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New technique finds farthest known planet

Researchers say they have spotted a planet thousands of light-years away by watching how it dims the light from the star it orbits.
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Researchers say they have spotted a planet thousands of light-years away by watching how it dims the light from the star it orbits. The technique could eventually be used to check millions of stars for the presence of Earthlike planets, a member of the research team said Monday.

“WE BELIEVE the door has been wide open to go and discover a new Earth,” Dimitar Sasselov, an astronomer at the Harvard-Smithsonian Center for Astrophysics, said at this week’s American Astronomical Society meeting in Seattle.

In the past decade, astronomers have detected 100 planets orbiting stars other than our own sun, primarily by analyzing the faint signs of a wobble in the stars’ motion that is caused by the planets’ gravitational pull. Increasingly powerful telescopes have made such discoveries almost routine. But the latest finding is notable for two reasons:

The researchers used an innovative technique to identify the planet, which involves looking for the faint, regular dimming of light as a planet makes its transit across the disk of the parent star. The transit method was used in 1999 to confirm the existence of a world called HD 209458b, but this time, scientists took the reverse route: They started out with the transit data and confirmed the initial discovery using the more traditional technique, known as the radial velocity method.

The newly identified planet, known as OGLE-TR-56b, is an amazing 5,000 light-years from Earth, easily setting a new distance record. “The new planet is 30 times farther away than the other extrasolar planets,” Sasselov said. In fact, it’s the first planet located outside the Orion spiral arm, our celestial neighborhood. OGLE-TR-56b is in the Sagittarius arm, an adjacent spiral arm that’s closer to the Milky Way galaxy’s center.

InsertArt(1747751)When the research team analyzed the gravitational wobble and the rise and fall of light levels, they determined that the planet’s minimum mass would be about 0.9 times that of Jupiter, and its diameter would be about 2.6 times Jupiter’s — which would mean the planet is a gas giant similar in density to Saturn, the team said. It appears to orbit its parent star every 29 hours, at an average distance of only 2.1 million miles.

That means the planet’s surface temperature would be about 3,100 degrees Fahrenheit (2,000 degrees Kelvin), Sasselov said. “It’s so hot, it rains iron there — unlike Seattle,” he joked.

The formal findings are to be published later this month in the journal Nature. Lead author of the paper is Maciej Konacki of the California Institute of Technology, and in addition to Sasselov, the research team includes Guillermo Torres, also of Harvard, and Saurabh Jha of the University of California at Berkeley.


The discovery wasn’t just a case of stumbling upon a cosmic dimmer switch. In fact, the tale reads more like a detective story. The researchers started out with 59 likely suspects — faint stars that a survey project called the Optical Gravitational Lensing Experiment identified as promising because they regularly dimmed and brightened. The scientists checked spectroscopic readings of the stars, using telescopes at the Whipple Observatory in Arizona and the Las Campanas Observatory in Chile. It turned out that, in most cases, a binary-star system rather than a transiting planet was responsible for causing the changes in brightness.

After eliminating those cases, five suspects were left. The remaining stars were examined using a spectrometer at the Keck Observatory in Hawaii. It took just four nights’ worth of observations to confirm that OGLE-TR-56 was a single star orbited by a planet, Sasselov said.

“Our success depended on efficiently eliminating binary stars using smaller telescopes,” Konacki said in a statement. “The remaining planetary candidates were then confirmed using the largest optical telescope in the world, the 10-meter Keck I telescope in Hawaii. Our time on Keck was critical to achieving this discovery.”


The results were well-received by others familiar with the field of planet-hunting. Melissa McGrath, a researcher at the Baltimore-based Space Telescope Science Institute, was impressed.

“Wow — look where we are: Just a little over 10 years ago, we didn’t know any more than a few planets very nearby in our own solar system ... and [now] we stand on the verge of hopefully soon being able to find planets like our Earth, and find out if there are planets like our Earth out there,” she said at Monday’s news briefing.

The transit method could be used to expand the search field from the current 40,000 stars to the 100 million stars or more within a 5,000-light-year radius, the research team said. What’s more, the process of spotting planets in that huge database could become much more efficient if candidates could be first screened using the transit method, then confirmed using the traditional radial velocity method.

If the measuring instruments become fine enough, astronomers could even watch for the extremely faint dimming of a star caused by much smaller planets, even one as small as Earth. But the effect of an Earth-size transit would be far more subtle - less than 1 percent of the effect observed in the case of the newly discovered planet. Sasselov acknowledged that such a feat was beyond the capability of present-day telescopes.

“This is exactly why you need to go to space,” he said. “You can’t do this from the ground, because of the atmosphere.” The Kepler spacecraft, a NASA probe due to be launched in 2007, could detect Earth-type worlds by making several years’ worth of observations, he said. Sasselov is a member of the science team for the $286 million Kepler mission.

Such observations could touch off a second wave of extrasolar planet discoveries, Sasselov said.

“Compared to the first one, it’s going to be a tsunami,” he said.