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Extrasolar planet
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Planet Fomalhaut b (inset against Fomalhaut's interplanetary dust cloud) imaged by the Hubble Space Telescope's coronagraph (NASA photo)
The three known planets of the star HR8799, as imaged by the Hale Telescope. The light from the central star was blanked out by a vector vortex coronagraph.
2MASS J044144 is a brown dwarf with a companion about 5-10 times the mass of Jupiter. It is not clear whether this companion object is a sub-brown dwarf or a planet.
Coronagraphic image of AB Pictoris showing a companion (bottom left), which is either a brown dwarf or a massive planet. The data was obtained on March 16, 2003 with NACO on the VLT, using a 1.4 arcsec occulting mask on top of AB Pictoris.
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of 708 such planets have been identified as of December 12, 2011. It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars. It follows that tens of billions of exoplanets must exist in the Milky Way Galaxy alone.For centuries, many philosophers and scientists supposed that extrasolar planets existed. But there was no way of knowing how common they were or how similar they might be to the planets of our Solar System. Various detection claims made starting in the nineteenth century were all eventually rejected by astronomers. The first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmed detection of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Due to improved observational techniques, the rate of detections has increased rapidly since then. Some exoplanets have been directly imaged by telescopes, but the vast majority have been detected through indirect methods such as radial velocity measurements.Most known exoplanets are giant planets believed to resemble Jupiter or Neptune. That reflects a sampling bias, since massive planets are easier to observe. Some relatively lightweight exoplanets, only a few times more massive than Earth, are known as well; statistical studies now indicate that they actually outnumber giant planets. Planetary-mass objects also exist that orbit brown dwarfs or that "float free" in space, although strictly speaking the term "planet" may not be applicable to them.The discovery of extrasolar planets has intensified interest in the possibility of extraterrestrial life. Several giant planets are now known that orbit in their star's habitable zone. Among the candidates are Gliese 581 d and HD 85512 b.In December 2011, NASA confirmed that 600-light-year distant Kepler-22b, at 2.4 times the radius of Earth, is potentially the closest match to Earth in terms of both size and temperature.
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