A nearly Earth-sized planet found, "the most exciting discovery"

"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 581 e 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, w hich NASA says could swallow more than 1,000 Earths.Gliese 581 e sits close to the nearest star, making it too hot to support life. Still, Mayor said its discovery in a solar system 20 1/2 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 581 d, 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 581 d 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.