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Carbon copy? The Gliese 581 solar system resembles our own but on a much smaller scale.

Zina Deretsky/National Science Foundation

Astronomers Find Most Earth-like Planet to Date

Survivalists take note. Astronomers may have found the most Earth-like alien planet to date, and it's located only a short distance away, cosmically speaking. The team says that the planet's proximity to its sun, coupled with the ease with which it was detected, suggests that the galaxy could be teeming with habitable worlds.

Most of the 500 or so planets astronomers have found orbiting other stars have fallen into the gas-giant class: very large worlds, some much bigger than Jupiter, that can't support life, because they lack solid surfaces, and because they orbit either too far from or much too close to their suns. The few rocky worlds discovered so far also orbit too near or far from stars to fall into what planetary scientists call the "habitable zone," in which liquid water—and therefore life—could exist.

But Gliese 581g looks like a game-changer. Detected from the minuscule amount of gravitational influence it exerts on its star, the planet lives a mere 20 light-years away in the constellation Libra. Gliese 581g is the sixth world discovered around its sun—and the fourth most distant. Yet its orbit brings it closer to its parent star than Mercury is to our sun. Still, it's squarely within the habitable zone, because the planet's star, which is a type known as a red dwarf, contains only about 30% of the sun's mass and shines with only about 1% of its brightness, the researchers will report in an upcoming issue of the Astrophysical Journal.

That said, if Gliese 581g harbors any kind of life, the creatures would inhabit a world quite different from our own. For one thing, explains astronomer and co-author Steven Vogt of the University of California, Santa Cruz, Gliese 581g is "tidally locked" to its star. Just like our moon, the length of Gliese 581g's day precisely matches the length of its year. Consequently, the planet rotates so that it always shows the same face to its sun. Another difference is that the star casts Gliese 581g in a soft, red light that would give the appearance of a perpetual sunset.

On the other hand, Vogt told reporters during an online briefing today, the perpetual days or nights would mean that the planet could develop highly stable habitats. Although the most comfortable areas for humans would lie along the so-called terminator—the zone between the world's dayside and nightside—other creatures with preferences for hotter or colder temperatures could find tolerable conditions elsewhere on the surface.

There's another advantage, says astronomer and co-author Paul Butler of the Carnegie Institution for Science in Washington, D.C. Because red-dwarf stars are essentially immortal—their life spans are much longer than our sun's—life could have many more chances of gaining a foothold on Gliese 581g. The key is whether liquid water exists on the surface, Butler explained at the briefing. The planet is about 3.1 times more massive than Earth—bulky enough to hold onto an atmosphere and to keep liquid water from evaporating into space. And "on Earth, anywhere you find liquid water you have life," Butler says.

But whether Gliese 581g harbors life may not be the most striking aspect of this find. Vogt says the discovery of a potentially habitable world less than 100 light-years away means that habitable worlds may be much more common than astronomers thought. Given the number of stars in the Milky Way, Vogt explains, that could mean there are "potentially billions" of Earth-like worlds out there.

Planetary scientist Jack Lissauer of NASA's Ames Research Center at Moffett Field, California, agrees. "It's a big breakthrough. Gliese 581g could be a big, rocky planet with oceans on its surface warmed by the faint but nearby red-dwarf star," he says. Or it could be a "mini-Neptune, hosting a massive atmosphere above a surface that is far too hot for life as we know it." But however it turns out, the discovery promises "even more exciting news in the near future."

This article has been corrected. Gliese 581g is the sixth planet discovered around its star, not the seventh.