Washington, Sep 30 (DPA) A group of US-based scientists has discovered what might be the first habitable planet outside Earth's solar system, the University of California at Santa Cruz announced late Wednesday.

The team of planet hunters found an Earth-sized planet - three times the mass of Earth - in orbit around a nearby star at a distance where liquid water could exist on the surface.

"Our findings offer a very compelling case for a potentially habitable planet," said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz. "The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common."

If the findings are confirmed by other astronomers, it would be the most Earth-like exo-planet yet discovered.

What astronomers identify as having the potential to sustain life would not necessarily mean hospitable for humans, but being in the right proximity - not too close, not too far - from the star it orbits is key, so that water can exist as a liquid on the surface. Some sort of protective atmosphere is important, too.

The discovery arose from 11 years of observations at Hawaii's WM Keck Observatory.

"Advanced techniques combined with old-fashioned ground-based telescopes continue to lead the exo-planet revolution," said Paul Butler of the Carnegie Institution. "Our ability to find potentially habitable worlds is now limited only by our telescope time."

The planet orbits the red dwarf star Gliese 581, which is 20 light years from Earth in the constellation Libra. The Earth-like planet, Gliese 581g, has a nearly circular orbit.

Gliese 581's six planets discovered so far are the most known outside Earth's own solar system.

The team of planet hunters concludes that the Gliese 581g discovery makes it mathematically likely that many more such solar systems and planets exist.

"If these are rare, we shouldn't have found one so quickly and so nearby," Vogt said. The number of systems with potentially habitable planets is probably on the order of 10 or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that's a large number. There could be tens of billions of these systems in our galaxy."

The team of scientists was led by astronomers from the University of California at Santa Cruz and the Carnegie Institution of Washington.

The teams work is to be published in the Astrophysical Journal and posted online at arXiv.org.