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Planet hunters find an Earth-mass planet and a potential water world

ESO/L. Calçada

(Read caption) Artist's rendering of the newly discovered planet Gliese 581e orbiting its M-dwarf "sun." The small blue dot is another planet, Gliese 581d, which sits in the star's habitable zone (see second image for a diagram). Astronomers speculate that it could be a "water world."

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One star, its fourth planet, and a lot of buzz.

That's what's happening as planet-hunters digest the news that European astronomers have detected planet No. 4 orbiting a star with the imaginative moniker: Gliese 581. The new planet has 1.9 times Earth's mass, making it the tiniest exoplanet astronomers have bagged to date.

The golden Easter egg in this hunt, of course, is the elusive Earthlike planet orbiting a star in its habitable zone. There, it's not too hot and not too cold, but just right for liquid water to gather and persist on the planet's surface.

To spot a planet this small, and use an Earth-based telescope to do it, is a big deal.

"This is really the most exciting new discovery in the field of exoplanets," enthused Debra Fischer, an astronomer at San Francisco State University and an active planet hunter, in an e-mail exchange.  It's an impressive achievement, she continues, in no small part because the planet's tell-tale signature is tiny compared with larger planets astronomers have found using the same detection technique.

Looking for stars that wobble

The approach detects the presence of a planet through the tiny tug its gravity imparts on its host star. This appears as a periodic wobble in the star's spectrum. The tool in this case was a sensitive planet-hunting spectrograph called HARPS bolted to the back end of the European Southern Observatory's 3.6-meter telescope at La Silla, Chile.

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