technique to detect smaller drops in light, and hence smaller planets, than is possible from the ground.

Masses of the first 3000 known extrasolar planets range from around 30 Jupiters (on the verge of being a brown dwarf) to a few tenths that of the Earth. Periods range from less than a day to several centuries, the longest being Fomalhaut b, nearly 900 years, although 98% are under 10 years. Orbital radii are 0.01 au to over 1000 au from their parent stars, although only a few are greater than 20 au.

http://exoplanet.eu Extrasolar planets encyclopedia, regularly updated.

https://exoplanets.nasa.gov NASA website on exoplanets, regularly updated.

http://www.gemini.edu/node/11126