From: Richard A. Kerr, ScienceNOW
Published April 22, 2009 08:08 AM

Closing In on the Next Earth

A group of astronomers today announced the discovery of the least massive planet yet detected outside of our solar system. It is lightweight enough--between two and three times the mass of Earth--to almost certainly be rocky like Earth rather than a huge ball of gas. Although the planet orbits too close to its star to be habitable, a new analysis of one of its neighbors suggests a world with deep oceans. All this raises the prospect of discovering an Earth-size planet orbiting at just the right distance from its star to give life a chance. "It's just a matter of time now," says exoplanet specialist Sara Seager of the Massachusetts Institute of Technology in Cambridge.

Planet hunters are an ambitious lot. They're after an Earth-sized, rocky planet orbiting at a comfortable, livable distance from its star. For ease of observation, the exoplanet should periodically pass between the star and astronomers on Earth. They've already bagged huge planets orbiting in a star's habitable zone--where any water would be liquid--and huge planets transiting in front of their stars. They've found small planets too: CoRoT-Exo-7b, whose discovery was announced in February, is twice the diameter of Earth but likely eight times its mass--and far from habitable.


What makes the new world so noteworthy is its small mass. At the Joint European and National Astronomy Meeting being held this week in Hatfield, U.K., a 13-member group led by planet hunter Michel Mayor of the Observatory of Geneva in Switzerland announced that Gliese 581 e, located only 20.5 light-years away in the constellation Libra, is so small that it isn't smothered with gas the way Jupiter or Neptune is.

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