Distant world could support life

Extrasolar planet has right conditions to be habitable

Scientists have spotted an Earth doppelgänger that may have the right specs to harbor life, in the Libra constellation just 20 light-years distant.

Although details about conditions on the planet’s surface remain a mystery, the find suggests that many more potentially habitable planets are likely to be found. The discovery is reported online September 29 at arXiv.org and will be described in an upcoming Astrophysical Journal.

Finding the planet so nearby and so soon after beginning the hunt for Earthlike worlds beyond the solar system suggests that the galaxy is teeming with them, said coauthor Paul Butler of the Carnegie Institution for Science in Washington, D.C. He noted that it has been only 15 years since astronomers first discovered any planets orbiting sunlike stars beyond the solar system.

“This is the first one, but the threshold has now been crossed,” Butler said in a September 29 press briefing. “Over the next 10 years I would be shocked if there weren’t many tens of these things.” The planet is one of six known to be orbiting the red dwarf star Gliese 581 — the current record for most planets orbiting a star other than the sun.

“It’s kind of a mini-version of our own solar system,” said study coauthor Steven Vogt of the University of California, Santa Cruz.
Vogt and his colleagues used more than 200 nights’ worth of data from a telescope at the W.M. Keck Observatory in Hawaii to track tiny wobbles of Gliese 581 caused by the gravitational tug of orbiting planets. Astronomers can use these wobbles to determine the masses and orbital paths of those unseen planets.

The researchers found that the planet Gliese 581g, estimated to be about three times more massive than Earth, orbits its sun about once every 37 days. The average surface temperature ranges from -24 degrees to 10 degrees Fahrenheit, but may vary greatly.

Just as the moon always keeps the same face toward Earth, one side of the planet always faces its sun. But the planet’s temperature range may not be as extreme as the moon’s, where the difference between the dark and illuminated sides can be 278 degrees Celsius, or more than 500 degrees Fahrenheit. Modeling experiments suggest that winds could help distribute heat on Gliese 581g, moderating extreme temperatures. “It would be quite a benign, comfortable place to live,” Vogt said, and one with “a lot of different niches for different kinds of life to evolve stably.”

The result is “fantastic news,” said astrophysicist Franck Selsis of the Laboratoire d’Astrophysique de Bordeaux in France. Selsis and his colleagues have proposed that two other Gliese-orbiting planets might be on the verge of being habitable; this newly discovered planet is right in between those candidates.  

A surface temperature that allows liquid water is considered a prerequisite for life. “At this point, we can’t say anything about the physical conditions on the planet,” Butler said. “We can’t say anything for sure about the atmosphere. We can’t say anything sure about water.”

Vogt points out that water is abundant in the galaxy. “It’s pretty hard to imagine that water wouldn’t be there,” he said. “Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say that my own personal feeling is that the chances of life on this planet are 100 percent,” Vogt said. “I have almost no doubt about it.”

But Selsis cautions that being in the all-important “habitability zone” where temperatures are right for life as we know it is “necessary, but certainly not sufficient.” The planet could have formed without any water. It could be regularly bombarded with objects capable of wiping out any life that develops. Or it could have such a heavy atmosphere that its surface would be extremely hot.

Astrophysicist and planetary scientist Sara Seager of MIT said that the authors don’t yet know if the greenhouse effect would be too strong on Gliese 581g for life to survive. For astronomers to get accurate atmospheric data, the planet would have to pass between its sun and Earth, which Gliese 581g doesn’t do. Another option would be to launch equipment into space to image the planet directly, but such a mission would take more than 200 years.

Still, the discovery is monumental, Seager said. “We definitely expected it, but that’s not the same thing as having it.” Like the first bird that arrives in spring, the planet is a harbinger of more, she said. “This is the first of many to come, so we get excited.”

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From Gliese 581g, you could see the Earth’s sun, about 20 light-years distant, and many of the stars (blue labels) and constellations (orange labels) visible from Earth. Credit:  D. Nash, www.astronexus.com, adapted by T. Dubé

What would it be like to live on Gliese 581g?
Eternal sunshine  The rocky planet is tidally locked to its star, so one side is perpetually sunny and the other is always dark and cold. There wouldn’t be time-dependent seasons, and a person sitting at a given longitude would experience the same temperature and weather patterns year-round.
Weighed down  The gravity of the planet is estimated to be slightly stronger than that of Earth. A 130-pound Earth woman would weigh between 143 and 221 pounds on Gliese 581g.
Big orange ball  Gliese 581g’s parent star would appear about twice as large as the sun appears to an Earth observer, and, depending on the atmosphere, the star might appear orange. The quality of the light on the surface of Gliese 581g, however, would probably appear whitish.
Visible planets  The three inner planets in the Gliese system would be visible with the naked eye from the surface, and they would have phases, similar to the progression of the moon cycle as seen from Earth.
Howdy neighbor  A person on Gliese 581g could see Earth’s sun with the naked eye. But red dwarfs don’t burn as brightly, so Earthlings must use a telescope to see Gliese 581.
Time to burn  Red dwarfs can persist for hundreds of billions of years — a life span much longer than other types of stars, including the sun. That means that any life on Gliese 581g will have a very long time to evolve.

ANOTHER WORLD Gliese 581 (upper left in this artist’s depiction) has six confirmed planets, including one (foreground) that orbits the star at a distance hospitable to life. Lynette Cook

LOOKS LIKE HOME This schematic diagram shows the Gliese 581 planetary system superimposed on the solar system. All but one of Gliese 581’s planets circle the star in orbits smaller than Mercury’s. But because Gliese 581 is substantially dimmer than the sun, planet g is at the right distance to have liquid water at its surface, a necessary requisite for life. Zina Deretsky, National Science Foundation

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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