Astronomers say they've found what may be the smallest and most distant planet known to be orbiting a star outside our solar system.
The find suggests that such small rocky or icy planets may be more common in the cosmos than Jupiter-sized gas giant planets, researchers reported this week in the journal Nature.
All such planets, known as exoplanets, discovered around distant stars have been larger than Earth. This one is about 5.5 times the mass of Earth -- much smaller than most of the 160 previous exoplanet discoveries.
The planet is probably too cold to sustain life, reaching no more than minus 364 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, about 20,000 light years from Earth, said study coauthor David Bennett of the University of Notre Dame.
Nearly all of the known exoplanets have been detected by their gravitational tugs on the stars they orbit, which makes the stars wobble. The new planet is the third to be uncovered by a technique that uses a celestial body's gravity to bend light like a cosmic glass lens.
If a planet and its star pass between Earth and a more distant star, this effect, called microlensing, gives a temporary but telltale boost to the brightness of the more distant star. This technique opens the door to finding relatively small planets with masses and orbits similar to Earth's, researchers said.
The astronomers named the planet OGLE-2005-BLG-390Lb. OGLE is the group of astronomers that monitors stars for evidence of microlensing.