NASA's Kepler space telescope has discovered a planet circling at just the right distance from another star, making for comfortable temperatures and supporting an environment for liquid water and possible life, scientists announced Monday.

Artist's concept of Kepler-22b, a newly-discovered planet positioned a comfortable distance from a sun-like star. Credit: NASA/Ames/JPL-Caltech
 
Orbiting a star much like the sun, the planet is about 2.4 times the radius of Earth and has a year lasting about 290 days. But researchers need to know its mass, density and composition before declaring the planet Earth-like.

Kepler has found 48 candidate planets in the habitable zone, the cosmic sweet spot where scientists believe conditions could support life. Planets closer to their parent star are too hot, causing water to evaporate and starving life of vital nutrients. Colder planets lie further away from the star.

"It's conceivable that any of these planets and their moons [in the habitable zone] could have life," said Bill Borucki, Kepler principal investigator at Ames Research Center in California.

The new discovey, named Kepler-22b, could be rocky, watery or made of gas. Astronomers will use ground-based telescopes next summer to determine the planet's composition, density and paint a clearer picture of its prospects for harboring life.

"We're certain that is in the habitable zone, it's not at the edge. And it if it has a surface, it ought to have a nice temperature," Borucki said.

Kepler-22b orbits a G-type star 600 light years from Earth. The star, which is slightly cooler and smaller than the sun, lies in the field of view of Kepler's photometer, a camera with detectors adding up to a resolution of 95 million megapixels.

"It's right in the middle of this habitable zone, right next to Earth," said Natalie Batalha, the deputy leader of the Kepler science team at Ames. "The other thing that's really exciting is it's orbiting a star very much like our own sun."

Jill Tarter, director of the Center for SETI Research, said astronomers will point radio telescopes toward planets discovered by Kepler and listen for evidence of intelligent life.

"My team is interested in using the Kepler data to find true Earth analogs," Tarter said. "We will give a higher priority to those worlds that our colleagues tell us at not too warm, not too cold, but just right."

SETI stands for the Search for Extraterrestrial Intelligence. The SETI Institute is a non-profit organization that scientifically addresses the question of the origin, nature, and prevalence of life beyond Earth.

Launched in March 2009, Kepler is stationed in an Earth-trailing solar orbit and aims its 3.1-foot telescope toward constellations Cygnus and Lyra, observing a 10-degree-wide field containing 4.5 million detectable stars. Kepler is focusing on approximately 156,000 stars for the purposes of its research.

Artist's concept of the Kepler spacecraft. Credit: NASA
 
"This is a major milestone on the road to finding Earth's twin," said Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington. "Kepler's results continue to demonstrate the importance of NASA's science missions, which aim to answer some of the biggest questions about our place in the universe."

Kepler's camera constantly monitors the brightness of each star in its target catalog, and the mission detects planets by measuring dips in the brightness of stars as bodies pass in front. By observing "transits" three times, scientists are close to confirming its a planet.

Further observations by NASA's Spitzer Space Telescope and ground facilities, extensive modeling, and reviews by independent researchers help nail down a candidate as a real planet and not just another star, according to Borucki.

Kepler has now discovered 2,326 candidate planets outside the solar system. NASA says 207 of the likely planets are Earth-sized, 680 are super Earth-sized, 1,181 are Neptune-sized, 203 are Jupiter-sized and 55 are larger than Jupiter.

Mission officials announced 1,094 new planet candidates Monday, adding to more than 1,200 potential discoveries unveiled in February. Kepler scientists are releasing data on the candidates for outside astronomers to observe and try to confirm as real planets.

The Kepler-22b announcement raises the mission's list of discoveries to 29 planets. Many of the verified planets are massive or large objects orbiting scorchingly close to their parent stars.

It takes several years to obtain the observations necessary to confirm an Earth-sized planet in the habitable zone. That's because such planets only transit a star approximately once per year.

Borucki said his team is preparing a proposal for review by NASA in February to extend Kepler's mission until 2015. The current mission is due to end at the end of 2012.