“The threshold has now been crossed,” said astronomer R. Paul Butler of the Carnegie Institution of Washington, one of the planet’s discoverers, in a press briefing September 29. “The data says this planet is at the right distance for liquid water, and the right mass to hold on to a substantial atmosphere.”

The discovery is both “incremental and monumental,” comments exoplanet expert Sara Seager of MIT, who was not involved in the new study. When a recent study predicted the first habitable world should show up by next May, Seager rightly said the real answer was more like “any day now.”

“We’ve found smaller and smaller planets that got closer and closer to the habitable zone,” she said. “But this is the first that’s in the habitable zone.”

The new planet is one of six orbiting the star Gliese 581, a red dwarf 20 light-years from Earth. Two of the planet’s siblings, dubbed planets c and d, have also been hailed as potentially habitable worlds. The two planets straddle the region around the star where liquid water could exist — 581c is too hot, and 581d is too cold. But 581g is just right. The discovery will be published in the Astrophysical Journal and online at arxiv.org.

The new planet is about 3 times the mass of Earth, which indicates it is probably rocky and has enough surface gravity to sustain a stable atmosphere. It orbits its star once every 36.6 days at a distance of just 13 million miles.

The surface of a planet that close to our sun would be scorching hot. But because the star Gliese 581 is only about 1 percent as bright as the sun, temperatures on the new planet should be much more comfortable. Taking into account the presence of an atmosphere and how much starlight the planet probably reflects, astronomers calculated the average temperature ranges from -24 to 10 degrees Fahrenheit.

But the actual temperature range is even wider, says astronomer Steven Vogt of the University of California, Santa Cruz, who designed some of the instruments that helped find the planet. Gravity dictates that such a close-in planet would keep the same side facing the star at all times, the same way the moon always shows the same face to Earth.

That means the planet has a blazing hot daytime side, a frigid nighttime side, and a band of eternal sunrise or sunset where water — and perhaps life — could subsist comfortably. Any life on this exotic world would be confined to this perpetual twilight zone, Vogt says, but there’s room for a lot of diversity.

“You can get any temperature you want on this planet, you just have to move around on its surface,” Vogt said. “There’s a great range of eco-longitudes that will create a lot of different niches for different kinds of life to evolve stably.”

Another advantage for potential life on Gliese 581g is that its star is “effectively immortal,” Butler said. “Our sun will go 10 billion years before it goes nova, and life here ceases to exist. But M dwarfs live for tens, hundreds of billions of years, many times the current age of the universe. So life has a long time to get a toehold.”

The discovery is based on 11 years of observations using the HIRES spectrometer at the Keck Telescope in Hawaii, combined with data from the HARPS (High Accuracy Radial velocity Planet Searcher) instrument at the La Silla Observatory in Chile.

Both instruments looks for the small wobbles stars make as their planets’ gravity tugs them back and forth. The HIRES project started looking for planets 25 years ago, back “when looking for planets made you look like a nut,” Butler said. At first the instruments could detect changes in a star’s velocity that were 300 meters per second or larger. That’s why the first extrasolar planets discovered were almost exclusively hot Jupiters — these monstrous planets that sit roastingly close to their stars exert a bigger gravitational pull.

Since then, techniques have improved so that changes as small as 3 meters per second can be seen. That wouldn’t be enough to see Earth from 20 light-years away, Butler says. Because red dwarfs are so small and their habitable zones so close, though, Earth-sized planets have enough gravitational oomph to make a difference.

“The excitement here is that by looking at stars that are small it’s much easier to find small planets,” said exoplanet expert David Charbonneau of Harvard, who is hunting for small planets that cross in front of dwarf stars. “I think it’s great news for those of us looking for this kind fo thing around this kind of star.”

But finding them takes a long time. In all, 238 measurements of the star’s wobbles, went into the discovery, and each measurement took a full night of observing.

For Butler and Vogt, though, 11 years wasn’t so long to wait. He’s actually surprised that a potentially habitable planet showed up so quickly and so nearby.

“The fact that we found one so close and so early on in the search suggests there’s a lot of these things,” Butler says. Only about 100 other stars are as close to Earth as Gliese 581, and only 9 of them have been closely examined for planets. Odds are good that 10 to 20 percent of stars in the Milky Way have habitable planets, Vogt says.

Finding them won’t take a huge advance in technology, he adds. It will just take more telescope time.

“I have suggested that we build a dedicated automated planet finder to do this kind of work 365 nights a year,” he said. “If we had something equivalent to Keck that we could use every night, these things would be pouring out of the sky.”

Image: 1) Lynette Cook. 2) The planetary orbits of the Gliese 581 system compared to those of our own solar system. Zina Deretsky, National Science Foundation.

See Also:

• First Habitable Exoplanet Could Be Discovered by May
• Potentially Habitable Planet Discovered Outside Our Solar System
• Astronomers Closer to Exoplanet ‘Holy Grail’
• Most Earth-Like Extrasolar Planet Found Right Next Door
• Smallest Exoplanet Is Most Earth-like Yet
• Name the New Super-Earth

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Authors: Lisa Grossman

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