Scientists analyzing data from NASA’s Kepler extra-solar planet-finding mission have confirmed their first Earth-like planet orbiting in the habitable zone of a star 600 light years from Earth, while almost doubling the number of candidates identified as they improve data-analysis techniques.

Researchers believe their growing skill at extracting meaningful information from the flood of data gushing down from the spacecraft will lead them to more planets where temperatures are neither too warm nor too cold for liquid water to exist on their surfaces. The new batch of candidates includes 10 that may also be Earth-like and in the habitable zone, but await confirmation by ground-based telescopes. Overall, the total number of Earth-like candidates is larger.

“What we see are 48 objects, planetary candidates, in the habitable zone between 185 and 303K,” said Bill Borucki of the NASA Ames Research Center in California, the Kepler principal investigator. “It’s conceivable that any, or many, of these planetary candidates, and their moons, could have life.”

Designated Kepler 22b, the planet lies in the direction of the constellations Lyra and Cygnus, circling a star that is only slightly smaller than the Sun and about 25% less luminous. The Kepler probe, launched March 7, 2009, detects extra-solar planets by measuring the dimming of light from a star as a planet passes in front of it.

Based on three observed transits — the first of them only three days after the spacecraft was declared operational — Kepler 22b orbits its star every 289.9 Earth days, crossing in front of it for 7 hr., 4 min., Borucki said in announcing the confirmation at a conference on Kepler science at Ames.

The relatively limited data do not allow a definitive characterization of the planet, including whether it is rocky, gaseous or has a liquid surface, and the nature of its atmosphere, if any. Its temperature, if there is an atmosphere, would be pleasant.

“The Earth’s temperature is 255K, Borucki said. “This is 262K. So if the greenhouse warming is similar on this planet, at its surface it would be something like 72F.”

The mass of the planet is not yet known, but based on the lack of a measurable wobble in its star, scientists know that it is less than 36 times that of Earth. Next summer, when the planet’s star is high in Earth’s sky, astronomers will have a better chance to measure its mass with large ground-based telescopes.

“You would expect it to have a lot of rocky material, and it probably has a lot of water as well,” Borucki says.

That possibility has raised the interest of the SETI Institute, which uses radio astronomy to scan the heavens for possible signals from extraterrestrial intelligence. Jill Tarter, director of the institute’s Center for SETI Research, says private funding has enabled reactivation of the Allen Telescope Array at the Hat Creek Observatory in Northern California, which shut down in April for lack of funding. The array’s 42 six-meter antennas will resume scanning Kepler 22b and other potential planets identified by the orbiting probe for “techno-signatures” that would confirm extraterrestrial intelligence, she says.

To date Kepler has identified 2,326 candidate extra-solar planets: 207 that are roughly the same size as Earth, 680 “super-Earths” up to 10 times more massive than Earth, 1,181 with the mass of Neptune, 203 with the mass of Jupiter and 55 more massive than Jupiter.