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Astronomers Find Smallest Planet Yet Outside Our Solar System

Astronomers report finding the smallest, possibly most Earth-like planet ever detected around a star outside our solar system. The planet is bigger than Earth, but the scientists say their technique is sensitive enough to identify many more planets about the size of our own.

Since the mid-1990s, astronomers have found more than 170 planets orbiting stars outside our solar system. But the latest one found in the center of our galaxy is different and gives scientists reason to believe there could be many Earths out there.

So far, the vast majority of planets found around normal stars are gaseous giants like Saturn and Jupiter. Several presumably rocky planets the size of Earth have been found, but they orbit dead, collapsed stars called neutron stars. Until now, only one rocky planet has been found around a normal star, but it is seven-and-a-half times Earth's mass. Furthermore, all the newly discovered planets orbit too close to their stars to harbor life.

The newest planet to be identified outside our solar system is a closer fit to Earth. The 73 scientists in 10 countries who tracked it estimate it to be just 5 1/2 times Earth's mass and farther away from its star than the others -- 2 1/2 half times the distance from Earth to the Sun.

Co-discoverer David Bennett of the University of Notre Dame in Indiana says this means it is beyond the habitable zone, registering minus 220 degrees Celsius on its surface. Still, he says it's a more appealing place than the other planets found sizzling outside our solar system.

"Basically we're claiming that we're opening a new window and so we're actually getting fairly close to Earth-like planets, although we are actually a bit more sensitive to planets that are a bit cooler than the Earth."

The discovery involved a novel search technique different from the one used to find most other planets. The older method did not see the planets, but inferred their existence by observing a wobble in the stars induced by the orbiting planet's gravity. This procedure tends to turn up the biggest, closest, and hottest planets probably unable to support life.

The new method used a quirk of nature called microlensing. In this technique, light from a distant star is magnified by the gravity of a nearby star, like the beam from a searchlight encountering a magnifying glass. If a planet orbits the background star, its gravity can magnify the brightness a bit more.

French astronomer Jean-Pierre Beaulieu of the Institute of Astrophysics in Paris and others looking at the star in question last July and August were surprised at an unexpected spike in its brightness.

"We were expecting the star to be a bit fainter than it was observed. So we just decided to take another measurement and then this second measurement was even brighter. Then you are very excited because it is exactly the kind of blip [spike] in the night that we have been looking for for a long time," he said.

The researchers says the advantage of microlensing is that it can detect low mass planets. Of course, this method can more easily observe big ones like Jupiter, but so far it has found only two. Bennett says if the super-sized planets were the most common in the universe, microlensing should have found many more. "The implication actually is that these planets of around five Earth masses are substantially more common than the Jupiter mass range, and if you extrapolate from that, then maybe Earth-mass planets would be even more common than these," he said.

Bennett and his colleagues, who report their finding in the journal "Nature," say microlensing is likely to turn up more low mass planets in the coming months.

At the U.S. agency that helped fund the research, the National Science Foundation, Michael Turner of the physical sciences directorate says the discovery is an important breakthrough in the effort to answer the question, "Are we alone in the universe?"

"Mankind has pondered this question for millennia. With the discovery of more than 170 extrasolar planets over the past 11 years, the scientific adventure of answering this question has begun," he said.