The new planet is a ball of rock coated in frozen oceans roughly five times the size of Earth. It is more than 20 thousand light-years away, impossibly out of reach for foreseeablehuman travel. But the manner of its detection suggests the universe could be full of mid-sized terrestrial planets not too different from our own.
"We think these planets are a lot more common," said David Bennett, an astrophysicist at Notre Dame. "We're opening a new window."
"It says in a sense that they are the tip of the iceberg, that it's likely that there are many more rocky planets," said Princeton planetary scientist Scott Tremaine.
In the last decade, discoveries of planets beyond the solar system exploded to more than 170, and the list grows almost every month. But most planet hunters use techniques best suited for big game, so overwhelmingly these new planets are Jupiter-class giants made of gas.
Scientists recently have found a few rocky planets, including one last year just seven times the size of Earth, but all are very close to their stars and so extremely hot. The planetary discovery announced Wednesday and reported in today's edition of the journal Nature is the smallest and coldest yet.
Its sun is a red dwarf near the center of the Milky Way, and the planet is more than double Earth's distance from its sun, so that it receives just a thousandth of its sun's light and, scientists calculate, has a temperature of minus 370 degrees.
"It is a new kind of beast," said Harvard University astronomer Dimitar Sasselov. "It's really coming close to the Holy Grail for us planet hunters, which is to find an earth analog. That's why everyone's so excited about it."
The planet, dubbed OGLE-2005-BLG-390Lb, is one of the first discovered by astronomers using a technique that is uniquely sensitive for detecting smaller planets. Albert Einstein realized in 1936 that stars and other cosmic bodies carried enough gravitational attraction to bend light. In recent decades, scientists have drawn on his idea, using stars as magnifying lenses to peer farther into space and study the surfaces of other stars or infer the presence of unseen objects in the heavens.
By scanning millions of stars every night, an international team of scientists is finding about 500 to 600 of these so-called "microlensing events" every year, when two stars align perfectly as seen from Earth. Ten years ago, other international teams turned the technique to hunting for planets. Scientists swivel their telescopes to watch about 50 of the lensing events a year, in hopes of seeing a tiny extra brightening that would betray the existence of a planet around the lens star.
The first two planets they found were close to Jupiter's mass. The third, spotted by an astronomer in Chile, was BLG-390Lb. The planet's gravity produced the telltale surge of light for only 10 hours, but enough for astronomers elsewhere to confirm the discovery.
Scientists say it is 50 times harder to detect a planet of that size than one of Jupiter class, suggesting that either the discovery was extraordinary luck or rocky, middle-orbit planets are more numerous than gaseous giants.
Kem Cook, astrophysics team leader at Lawrence Livermore National Laboratory and part of the team that discovered the planet, is betting that there are thousands more rocky planets to be found.
"The fact that we found one rocky, icy planet out of three found by microlensing suggests that they are quite common, that they are more common than the large gas giants, because the gas giants are just easier to find," said Cook.
Microlensing only finds planets thousands of light years away because the physics of the bending of starlight is subtle and can only be appreciated at a distance. But next year, NASA plans to launch the Kepler mission to search for Earth-like planets nearby. It will use another method that scans thousands of stars for the slight dimming that occurs when a planet passes in front of the star.
"If I were a senior administrator at NASA, I would be heaving a sigh of relief, because it means or suggests that when they go out and look for those planets that they actually will be there," said Princeton's Tremaine.
But scientists say the microlensing technique could find many more planets if more and larger telescopes worldwide were looking all night long, rather than "an underfunded but determined, rag-tag group of people" borrowing time on a few telescopes, as Livermore's Cook puts it.
Andrew Gould, an astronomy professor at Ohio State University, agrees. "Then we could find dozens or hundreds of these planets."
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