New planet may be habitable, but don't start packing

WASHINGTON — For the first time, astronomers have discovered a planet outside our solar system that is potentially habitable, with Earth-like temperatures — a key step toward answering whether our cradle of life is unique in the universe.

The planet circles the star Gliese 581, which at 20.5 light years away is among the 100 stars closest to Earth. Dubbed Gliese 581c, the planet orbits very close to its star — closer than Mercury is to our sun. But astronomers with the European Southern Observatory say the star is dim enough that temperatures on the planet's surface likely range from 32 degrees to 104 degrees Fahrenheit, within the crucial range in which water can exist as a liquid.

If the planet has water — a big unknown — its size and climate could make it habitable, experts said. The planet appears to be about 50 percent larger than Earth and has five times more mass, making it one of the smallest far-off planets ever detected.

"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."

A few crucial details

But before you book your extrastellar flight to Gliese 581c, a few caveats about how alien that world probably is: Anyone sitting on the planet would get heavier quickly, and birthdays would add up fast, since it orbits its star once every 13 days.

An Earth-like planet?

Location: 20.5 light-years from Earth, in the Libra constellation

Size: 50 percent bigger and five times the mass of Earth

Gravity: 1.6 times as strong as Earth's

Orbit: Every 13 Earth days

Temperature: Average 32 to 104 degrees F

Star: Gliese 581, a red dwarf

Sources: Los Angeles Times, Christian Science Monitor, The Associated Press

Gravity is 1.6 times as strong as Earth's, so a 150-pound person would feel as if he weighed 240 pounds.

But oh, the view! The planet is 14 times closer to the star it orbits than Earth is. Swiss astronomer Stephane Udry, the discovery team's lead author, figures the red dwarf star would appear in the sky at a size 20 times larger than our moon. And astronomers think the planet doesn't rotate, so one side would always be sunlit and the other dark.

Distance is a problem. "We don't know how to get to those places in a human lifetime," said retired NASA astronomer Steve Maran.

Still, the conditions look promising enough that officials with the California-based SETI Institute, which looks for signs of radio communication from alien civilizations, said they hope to give the planet a fresh look this summer. Radio observations of Gliese 581 in the 1990s turned up nothing unusual.

More like this out there?

The finding is a milestone in any case because it suggests that Earth-like planets may be common throughout the universe, astronomers said.

Planets have been detected at about 5 percent of the stars in our galaxy where astronomers have looked, and the discovery of Gliese 581c suggests that at least 1 percent of those stars have Earthlike planets. So amid the 200 billion stars of the Milky Way galaxy, about 100 million may have planets that meet the bare requirements for life to emerge.

"This is a marvelous discovery ... the best case for a habitable planet" so far, said astronomer Geoff Marcy, a University of California, Berkeley, astronomer who has discovered most of the 227 so-called exoplanets orbiting other stars. Marcy's group had been racing the Europeans to find the first potentially Earth-like planet.

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

Eugene Chiang, a planetary specialist at UC Berkeley, said the Europeans have no proof that the planet has a solid surface, or that their estimates of the surface temperature are accurate.

"This is a step beyond what's been done," he said of the discovery. "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 has created an unlivable 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.

"A rocky object"

Some experts said the planet could be a smaller version of gaseous planets such as Neptune, with a deep atmosphere and little chance for life to take hold. But models of planetary formation suggest that's unlikely, said Bruce Jakosky, a planetary scientist at the University of Colorado.

"We would lean pretty strongly toward the idea that this has to be a rocky object," said Jakosky, who specializes in astrobiology, the study of how life might arise on other planets.

As with nearly all such planetary discoveries, astronomers could not observe Gliese 581c directly because it is invisible in the glare of its sun. Instead, the European group led by Udry and Michael Mayor calculated the planet's presence from its tiny gravitational tug on the star.

Until the last few years, the only planets found in this way were huge gas giants, similar to Jupiter or even larger. None approached Earth's relatively small size.

To find Gliese 581c, Mayor's group used a cutting-edge planet-seeking instrument at an observatory in La Silla, Chile. The telescope has a device called a spectrograph that can detect small shifts in a star's light as it moves in relation to Earth. When the star edges toward Earth, its light shifts to the blue end of the spectrum; when the star moves away, the light undergoes a red shift.

In the case of Gliese 581c, the planet's pull on the star produced minute shifts in light. The star's movement was as slow as 2 meters per second — about the speed of a person walking.

"We're to the point where we can see if stars are strolling toward us or strolling away from us," said Steve Vogt, a professor of astronomy and astrophysics at the University of California, Santa Cruz.

The results of the discovery have not been published but have been submitted to the journal Astronomy and Astrophysics.

Beyond listening for radio transmissions, learning more about this planet and any others similar to it will be difficult. It may be decades or longer before astronomers learn how to cancel out the glare of stars and observe far-off planets directly. And although Gliese 581c is our close neighbor in galactic terms, it would take thousands of years for the fastest modern spacecraft to reach it.

Compiled from reports by the Chicago Tribune, The Associated Press and the Los Angeles Times