European astronomers announced Tuesday that they had discovered the first planet beyond our solar system that orbits in a "sweet spot" zone where life could exist.
The planet, about five times as massive as Earth, orbits Gliese 581, a red dwarf star about 20 light-years from our solar system.
The team of Swiss, French and Portuguese scientists who found the planet estimate its surface temperature at freezing to 104 degrees Fahrenheit, a 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," said Xavier Delfosse, an astronomer from Grenoble University in France.
"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, which analyzes light from distant astronomical objects.
Geoffrey Marcy, a UC Berkeley astronomer who has discovered most of the so-called exoplanets orbiting other stars, called it "a marvelous 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."
If alien astronomers saw Venus from afar, for instance, they would not be able to tell that a runaway greenhouse effect had created an unlivable hell of a world, with temperatures of 750 degrees Fahrenheit.
"The surface temperature depends on the atmosphere, and they don't know what the atmosphere is made of," Chiang said of the new planet.
More than 200 exoplanets have been found, most by Marcy's team and the European team that announced this latest find.
The vast majority have been gas giants, orbiting so close to their home stars that anything alive, at least as we understand it, would be cooked.
Most exoplanets are too distant to be seen directly. Their existence must be inferred by observing the slight wobble they cause in their home stars.
Because of this, most of the exoplanets detected are huge and very close to their stars.
The holy grail for planet hunters is a rocky planet about the size of Earth orbiting a star similar to the sun in the so-called Goldilocks zone -- not too hot, not too cold.
The European team said the new planet was the closest match so far. Even though the planet is closer to its star than Mercury is to our sun, Gliese 581 is a much cooler star.
The new planet, which completes one orbit every 13 days, is the third found around Gliese 581, including one Neptune-size world that completes an orbit every five days.
Xavier Bonfils of Lisbon University said red dwarfs were ideal targets for planet-finding efforts "because they emit less light and the habitable zone is thus much closer than it is around the sun."
Gliese 581 is among the 100 stars closest to our solar system. The vast majority are red dwarfs. The Sun is a medium-size yellow star.
Some planet hunting experts think it will take years and new technology to find an ideal world for life.
One proposed mission is NASA's Terrestrial Planet Finder, which would use a coronagraph to block the light from the star so that any surrounding planets could be seen. No launch date has been set.
But even then, it would not be easy. Imaging a planet near its star is like taking a picture from New York of a person holding a candle in San Diego while standing next to a bonfire, experts say.
So far, 13 exoplanets have been discovered with masses below that of 20 Earths. The lowest-mass planet previously known was 5.9 Earth masses, which was discovered by Marcy's team. It orbits the star Gliese 876.
"We are confident that, given the results obtained so far, Earth-mass planets around red dwarfs are within reach," said Michel Mayor of Geneva Observatory in Switzerland.
Despite the long odds, SETI, the Bay Area organization that has long led the search for intelligent life in other star systems, turned its radio antenna at its Hat Creek observatory in Northern California toward Gliese 581 after the Europeans' announcement.
Seth Shostack, SETI's senior astronomer, said the organization zeroed in on Gliese 581 twice in the 1990s but found nothing.