|This artists rendering released by European Southern Observatory, shows the planetary system around the red dwarf Gliese 581|
European astronomers announced they have discovered the smallest and potentially most Earth-like planet yet outside the solar system, according to media reports Wednesday.
Fives times as massive as Earth, it orbits a relatively cool star at a distance that would provide earthly temperatures as well, signaling the possibility of liquid water and, potentially, life.
"The separation between the planet and its star is just right for having liquid water at its surface," said astronomer and team spokesperson Stephane Udry of the Observatory of Geneva in Versoix, Switzerland. "That's why we are a bit excited."
But researchers do not yet know if the planet contains water, if it is truly rocky like Earth, which might make it hospitable to life as we know it, or whether it is blanketed by a thick atmosphere.
The researchers said they detected the presence of two new extrasolar planets (exoplanets) around a red dwarf star, Gliese 581, 20.5 light-years away in the constellation Libra, based on slight motions of the star. Their discovery brings the total number of planets orbiting Gliese 581 to three; two years ago they made the initial finding of a planet there.
They believed that many smallish exoplanets exist, but so far they have only found 13 "super Earths" weighing in at less than 20 Earth masses, compared with more than 200 heavier gaseous planets. Udry's group searched for smaller planets using a telescope called HARPS (High Accuracy Radial velocity Planet Searcher), which looks for stars that wobble slightly.
The smaller of the new planets, dubbed Gliese 581 c, orbits at one fourteenth the distance between Earth and the sun. But the red dwarf is 50 times cooler than the sun. The group estimates that the planet would experience temperatures in the zero-to-40-degree-Celsius (32 to 104 Fahrenheit) range.
The big question is whether there really is water on Gliese 581 c's surface, which requires that its surface be solid. Udry said planets smaller than 15 Earth masses are likely to be rocky or icy.
David Charbonneau, astronomer of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., is more cautious. A five-Earth-mass planet "sort of looks like Earth, but it sort of looks like Neptune. So which is it?" he said. "There's just no way to know."