Astronomers Find Planet Closer to Size of Earth

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http://www.nytimes.com/2009/04/22/science/22planet.html


European astronomers said Tuesday that they had discovered the smallest planet yet found orbiting another star. The planet could be as little as only 1.9 times as massive as the Earth and belongs to a dim red star known as Gliese 581, which lies about 20 light-years from Earth in the constellation Libra.


The star was already know to harbor at least three more massive planets. The new planet, known as Gliese 581e, is probably rocky like the Earth, but it lies in such a close orbit — only three million miles from its star — that it is surely blasted with too much radiation and heat to be livable.

Michel Mayor, of Geneva Observatory, and his colleagues announced their results at a conference at the University of Hertfordshire in Britain and in a paper submitted to the journal Astronomy and Astrophysics.

Astronomers said the discovery was more encouragement that the galaxy was full of small-mass planets and that with more time and improved instruments like the Kepler satellite, recently launched by NASA, they would eventually find Earth-like planets in orbits suitable for life around other stars.

“Finding Earth-like planets with lukewarm temperatures is the next great goal,” Geoff Marcy, of the University of California, Berkeley, a planet-hunting rival of Dr. Mayor’s, said in an e-mail message.

“This is the most exciting discovery in exoplanets so far,” Dr. Marcy said.

Dr. Mayor’s group also discovered the first exoplanet, a gas giant 160 times the mass of the Earth, in 1995, using a technique known popularly as the “wiggle” method that detects planets by a slight gravitational tug they give their stars. The method is most sensitive to massive planets in close orbits. In a statement, Dr. Mayor noted that the new planet is only one-eightieth of the mass of the first one, saying, “This is tremendous progress in 14 years.”

The discovery also cements the Gliese system as one of the most promising exoplanet systems. Two years ago, the third planet from that star was hailed as a ”Goldilocks” planet, where liquid water and thus life might be possible, until calculations showed that the greenhouse effect would broil it.

But the new data also shifted the orbit of the star’s outermost planet inward so that it now appears to revolve in the so-called habitable zone where liquid water is possible, according to Stéphane Udry of Geneva University, one of the team members.

That planet, 581d, is about seven times as massive as the Earth, Dr. Udry explained, which is too big to be just rock. It probably formed as a combination of ice and rock farther out in the Gliese system and then migrated inward, according to various planetary formation models, and melted. He called it the first serious “water world candidate.”

Sara Seager, a planet theorist at the Massachusetts Institute of Technology, said in an e-mail message that the Gliese planetary system “is like the gift that keeps on giving.”