HABITABLE? A planet about five times as heavy as Earth orbits the red dwarf star Gliese 581, in this artist's depiction. On the orb's surface, water would probably be liquid.ESO

It's not quite like home, but astronomers this week announced that they had found Earth's closest known analog outside the solar system. The newly discovered planet, which orbits a neighbor of the sun, is about five times as heavy as Earth and probably has an average surface temperature at which water would be liquid.

The planet, which is too small to be imaged, completes one orbit of its star in just 13 days. The distance between this planet and its star is only one-fourteenth the distance between Earth and the sun. But Stephane Udry of the Geneva Observatory in Sauverny, Switzerland, and his colleagues calculate that because the planet's star is less massive and cooler than the sun, the planet's close-in location puts it in the "Goldilocks zone"—neither too hot nor too cold, but just right for liquid water.

This is "arguably the first habitable planet" ever identified, says theorist Alan Boss of the Carnegie Institution of Washington, D.C.

Planet-formation models indicate that a body of this mass would have a solid surface, perhaps rocky or watery, and a diameter 1.5 times that of Earth, Udry's team notes.

The planet's parent star, the red dwarf Gliese 581, resides just 20.5 light-years from Earth and is one-third as heavy as the sun. Dwarf stars are the most common type of star in the galaxy, and the discovery suggests new places to search for life, Udry says.

A decade ago, researchers had ruled out dwarf stars as hosts of habitable worlds, notes astronomer Jill Tarter of the SETI Institute in Mountain View, Calif. One problem, theorists argued, is that a close-in planet occupying the habitable zone around a dwarf would be vulnerable to outbursts of radiation from the star. But a reappraisal now suggests that this problem and others may not be deal breakers and that planets around dwarf stars could indeed support life, Tarter and other researchers report in the spring issue of Astrobiology.

Udry's team discovered the Gliese 581 planet indirectly by using the standard technique of searching for back-and-forth motions of the parent star. From a star's tiny wobble, astronomers can infer the minimum mass and orbit of an unseen planet tugging on it. Among the more than 225 extrasolar planets now known, most were discovered using this method.

In their initial study of Gliese 581, Udry and his colleagues found a closer-in planet about 15 times as massive as Earth. Those measurements, obtained with a sensitive spectrometer on the European Southern Observatory's 3.6-meter telescope in La Silla, Chile, showed signs of another planet.

Follow-up studies have now revealed the new "super-Earth" as well as indications of a third, more distant planet, Udry's team reports.

Without direct measurements of size and mass, it's difficult to know "if the planet is indeed rocky or icy, or whether it has a significant gas atmosphere," cautions theorist Sara Seager of the Massachusetts Institute of Technology. For example, if the planet's atmosphere were thicker than that of Venus, "the surface would likely be too hot for liquid water," she notes.

That's why Seager is looking forward to the discovery of a super-Earth that periodically passes between its star and Earth, blocking some of the starlight. These mini-eclipses, she notes, would reveal the planet's density and composition, indicating its potential to support life.

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