European astronomers have found what could be the first habitable planet outside our solar system, a sphere a bit bigger than Earth covered by rocks or oceans, 20.5 light-years away.
Researchers aren't sure whether the planet has oxygen, carbon or other essential building blocks of life. But it orbits at the right distance from its star to make conditions ripe for an essential ingredient to life as we know it.
"The temperature is right to have water," said Stephane Udry, an astronomer at the Geneva Observatory in Switzerland and lead author of the report published today as a letter to the editor in the journal Astronomy and Astrophysics.
Experts say the discovery represents a breakthrough because even if life is not found on the planet, astronomers will soon find others like it.
"It's the dawn of the era of finding planets that are habitable," said Sara Seager, a planetary scientist at the Massachusetts Institute of Technology.
So little is known about the planet that it is difficult to speculate about the possibility of life there, scientists say. In fact, the search for extrasolar planets is a relatively new field for astronomers: The first weren't discovered until the 1990s.
Since then, more than 200 have been detected, but until now, none has been anything like Earth. Almost all are gas giants the size of Jupiter, but orbiting so close to their stars that their surfaces are as hot as skillets.
Seager cautioned that details about the size and temperature of the planet are based on computer models generated from data collected by observations of the star the planet orbits - and not by observing the planet itself. She put the odds of its being habitable at 50-50.
"Before you get too excited, you should call it 'the first potentially habitable planet,'" she said.
The planet is designated as Gliese 581c, based on the red dwarf star in the constellation Libra that it orbits every 13 days. The planet's radius is about 50 percent greater than the Earth's, and its mass is five times greater.
The distance from Gliese 581c to the star it orbits is about 6.6 million miles. Earth, by comparison, is 93 million miles from the sun.
Because the red dwarf is dimmer and cooler than our sun, researchers estimate that temperatures on the planet range from 32 degrees to 104 degrees Fahrenheit. They estimate that the planet's gravity is about twice that on Earth.
That means your weight would double there - and that many creatures probably would be "stockier" than our life forms, said Frank Drake, director of the Search for Extra Terrestrial Life Institute's Center for the Study of Life in the Universe.
"Birds would have a harder time flying. They'd need larger wings," Drake said. "But fish would be largely unaffected."
"You wouldn't want to fall down in that gravity," said Marc Kuchner, who studies distant planets at the NASA Goddard Space Flight Center in Greenbelt. "Snakes would be OK."
The discovery of a potentially habitable planet orbiting a red dwarf is encouraging because scientists weren't sure whether the so-called "M Class" stars had orbiting planets.
"Because this planet was found orbiting a red dwarf, a common type of star, it increases the likelihood that there are other planets like it elsewhere in the universe," Drake said.
The planet's strong gravity probably gives it a slight advantage in fostering life, because that makes it more likely to retain water molecules and life-forming elements such as oxygen and carbon, said Dirk Schulze-Makuch, an astrobiologist at Washington State University. Mars, for example, lost most of its atmosphere in its youth because its small size meant less gravitational force, so that many life-forming molecules were dispersed into space.
But having an atmosphere doesn't necessarily mean a habitable planet, he said. Venus has a carbon dioxide atmosphere that produces such pronounced greenhouse effects that its surface is far too hot for life.
"This is a great discovery, a really great discovery," he said. "It has a potential for life to be there. But more has to be known for us to determine if life even exists there and what kind of life it may be."
The planet's proximity to its star also means that it could be tidally locked, keeping the same face toward it at all times. That would diminish - but not rule out - the chance of life.
"We've found that life finds a way to make it in some pretty extreme conditions," said Nancy Y. Kiang, a biometeorologist who has studied the potential color schemes of extraterrestrial plants at NASA's Goddard Institute for Space Studies.
With light from a red dwarf star, plants on Gliese 581c would have dark pigments that probably would look black or very dark to the human eye, Kiang said.
To harbor life, the planet would have to be a certain age. Photosynthesis started things brewing on Earth about 3.8 billion years ago, almost a billion years after our planet was formed, Kiang said.
Like most of the extrasolar planets found so far, the newly discovered planet was detected by measuring the radial velocity, or "wobbling," of light from the star that it orbits.
"The only light we get is from the star, not from the planet itself. We measure the planet from its pull on the star," said Thierry Forveille, a co-author from the Grenoble Observatory in France.
Orbiting instruments such as the Hubble Space Telescope are unrivaled at peering deep into the universe and reading ultraviolet light from distant galaxies, experts say. In recent years, the Hubble Telescope has confirmed theories about planet formation, showing that discs of matter cast off by stars are the building blocks of planets, said Steve Maran, a retired astronomer from the Goddard Space Flight Center and the author of Astronomy for Dummies.
But to detect planets, astronomers have been working largely with ground-based telescopes that feed as much light as possible into spectrographs, which spread light into color patterns that can track movements created by the gravitational pull on stars from the planets that orbit them.
For its discovery, the European team used a spectrograph that read light collected over three years by the European Southern Observatory's 3.6-meter telescope in La Silla, Chile. Two years ago, the instrument found a Neptune-size planet orbiting the same star every five days.
The habitable planet was detected during follow-up observations of the star, Udry said.
"It was in the same direction as the models had predicted," Udry said. "It means that there are more of these small planets out there. We just have to go and find them."
The European team's HARPS spectrograph had been planet-hunting for 100 nights a year for the past three years, researchers said.
"It shows how much the Europeans have caught up and, some would say, taken the lead in observational astronomy," Maran said.
A U.S. team made a similar discovery two years ago using the Keck Observatory in Hawaii, but the planet was larger and had temperatures around 700 degrees Fahrenheit, too hot to be habitable.
At Keck and other observatories, U.S. planet hunters must compete for observation time with scientists working on other projects.
"There's a lot of things happening in astronomy, and everybody wants a piece of the pie," Seager said.