Smallest habitable world around sun-like star found

Astronomers have found the smallest planet ever detected in the habitable zone around a star like the sun.

The new planet was found with the Kepler telescope, which searches for signs that a star's light has dimmed because a planet has passed between it and the telescope ? an event called a transit.

See graphic: Planet hunter

"This discovery supports the growing belief that we live in a universe crowded with life," team member Alan Boss of the Carnegie Institution for Science said in a statement. "Kepler is on the verge of determining the actual abundance of habitable, Earth-like planets in our galaxy."

The planet, named Kepler-22b, lies 600 light years away around a star of the same type (called G) as the sun. It is about 2.4 times as wide as Earth and orbits its star every 290 days, right in the middle of its star's habitable zone, where liquid water can exist on an object's surface.

Transit observations cannot pinpoint its mass, however. Astronomers have used other telescopes to search for signs that the planet's gravitational tugs are causing its host star to wobble, but so far have not detected any wobbles. That means the planet's mass must be less than 36 times that of the Earth.

It is close in size to a class of planets called super-Earths, which are up to about 2 times as wide as Earth. "We have no planet like this in our solar system," says Bill Borucki, Kepler's chief scientist at NASA's Ames Research Center in Moffett Field, California. He announced the find on Monday at the Kepler Science Conference at NASA Ames.

Just right

If the planet is on the smaller end of the allowed mass range, it could be rocky and could contain water, Borucki says. Ground-based observations in mid-2012, when the patch of sky where the planet lies is more easily visible, could help astronomers nail down the planet's mass. That will help them identify its composition.

Two previous rocky planet candidates have been found in the habitable zones of their stars, but in both cases the stars were cooler than the sun.

And neither candidate was found right in the middle of its star's "Goldilocks" zone, which could boast the best conditions for hosting life as we know it. Kepler-22b's surface is probably a balmy 22??C, Borucki said.

Scanning for ET

The Kepler telescope has been staring at more than 150,000 stars between the constellations Cygnus and Lyra for the past 1000 days. The Kepler team has now found more than 2300 candidate exoplanets, about 1000 more than it reported in February. Ten of those span no more than about twice Earth's width.

To confirm a new planet, scientists must observe three of its transits. Mission scientists saw the first transit of Kepler-22b three days after Kepler began collecting data in 2009. The third transit appeared in December 2010. "It's a great gift," Borucki said. "We consider this our Christmas planet."

"It's conceivable that these new planet candidates and their [potential] moons could have life," Borucki said.

The SETI Institute in Mountain View, California, will observe the new candidates with its Allen Telescope Array of radio telescopes in California in the hopes of detecting signals from any extraterrestrial civilisations there, said the institute's Jill Tarter. The array had been offline since April due to budget cuts but restarted observations on Monday after raising funds by partnering with the US air force and crowdsourcing donations.

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