Extrasolar planets

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 415 extrasolar planets discovered by December 2009, 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.^[73] The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12.^[74] There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses,^[73] such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436.^[75] These planets have been nicknamed "Neptunes" because they roughly approximate that planet's mass (17 Earths).^[76] Another new category are the so-called "super-Earths", possibly terrestrial planets far larger than Earth but smaller than Neptune or Uranus. To date, six possible super-Earths have been found: Gliese 876 d, which is roughly six times Earth's mass,^[77] OGLE-2005-BLG-390Lb and MOA-2007-BLG-192Lb, frigid icy worlds discovered through gravitational microlensing,^[78][79] COROT-Exo-7b, a planet with a diameter estimated at around 1.7 times that of Earth, (making it the smallest super-Earth yet measured), but with an orbital distance of only 0.02 AU, which means it probably has a molten surface at a temperature of 1000–1500 °C,^[80] and two planets orbiting the nearby red dwarf Gliese 581. Gliese 581 d is roughly 7.7 times Earth's mass,^[81] 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.^[82] 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.^[83]

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.^[84]

More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes. Currently the COROT and Kepler spacecraft are searching for stellar luminosity variations due to transiting planets. Several projects have also been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. These include the proposed NASA's, Terrestrial Planet Finder, and Space Interferometry Mission programs, and the CNES' PEGASE.^[85] The New Worlds Mission is an occulting device that may work in conjunction with the James Webb Space Telescope. However, funding for some of these projects remains uncertain. The first spectra of extrasolar planets were reported in February 2007 (HD 209458 b and HD 189733 b).^[86][87] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.^[88]

Planetary-mass objects

Main article: Planetary-mass object

[edit] Rogue planets

Main article: Rogue planet

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.^[89] Some scientists have argued that such objects found roaming in deep space should be classed as "planets," although others have suggested that they could be low-mass stars.^[90][91]

[edit] Sub-brown dwarfs

Main article: Sub-brown dwarf

Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud-collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Sub-brown dwarfs may be free-floating such as Cha 110913-773444, or orbiting a larger object such as 2MASS J04414489+2301513.

For a brief time in 2006, astronomers believed they had found a binary system of such objects, Oph 162225-240515, which the discoverers described as "planemos", or "planetary mass objects". However, recent analysis of the objects has determined that their masses are probably each greater than 13 Jupiter-masses, making the pair brown dwarfs.^[92][93][94]

Attributes

Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whilst others are also common to extrasolar planets.