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December 21, 2012

Another Earth just 12 light-years away?

Astronomers have discovered what may be five planets orbiting Tau Ceti, the closest single star beyond our solar system whose temperature and luminosity nearly match the sun's. If the planets are there, one of them is about the right distance from the star to sport mild temperatures, oceans of liquid water, and even life. Don't pack your bags just yet, though: The discovery still needs to be confirmed.

Tau Ceti is only 12 light-years from Earth, just three times as far as our sun's nearest stellar neighbor, Alpha Centauri. Tau Ceti resembles the sun so much that astronomer Frank Drake, who has long sought radio signals from possible extraterrestrial civilizations, made it his first target back in 1960. Unlike most stars, which are faint, cool, and small, Tau Ceti is a bright G-type yellow main-sequence star like the sun, a trait that only one in 25 stars boasts. Moreover, unlike Alpha Centauri, which also harbors a G-type star and even a planet, Tau Ceti is single, so there's no second star in the system whose gravity could yank planets away.

Astronomer Mikko Tuomi of the University of Hertfordshire in the United Kingdom and his colleagues analyzed more than 6000 observations of Tau Ceti from telescopes in Chile, Australia, and Hawaii. As the researchers will report in Astronomy & Astrophysics, slight changes in Tau Ceti's motion through space suggest that the star may be responding to gravitational tugs from five planets that are only about two to seven times as massive as Earth.

If that's right, all five planets lie closer to their star than Mars does to ours; however, Tau Ceti emits only 45% as much light as the sun, so each planet receives less warmth than a planet would at the same distance from our sun. Tau Ceti's three innermost planets-designated b, c, and d-are probably too hot to support life, being so close to the star that they require only 14, 35, and 94 days to complete an orbit. The farthest of the three, d, is about as close to Tau Ceti as Mercury is to the sun.

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