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Vol. 38, No. 18 |
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2-19-2008 Finding suggests we are not unique
An international team of astronomers has discovered two planets that look like smaller versions of Jupiter and Saturn in a solar system nearly 5,000 light years away. The find suggests our galaxy may host many star systems like our own, said Scott Gaudi, assistant professor of astronomy at Ohio State University. He and his colleagues reported their results in the Feb. 15 issue of the journal Science. The two planets were revealed when the star they orbit crossed in front of a more distant star, as seen from Earth. For two weeks from late March through early April of 2006, the nearer star magnified the light shining from the farther star. The phenomenon is called gravitational microlensing, and this was a dramatic example: The light from the more distant star was magnified 500 times.
The Optical Gravitational Lensing Experiment (OGLE) first detected the event on March 28, 2006. The Microlensing Follow Up Network (MicroFUN), led by Andrew Gould, professor of astronomy at Ohio State, then joined with OGLE to organize astronomers worldwide to gather observations of it (see sidebar, “Worldwide Effort”). Gaudi took the lead in analyzing the data as they came in. As he studied the light signal, he saw a distortion he thought was caused by a Saturn-mass planet. Then, less than a day later, came an additional distortion he wasn’t expecting: A “blip” in the signal that appeared to be caused by a second, larger planet orbiting the same star. Over the next few months, Gaudi showed this two-planet interpretation was correct. Then David Bennett, a research associate professor of astrophysics and cosmology at the University of Notre Dame, refined Gaudi’s preliminary model using sophisticated software to reveal more details about the system. This is the third time a Jupiter-mass planet has been found by microlensing, Gaudi said. In the previous two cases, additional planets would have been difficult to detect, had they been there. “This is the first time we had a high-enough magnification event where we had significant sensitivity to a second planet — and we found one,” Gaudi said. “You could call it luck, but I think it might just mean that these systems are common throughout our galaxy.” Astronomers have found two planets at once before, “but using other techniques that don’t pick up on solar systems like ours,” he said. The newly discovered planets appear to be gaseous planets like Jupiter and Saturn — only about 80 percent as big — and they orbit a star about half the size of the sun. The star is dim and cold compared to ours, issuing only 5 percent as much light. Still, the new solar system appears to be a smaller version of our own. The larger planet is about as massive compared to its star as Jupiter is to ours. The smaller planet shares a similar mass ratio with Saturn. Also, the smaller planet is roughly twice as far from its star as the larger one, just as Saturn is roughly twice as far away from the sun as Jupiter. Although the star is much dimmer than our sun, temperatures at both planets are likely to be similar to that of Jupiter and Saturn because they are closer to their star. “The temperatures are important because they dictate the amount of material that is available for planet formation,” Gaudi said. “Theorists have wondered whether gas giants in other solar systems would form in the same way as ours did. This system seems to answer in the affirmative.” The fact that astronomers found the planets during the first event that allowed such a detection suggests that these scaled-down versions of our solar system are common, he added. Ohio State coauthors on the Science paper included Darren DePoy and Richard Pogge, both professors of astronomy; and Subo Dong and Stephan Frank, both graduate students. This work was sponsored by the National Science Foundation and NASA, among others.
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