Scientists discover nearly Earth-size planet

HATFIELD, England — In the search for Earth-like planets, astronomers zeroed in Tuesday on two places that look awfully familiar to home. One is close to the right size. The other is in the right place.

European researchers said they not only found the smallest exoplanet ever, called Gliese 581e, but realized that a neighboring planet discovered earlier, Gliese 581d, was in the prime habitable zone for potential life.

"The Holy Grail of current exoplanet research is the detection of a rocky, Earth-like planet in the 'habitable zone,' " said Michel Mayor, an astrophysicist at Geneva University in Switzerland.

An American expert called the discovery of the tiny planet "extraordinary."

Gliese 581e is only 1.9 times the size of Earth — while previous planets found outside our solar system are closer to the size of massive Jupiter, which NASA says could swallow more than 1,000 Earths.

Gliese 581e sits close to the nearest star, making it too hot to support life. Still, Mayor said its discovery in a solar system 20 light years away from Earth is a "good example that we are progressing in the detection of Earth-like planets."

Scientists also discovered that the orbit of planet Gliese 581d, which was found in 2007, was located within the "habitable zone" — a region around a sun-like star that would allow water to be liquid on the planet's surface, Mayor said.

He spoke at a news conference Tuesday at the University of Hertfordshire during the European Week of Astronomy and Space Science.

Gliese 581d is probably too large to be made only of rocky material, fellow astronomer and team member Stephane Udry said, adding it was possible the planet had a "large and deep" ocean.

"It is the first serious 'water-world' candidate," Udry said.

Mayor's main planet-hunting competitor, Geoff Marcy of the University of California, Berkeley, praised the find of Gliese 581e as "the most exciting discovery" so far of exoplanets — planets outside our solar system.

"This discovery is absolutely extraordinary," Marcy told The Associated Press by e-mail, calling the discoveries a significant step in the search for Earth-like planets.