One of Us?

The 228th time was a charm.

Since 1992, when astronomers discovered the first planet outside our solar system, the hunt has been on for one that orbits its star not too closely and not too distantly, one that is not too hot and not too cold—one that is, in short, like Earth. Now astronomers, using the 141-inch telescope at the European Southern Observatory in La Silla, Chile, have hit pay dirt: the most Earth-like planet ever detected beyond the solar system. The discovery immediately boosted the odds that humans are not alone in the universe. London bookies duly took note, lowering the odds on extraterrestrial life from 1,000-to-1 to 100-to-1.

The planet, named Gliese 581c because it orbits the star Gliese 581, has a radius just 50 percent larger than the Earth, a mass five times greater, orbits its star in 13 days and—this is the crucial part—had the foresight to pick a star that has just the right characteristics to make its retinue of planets, well, interesting. The planet orbits at a distance 14 times closer to Gliese 581 than Earth is from our sun. But Gliese 581 is a red dwarf, a class of star that is smaller and colder than our sun—50 times fainter than our sun, to be precise. That means that although the planet orbits a mere 7 million miles away—which ordinarily would be in a zone so torrid that liquid water could not exist and any organic molecules brave enough to form (or land from a passing comet or meteor) would be instantly fried and vaporized—it nevertheless lies in the “habitable zone,” say astronomers.

“We have estimated that the mean temperature of this super-Earth lies between zero and 40 degrees Celsius, and water would thus be liquid,” said Stephane Udry of the Geneva Observatory, one of the discoverers. Models of planetary formation predict that a planet of this size and orbital distance “should be either rocky, like our Earth, or covered with oceans,” he said.

That makes Gliese 581c more intriguing, from an ET point of view, than the other gaseous giants among the exoplanets. “Because of its temperature and relative proximity,” said astronomer Xavier Delfosse of France’s Grenoble University, who was on the discovery team, “this planet will most probably be a very important target of the future space missions dedicated to the search for extraterrestrial life. On the treasure map of the universe, one would be tempted to mark this planet with an X.”

Astronomers not associated with the discovery are withholding judgment on whether Gliese 581c harbors liquid water—more observations will be needed to confirm that-but the basics are not in dispute. Planet c was discovered using the technique that has detected the vast majority of exoplanets, namely, wobble. The gravitational pull of even a puny object like Gliese 581c causes its host star to wobble a bit in its own orbit. The technique also allows astronomers to infer the time it takes the (invisible) planet to orbit the star, as well as its orbital distance and mass. Gliese 581 is among the 100 closest stars to Earth, 20.5 light-years away in the constellation Libra. That means it is in the zone that has received radio transmissions from Earth up through late 1986 (sorry about the big-hair bands, neighbors). Gliese 581 has two other planets, one 15 times the mass of Earth and the other eight times our mass.

The ESO astronomers plan to keep an eye on Gliese 581c, watching for it to pass in front of its star in such a way that will allow them to infer its size more precisely and, knowing its mass, its density. That will suggest what it’s made of. The observation of such a “transit” can also reveal if the planet has an atmosphere, and if so what it’s made of and how heavy it is. A crushing atmosphere such as that on Venus would make liquid water, at least any on the planet’s surface, impossible. Whatever the details of Gliese 581c, the odds on finding planets that are not just hospitable to life, but where organic molecules have taken advantage of that hospitality, are now a whole lot more interesting.

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