Earth-like planet discovered in 'habitable' zone

— -- Astronomers on Monday reported the discovery of an Earth-like planet outside the solar system whose size and distance from its own star put it in the "habitable" zone and make for a surface temperature perhaps averaging a balmy 72 degrees.

The planet, Kepler 22b, about 2.4 times wider than Earth, circles a star about 600 light years away, close by astronomical standards. The Kepler space telescope discovery team announced the find at a briefing at NASA's Ames Research Center in Moffett Field, Calif.

"It is right smack in the middle of the habitable zone," Kepler scientist Natalie Batalha says. Launched in 2009, the $591 million Kepler space telescope has detected more than 2,000 possible planets observed among about 150,000 stars within 3,000 light years of Earth along the "Orion spur" of our Milky Way galaxy. Kepler 22b's discovery caps a half-decade of astronomers searching for a "Goldilocks" planet — not too hot or not too cold to harbor oceans on its surface, like Earth. Liquid water is considered key for development of life.

"This is a phenomenal discovery in the course of human history," says planet hunting pioneer Geoff Marcy of the University of California-Berkeley, a Kepler investigator. European astronomers discovered the first planet confirmed orbiting a nearby star in 1995, spurring a gold rush of planet discoveries, mostly jumbo planets the size of Jupiter or larger.

Kepler 22b "is the smallest, most nearly Earth-size, planet ever found in the lukewarm zone around another sun where life could thrive."

Kepler spotted the planet from tiny dips in starlight caused by partial eclipses, or transits, of the planet in front of the star. It travels on a 290-day orbit around a sun-like star, dubbed Kepler 22a, nearly as bright and warm as our own. Astronomer Francesco Pepe of Switzerland's University of Geneva, says he is "convinced that the report will be solid and the data impressive" from the Kepler 22b discovery. Pepe is a member of a competing European Southern Observatory planet-hunting team that announced an Earth-sized planet on the edge of another star's habitable zone in September.

The transit detection method yields only a width and orbit time for planets, instead of a weight. Preliminary telescope observation of the planet's star for gravitational wobbles induced on it by the planet indicate only that Kepler 22b cannot weigh more than 36 times more than Earth.

"There is absolutely no doubt about the reality of this planet," Marcy says, given that the Kepler team has detected three 7.9-hour dimmings of the star, reoccurring at 290-day crossings in front of the star.

"This discovery is rock solid, even if the planet isn't," Marcy says.

From early observations, the Kepler astronomers cannot tell whether Kepler 22b is a rocky world, a water-covered world or something else, says team leader William Borucki of NASA's Ames Research Center. Borucki says observations show no signs of other planets in the star's solar system. Pepe estimates that the world would weigh about 14 times as much as Earth if it is built the same way.

Most likely, Marcy says, "this planet is probably rocky with a thick layer of water and gas, making it more like Neptune in our solar system."

Among the more than 2,000 possible planets spotted by Kepler are 10 planets less than 2.4 times as wide as Earth that linger in the ocean-friendly "habitable zone" distances from their stars, awaiting confirmation. "(W)e Homo sapiens are straining our reach into the universe to find planets that remind us of home," Marcy says by e-mail. "We are almost there."