Extrasolar Planets

Extrasolar planets are planets that orbit stars other than the Sun; a "planet" is defined as an object too small for gravitational pressure at its core to ignite the deuterium-fusion reaction that powers a star. (There is no generally-agreed-upon lower limit on the size of a planet.) The existence of extrasolar planets has been suspected since at least the time of Dutch astronomer Christian Huygens (1629–1695). The ancient Greek astronomer Aristarchus (late third century B.C.) may have developed the concept over 2,000 years ago, although this is not known certainly. However, extrasolar planets remained hypothetical until recently because there was no way to detect them. Extrasolar planets are difficult to observe directly because planets shine by reflected light and so are only about a billionth as bright as the stars they orbit. Their light is either too dim to see at all with present techniques, or is lost in their stars' glare. Since 1995, thanks to new, indirect observational techniques, over 100 extrasolar planets have been discovered, with masses ranging from that of Jupiter to the upper size limit for a planet (about 15 Jupiter masses).