Extrasolar planets
Extrasolar planets or exoplanets are planets that orbit stars other than the sun.
Planets have no light source of their own, and most extrasolar planets are too far away to be seen by the light they reflect from their own suns, however some have been detected by the methods described below.
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Methods of detection
Astrometry
A star and planet orbit about a spot that is the center of gravity of the system known as the barycenter. For the Sun and Jupiter, the barycenter is 1.068 solar radii. With larger planets orbiting closer to the star, it is possible to detect the movement of the star against the distant background. This method was used to detect a planet orbiting Gliese 876.
Radial velocity
Instead of mapping the position of the star against the background, it is also possible to detect the minute Doppler shift in the star's spectrum as it is tugged by the planet. This method is responsible for the majority of extrasolar planet detection.
Transit method
When a planet passes in front of its star, it blocks some of the light of the star. By measuring this periodic momentary dimming of the light, it is possible to identify a planet that is properly aligned. This method does not work if the planet itself does not eclipse the star from our perspective.
Direct imaging
In the situation where a star is not extremely bright, it is possible to detect a planet orbiting it. Such was the case with the brown dwarf 2M1207 which had a detectable companion that was named 2M1207b.
Current count
As of 2008, the number of extrasolar planets detected stands at more than 300. Since the detection methods devised to date favor large, gaseous planets, most detected planets are quite dissimilar to Earth, and they are not thought to be good candidates for finding life. However, the planet Gliese 581c orbiting the red dwarf star Gliese 581 (approximately 20 light-years from earth) appears to be a terrestrial (that is, rocky) planet that orbits in the "habitable zone" of space surrounding its star. The term "habitable" in this context simply means that the stellar light flux it receives is similar to that of Earth, and it therefore might have an Earthlike temperature. Whether it is in fact habitable depends on many other factors, and is pure speculation at present.
