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Four planets in 'habitable zones' spotted within spitting distance of Earth

Astronomers say they used a new statistical technique to find four possible super-Earths orbiting in the habitable zone of two stars within 22 light-years of Earth, Gliese 667C and tau Ceti.

By Staff writer / December 19, 2012

This diagram shows an artist's rendering comparing our own solar system to Kepler-22, a star system containing the first 'habitable zone' planet discovered by NASA's Kepler mission. The 'habitable zone' is a region where under the right conditions, liquid water can form stable pools on the surface.

NASA/Ames/JPL-Caltech

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Astronomers say they have uncovered evidence for what could be four super-Earth planets orbiting within the habitable zones of two stars within 22 light-years of Earth.

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Three of those candidate planets are among a tightly packed clutch of five that orbit Gliese 667C, part of a triple-star system 22 light-years away in the constellation Scorpius. The other possible planet is one of five orbiting tau Ceti, a sun-like star 12 light-years away in the constellation Cetus.

Taken together, the detections not only add to accumulating evidence that planets look to be more common than stars – and that planets in habitable zones could be more common than previously thought, some of researchers reporting the finds say.

The finds also illustrate the power of improved statistical tools to boldly uncover candidate planets where no planet had been found before.

The evidence for these candidate planets requires independent confirmation, the researchers caution. Still, the tools represent "a real breakthrough," says Steven Vogt, an astronomer at the University of California at Santa Cruz and a member of the team reporting the results for tau Ceti. The approach the team took leaves only about one chance in 3 million that the detections could herald something other than a planet.

Since the mid-1990s, astronomers have bagged more than 850 extra-solar planets. The ultimate goal is to find rocky planets with Earth-like masses orbiting within their stars' habitable zones – a region where under the right conditions, liquid water can form stable pools on the surface. Liquid water is considered an essential ingredient for organic life.

Different groups of astronomers had aimed three telescopes for various lengths of time at tau Ceti and found nothing. Led by Mikko Tuomi at the University of Hertfordshire in Britain, the team reporting this latest analysis applied relatively new statistical tools to the combined data from these telescopes.

The result: "Five planets came out: boom, boom, boom, boom, boom ... as clear as a bell," Dr. Vogt says.

Tau Ceti has about 78 percent of our own sun's mass, but its composition is quite similar, Dr. Tuomi's team reports. Its candidate planets range from 2 to 6.6 times Earth's mass.

The innermost object orbits the star once every 14 days, while the outermost takes 642 days to make its circuit. The fourth planet from the star, with a 168-day orbit, travels well within a zone where liquid water could remain stable on the planet's surface, the team estimates. However, the results don't speak directly to what the planets are made of. 

A similar story has played out for Philip Gregory, an astronomer at the University of British Columbia in Vancouver. Previous researchers had found two planets orbiting Gliese 667C, a red dwarf with 31 percent of the sun's mass. Using a broadly similar statistical approach, he reports detecting the initial two, plus three more planets. Three of the five fall within the star's habitable zone, he estimates.

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