Using a research technique called gravitation microlensing, the team determined that the newfound planet weighs about 13 times Earth's mass, probably comprises a mixture of rock and ice, and has a diameter several times bigger than Earth's. It orbits its star at the distance of the asteroid belt between Mars and Jupiter, or about 250 million miles (390 kilometers) from the Sun.

The planet's distant orbit chills it to about -330 degrees Fahrenheit (-200 degree Celsius), suggesting it is similar in structure to the Earth but too cold for liquid water or life. The research team said the giant icy planet probably never accumulated enough gas to grow into a Jupiter. Instead, its primordial disk of material likely dissipated, starving it of raw materials.

"This is a solar system that ran out of gas," said astronomer Scott Gaudi of the Harvard-Smithsonian Center for Astrophysics and a member of the MicroFUN collaboration that spotted the planet.

Gaudi performed the data analysis that confirmed the planet's existence. The same analysis also ruled out the presence of any Jupiter-sized world in the super-Earth's solar system.

Andrew Gould of Ohio State University in Columbus and leader of MicroFUN, said the discovery contains two key implications. "First," Gould said, "this icy super-Earth dominates the region around its star that in our solar system is populated by the gas-giant planets, Jupiter and Saturn. We've never seen a system like this before, because we've never had the means to find them."

Second, he added, "these icy super-Earths are pretty common - roughly 35 percent of all stars have them."

Gravitational microlensing is an effect predicted by Albert Einstein. It occurs when a massive object such as a star crosses in front of another star shining in the background. The foreground object's strong gravity bends the light rays from the more distant star and magnifies them like a lens. Astronomers see the magnified star grow brighter as the lens star crosses in front of it, and then fade as the lens gets farther away.

If the foreground star possesses a planet, the planet's gravity can distort the light further, thereby signaling its presence. The precise alignment required for the effect means each microlensing event lasts for only a brief time, so astronomers must monitor many stars closely to detect such events.

Microlensing is sensitive to less massive planets than the more common planet-finding methods of radial velocity and transit searches - also known as "wink and wobble."

"Microlensing is the only way to detect Earth-mass planets from the ground with current technology," Gaudi said. "If there had been an Earth-mass planet in the same region as this super-Earth, and if the alignment had been just right, we could have detected it. By adding one more two-meter telescope to our arsenal, we may be able to find up to a dozen Earth-mass planets every year."

The Optical Gravitational Lensing Experiment collaboration initially discovered the microlensed star in April 2005 while peering in the direction of the galactic center, where both foreground and background stars are widespread.

The discovery was shared by 36 astronomers, including members of the MicroFUN, OGLE and Robonet collaborations. The name of the planet is OGLE-2005-BLG-169Lb, which refers to the 169th microlensing event discovered by the OGLE Collaboration toward the galactic bulge in 2005. The suffix "Lb" refers to a planetary mass companion to the lens star.

Piecing together their observations, Gould and OGLE leader Andrzej Udalski of Warsaw University Observatory in Poland suddenly realized on May 1 the star was brightening extremely quickly, meaning it would provide exceptionally fertile ground for planet hunting.

"It was 4:00 a.m.," Gould said. "I was very excited and frantic to get someone to observe that star."

Gould called the MDM observatory at Kitt Peak in Arizona, Ohio State graduate student Deokkeun An was on duty. Gould asked An to spare a few minutes during his night's work to occasionally measure the star's brightness, but when An and his co-observer Ai-ying Zhou of Missouri State University heard how intense the signal was, they put aside their own project to take more than 1,000 measurements of the event.

"It's a good thing that they did," Gould said. "Their observations turned out to be critical to our determination that there was a planet."

An said the microlensing provided "a good chance to take many images of the event, to erase any doubt as to whether this was a planet signal. Since the target could only be seen through the telescope during a short time window, we did not hesitate to follow it."

Astronomers in New Zealand and Hawaii also observed the microlensing event, which turned out to produce a tiny warping in the signal could not have been caused by a planet. So OSU graduate student Subo Dong designed special software to help the MicroFUN computer model weed out the other possibilities.

With the new software, the team was able to confirm the presence of a Neptune-mass planet, about 13 times heavier than Earth, orbiting a star about half as big as the Sun.

"This icy super-Earth dominates the region around its star that, in our solar system, is populated by the gas giant planets," Gould said.

The team also calculated that about one-third of all main sequence stars may have similar icy super-Earths. Theory predicts that smaller planets should be easier to form than larger ones around low-mass stars. New research by Charles Lada of CfA has shown that most Milky Way stars are red dwarfs, so solar systems dominated by super-Earths may be more common in the Galaxy than those with Jupiters.

"Our discovery suggests that different types of solar systems form around different types of stars," Gaudi explained. "Sun-like stars form Jupiters, while red dwarf stars only form super-Earths. Larger A-type stars may even form brown dwarfs in their disks."

The discovery could lead to new understanding about the process of solar-system formation. Material orbiting a low-mass star accumulates into planets gradually, leaving more time for the gas in the protoplanetary disk to dissipate before large planets have formed. Low-mass stars also tend to have less massive disks, offering fewer raw materials for planet formation.

Until a decade ago, astronomers had no evidence of what other solar systems were like. Since then, however, they have discovered some 170 extrasolar planets, and most of them have been gas giants similar to Jupiter.

So far, only a handful of Neptune-mass planets have been detected - only two of them in the cold outer regions of their solar systems.

"The next step is to push the sensitivity of our detection methods down to reach Earth-mass planets," Gould said, "and microlensing is the best way to get there."

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