Extrasolar planets

Main article: Extrasolar planet

Exoplanets, by year of discovery, through 2010-10-03.

The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. Of the more than 500 extrasolar planets discovered by December 2010, most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass have been observed.^[75] The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12.^[76]
There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses,^[75] such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436.^[77] These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths).^[78]
and five of the six planets orbiting the nearby red dwarf Gliese 581. Gliese 581 d is roughly 7.7 times Earth's mass,^[82] while Gliese 581 c is five times Earth's mass and was initially thought to be the first terrestrial planet found within a star's habitable zone.^[83] However, more detailed studies revealed that it was slightly too close to its star to be habitable, and that the farther planet in the system, Gliese 581 d, though it is much colder than Earth, could potentially be habitable if its atmosphere contained sufficient greenhouse gases.^[84]

Size comparison of HR 8799 c (gray) with Jupiter. Most exoplanets discovered thus far are larger than Jupiter, though discoveries of smaller planets are expected in the near future.

It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. There may also exist a class of hot Jupiters, called Chthonian planets, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine Chthonian planets have been discovered.^[85]