For the first time, astronomers using NASA's Kepler space telescope have confirmed a roughly Earth-size planet orbiting a sun-like star in the so-called "Goldilocks" zone where water can exist in liquid form on the surface and conditions may be favorable for life as it is known on Earth.

Along with the confirmed extra-solar planet, one of 28 discovered so far by Kepler, researchers today also announced the discovery of 1,094 new exoplanet candidates, pushing the spacecraft's total so far to 2,326, including 10 candidate Earth-size worlds orbiting in the habitable zones of their parent stars.

Additional observations are required to tell if a candidate is, in fact, an actual world. But astronomers say a planet known as Kepler-22b, orbiting a star some 600 light years from Earth, is the real thing.

NASA

"Today I have the privilege of announcing the discovery of Kepler's first planet in the habitable zone of a sun-like star, Kepler-22b," Bill Borucki, the Kepler principal investigator at NASA's Ames Research Center, told reporters. "It's 2.4 times the size of the Earth, it's in an orbital period (or year) of 290 days, a little bit shorter than the Earth's, it's a little bit closer to its star than Earth is to the sun, 15 percent closer.

"But the star is a little bit dimmer, it's a little bit lower in temperature, a little bit smaller. That means that planet, Kepler-22b, has a rather similar temperature to that of the Earth...If the greenhouse warming were similar on this planet, its surface temperature would be something like 72 Fahrenheit, a very pleasant temperature here on Earth."

It is not yet known whether Kepler-22b is predominantly rocky, liquid, or gaseous in composition, but the finding confirms for the first time the long-held expectation that Earth-size planets do, in fact, orbit other suns in the habitable zones of their host stars.

That, in turn, greatly improves the odds for the existence of life, as it is commonly defined, beyond Earth's solar system.

"I think there are two things that are really exciting about Kepler-22b," said Natalie Batalha, the deputy science team lead at Ames. "One is that it's right in the middle of this habitable zone.

"The second thing that's really exciting is it's orbiting a star very, very similar to our own sun. This is a solar analogue, almost a solar twin, very similar to our own sun and you've got a planet 2.4 times the size of the Earth right smack in the habitable zone."

Equipped with a 95-megapixel digital camera, Kepler was launched from Cape Canaveral on March 6, 2009. The camera is aimed at a patch of sky in the constellation Cygnus that's the size of an outstretched hand that contains more than 4.5 million detectable stars.

Of that total, some 300,000 are believed to be the right age, have the right composition and the proper brightness to host Earth-like planets. More than 156,000 of those, ranging from 600 to 3,000 light years away, will be actively monitored by Kepler over the life of the mission.

To find candidate planets, the spacecraft's camera monitors the brightness of target stars in the instrument's wide field of view, on the lookout for subtle changes that might indicate a world passing between the star and the telescope. By studying the slight dimming--comparable to watching a flea creep across a car's headlight at night--and by timing repeated cycles, computers can identify potential extra-solar worlds even though the planets themselves cannot be seen.

NASA

But it's a challenging observation. For a planet like Earth passing in front of a star like the sun, the sun's light would dim by just 84 parts per million. To make sure an observation indicates the presence of a real planet and not some other phenomena, measurements over multiple orbits are required. For Earth-like planets in habitable-zone orbits, a full three years is needed to confirm an initial observation.

In June 2010, the Kepler team announced 312 planet candidates, most smaller than Neptune, in data collected over the first four months of the mission. In February 2011, based on 13 months of data, the number grew to 1,235 potential planets orbiting 997 stars.

The latest announcement pushes the total number of candidates to 2,326 possible planets orbiting 1,792 stars. Of that total, 367 stars--about 20 percent--show signs of multiple planet candidates.

Twenty-eight confirmed planets have been found in the Kepler data. Including Earth-based telescopes, more than 600 extrasolar planets have been found to date. But most of them are huge Jupiter-class worlds orbiting well outside the habitable zone.

With Kepler, "we're getting very close, we are really homing in on the true Earth-size habitable planets," Batalha said.

Also in the hunt: The SETI Institute, or the Search for Extraterrestrial Intelligence, in Mountain View, Calif.

Jill Tarter, director of the Center for SETI Research at the institute, said a radio telescope array that was looking for signs of radio signals in the Kepler field of stars that might indicate the presence of intelligent life is back in operation after a budget-driven hiatus earlier this year.

"I'm really pleased to announce as of 6:18 this morning, as the Kepler field rose over the observatory, the ATA (Allen Telescope Array) was back on the air, continuing the search for Earth analogues."

The Allen Telescope Array, originally funded by Microsoft co-founder Paul Allen, is being used to make systematic observations of stars in the Kepler field, on the lookout for any signs of artificial signals.

Citing a 1993 paper by Carl Sagan and four colleagues that used data from NASA's Jupiter-bound Galileo spacecraft as a test for detecting life on Earth, Tarter said "one of the strongest pieces of evidence for life, indeed intelligent life on Earth, was the presence of narrow-band pulse-amplitude-modulated radio transmissions."

"While there may be some uncertainty about how to define the habitable zone, an exoplanet that could be detected through the techno-signatures of its inhabitants would surely qualify as an Earth analogue," she said.