Scientists find most Earth-like planet ever

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.

"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 581 e 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.

While Gliese 581 e is too hot for life "it shows that nature makes such small planets, probably in large numbers," Marcy commented. "Surely the galaxy contains tens of billions of planets like the small, Earth-mass one announced here."