Astronomers say they've found what may be the smallest and most distant planet known to be orbiting a star outside our own solar system.
The work suggests that such small rocky or icy planets may be more common in the cosmos than Jupiter-sized gas giant planets, researchers said. The discovery also indicates the power of a relatively new method of finding such "exoplanets."
All of the exoplanets discovered so far around distant stars are larger than Earth. The newly found planet is about 5.5 times the mass of Earth, making it much smaller than most of the 160 previous exoplanet discoveries.
It appears to be less massive than another small planet found recently, but because of uncertainties in estimating mass, that isn't known for certain, the scientists said.
International discovery
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The finding was reported in the journal Nature by a team of researchers from 12 countries.
Similarly, at a distance of more than 20,000 light years from Earth, it is probably the most distant such planet yet found, said study co-author David Bennett of the University of Notre Dame. But distance estimates are too uncertain to be highly confident of that, he said.
In any case, the planet appears to be much too cold to sustain life, probably reaching no more than minus 360 degrees, the researchers said. It orbits its star about 2.6 times the distance between Earth and the sun.
The planet lies in the constellation Sagittarius, close to the center of the Milky Way.
Nearly all the known exoplanets have been detected by their gravitational tug on the stars they orbit, which makes the stars wobble. The new planet is the third to be uncovered by a different technique, microlensing, which uses the fact that a celestial body's gravity bends light like a cosmic glass lens.
The planet was named OGLE-2005-BLG-390Lb. "OGLE" is the group of astronomers who monitor stars for microlensing.
