The team of Swiss, French and Portuguese scientists who found the planet estimate its surface temperature ranges from freezing to 104 degrees Fahrenheit, the crucial range in which water can exist as a liquid.

"Because of its temperature and relative proximity, this planet will most probably be a very important target of the future space missions dedicated to the search for extraterrestrial life," Xavier Delfosse, an astronomer from Grenoble University in France, said on Tuesday. "On the treasure map of the universe, one would be tempted to mark this planet with an X."

The discovery was made with the European Southern Observatory's 12-foot diameter telescope in La Silla, Chile. The planet was first detected by the telescope's HARPS Spectrograph, a sensitive device for analysing the light from distant astronomical objects.

Geoffrey Marcy, a University of California, Berkeley, astronomer who has discovered most of the 200 or so "exoplanets" orbiting other stars, called it "a marvellous discovery ... the best case for a habitable planet" so far.

But Marcy and other scientists cautioned against reading too much into the discovery.

Eugene Chiang, a planetary specialist at UC Berkeley, said the Europeans had no proof the planet was rocky, or that their estimates of the surface temperature were accurate.

"This is a step beyond what's been done," he said. "But to say this planet is habitable is a real stretch."