WASHINGTON — Astronomers have discovered what may be the smallest alien planet yet — a rocky ``SuperEarth'' only four times heavier than our home planet.
It's orbiting a small star at a distance that puts it in the so-called ``habitable zone'' — a region neither too hot nor too cold for liquid water and therefore suitable for possible life.
Scientists believe such Earth-like planets are the best hope for detecting evidence of living organisms beyond our solar system.
The tentative finding, which has yet to be confirmed, was reported during the May meeting of the International Astronomical Union in Cambridge, Mass.
The object is one of 45 potential new planets in the Milky Way galaxy recently discovered by the European Southern Observatory's 140-inch telescope perched on a mountaintop at La Silla, Chile.
"The mass of the planets and the sheer number of them represents a huge step towards finding planets of the Earth's mass, and ones that might be suitable for life as we know it," Sara Seager, an astronomer at the Massachusetts Institute of Technology in Cambridge, said in an e-mail interview. "What amazes me is that these planets may be very, very common."
The European astronomers used the HARPS (High Accuracy Radial Velocity Planet Searcher) system, which measures tiny wobbles in a star's motion as a planet swings around it.
If the findings are confirmed, the 45 objects would make up the largest single haul of extra-solar planets yet reported. Previously, astronomers have confirmed the existence of 293 so-called ``exoplanets'' orbiting other stars. Most of them have been huge gas giants like Jupiter, plus a handful of Saturn or Neptune-sized objects.
(Jupiter is 318 times more massive than Earth, Saturn 95 times and Neptune 17 times.)
Confirmation of some of the 45 candidate planets is expected at a conference on Super-Earths in Nantes, France, in mid-June, according to Stephane Udry, a member of the HARPS team in Geneva, Switzerland. Others may require another year of observation before their status is clear, Udry said.
"There's no doubt that the majority of them will turn out to be real," Christophe Lovis, an astronomer at the University of Geneva, told the British journal, NewScientist.
The smallest rocky planet previously confirmed is five times more massive than Earth. It was discovered by the HARPS survey in April 2007.
The candidate planets found in the latest survey are 4 to 30 times bigger than Earth. Some of them travel in highly eccentric orbits, ranging very close to their star and very far away, which probably makes life impossible. The smallest planet, however, revolves in a circular orbit in the habitable zone, where liquid water, a necessity for life, is possible.
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See a catalog of known exoplanets.