A team of planet hunters from the University of California and the Carnegie Institution of Washington have discovered a planet with three times the mass of Earth orbiting a nearby star at a distance that places it squarely in the middle of the star's "habitable zone."

The new findings are based on 11 years of observations of the nearby red dwarf star Gliese 581 using the HIRES spectrometer on the Keck I Telescope in Hawaii. The spectrometer allows precise measurements of a star's motion, which can reveal the presence of planets.

The planet lies in an area where liquid water could exist on the planet's surface. If confirmed, it would be the most Earth-like exoplanet ever discovered and the first strong case for a potentially habitable one. Its mass indicates that it is probably a rocky planet with a definite surface and enough gravity to hold on to an atmosphere.