The first extrasolar planets were discovered in 1992, and more than 4,100 such planets are now known. Over 600 of these systems have more than one planet. Because planets are much fainter than their stars, fewer than 100 have been imaged directly. Most extrasolar planets have been found through their transit, the small dimming of a star’s light when a planet passes in front of it.
Many of these planets are unlike those of the solar system. Hot Jupiters are large gas giants that orbit very close to their star. For example, HD 209458b is 0.69 times the mass of Jupiter and orbits its star every 3.52 days. Hot Neptunes are large ice giants about 10 percent of Jupiter’s mass that also orbit very close to their star. Super-Earths are planets that are likely rocky like Earth but several times larger.
A primary goal of extrasolar planet research has been finding another planet that could support life. A useful guide for finding a life-supporting planet has been the concept of a habitable zone, the distance from a star where liquid water could survive on a planet’s surface. About 20 planets have been found that are roughly Earth-sized and orbit in a habitable zone.
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