Earth-Like Planet Found Outside Solar System

By Guy Gugliotta
Washington Post Staff Writer
Thursday, January 26, 2006

Scientists using a technique derived from Albert Einstein's theories said yesterday they have detected the most Earth-like planet ever discovered outside the solar system -- an icy, rocky world 5 1/2 times the size of Earth orbiting a star 28,000 light-years away.

The new planet, given the identifier OGLE-2005-BLG-390Lb, was discovered last summer by teams of researchers using telescopes throughout the world. The scientists said the discovery suggested Earth-size planets are quite common in the universe.

"We're opening a new window," said University of Notre Dame astrophysicist David P. Bennett in a webcast news conference hosted by the National Science Foundation, a major sponsor of the research. In the future, he added, "we will be able to detect planets down to Earth mass."

In the past decade, scientists have found more than 160 extra-solar planets -- planets circling stars outside our solar system. Most of the discoveries were made indirectly, by measuring the "wobble" in a star's motion caused by the gravitational pull of its planet.

To make a star wobble, the planet has to be very large -- many are several times the size of Jupiter -- or very close to the star, as Mercury is to the sun. None of the planets discovered by this method are believed capable of sustaining life.

The current discovery, reported in today's issue of the journal Nature in a paper signed by 73 researchers, took advantage of a relatively new technique called gravitational microlensing, based on Einstein's precept that light from a distant star is bent by the gravitational pull of intervening stars, or other large bodies, on its way toward Earth.

The "lensing incident" distorts the incoming starlight, causing it to brighten for about a month. If the intervening star has an orbiting planet -- even a small one -- it will cause a supplemental "blip" in brightness. To analyze these planets, astronomers must observe the microlensing event continuously for as long as it lasts.

On July 11, the Optical Gravitational Lensing Experiment search team, which surveys millions of stars, detected a microlensing event in the constellation Sagittarius, prompting two telescope networks -- the Probing Lensing Anomalies Network, or Planet, and RoboNet -- to begin observations.

On Aug. 10, astronomers at La Silla Observatory in Chile saw what they had been looking for: "This small brightening was . . . our blip in the night," said Nature lead author Jean-Philippe Beaulieu of Planet and the Institut d'Astrophysique in Paris.

Bennett, another Planet researcher, added that there will be more small planets to come. Microlensing has so far detected three exoplanets -- two giants and OGLE-2005-BLG-390Lb -- and "higher-mass planets are more easy to discover," he said. So if scientists have already found a small one, "these must be substantially more common."

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