Automated Astronomy Group

New planet around a distant star has implications for finding other earths

(June 30, 2005, Nashville, TN)--An international team of researchers, including an astronomer at Tennessee State University in Nashville, has discovered a new planet that periodically blocks light from its host star and so reveals some of the secrets of its composition. Its structure implies that it formed in a manner similar to the way astronomers theorize the Earth formed in the early days of our Solar System but with a strange twist. The details of the discovery are being published in the Astrophysical Journal.

"While more than 150 extrasolar planets have been found by observing the wobbles they cause in the stars they orbit, it is difficult to glean direct information about the structure of the planets," said Debra Fischer, a professor of astronomy at San Francisco State University who organized the international team of astronomers searching for new planets. "In rare cases like this one, the planet periodically passes in front of its star and dims the starlight, an event called a planetary transit," said Gregory Henry, an astronomer at Tennessee State University who discovered the transits of the new planet. "When that happens," said Henry, "we can calculate the physical size of the planet from the amount of dimming of the starlight and determine whether or not the planet has a solid core and also learn something about the planetary atmosphere."

The planet, orbiting the sun-like star HD 149026, takes just 2.87 days to circle around its star, and the temperature in the upper atmosphere of the planet is a whopping 2000 degrees F. Wobbles of the star implying the existence of the planet were first detected with the Japanese 8.2 m Subaru telescope on the island of Hawaii and confirmed with the 10 m Keck telescope also on Hawaii. The dimming of the starlight caused by the orbiting planet was subsequently discovered with several 0.8 m robotic telescopes operated by Tennessee State University at Fairborn Observatory in the Patagonia Mountains of southern Arizona. "The planet has about the same mass as Saturn," said Fischer, "but the small amount of dimming during the transit reveals it has a significantly smaller diameter." Modeling of the planet's structure by Peter Bodenheimer at the University of California Santa Cruz shows that the planet has a heavy core that is about 67 times the mass of the Earth. "For theorists, the discovery of a planet with such a large core is as important as the discovery of the first extrasolar planet around the star 51 Pegasi," said Shigeru Ida, a theorist from the Tokyo Institute of Technology. "We all expected a much larger planet, similar to Jupiter. None of our planetary formation models predicted that Nature could make a planet like this," said Bun'ei Sato, a postdoctoral fellow at Okayama Astrophysical Observatory in Japan. "We sure didn't expect anything like this," said Fischer.

There are two competing theories for planet formation. In the first, planets form when a dense cloud collapses. In the second, planets start as small rock-ice cores that grow as they gravitationally acquire additional mass. Without observational evidence, either theory is viable. However, the large core of the planet around HD 149026 could not have formed by cloud collapse; it must have grown a core first then acquired additional gas. "The confirmation of core accretion as a mechanism for planetary formation gives us the first "solid" evidence that terrestrial-type planets may exist in large numbers in other planetary systems," said Henry.

"This discovery is similar to an anthropologist's find of ancient hominid fossils. It distinguishes which formation path planets take," said Greg Laughlin, a professor at UC Santa Cruz. "Our team members have bounced some pretty wild hypotheses back and forth across the Pacific until we hit on several formation mechanisms that might just work."

In one of the most dramatic but, nevertheless, plausible scenarios, the planet formed from the collision and merger of two conventional protoplanets, each about 35 times the mass of the Earth. If the collision hypothesis is correct, then the orbit will likely be tilted with respect to the rotation axis of the star, leading to measurable changes in the stellar spectrum during transits. Observations are currently planned at Keck, Subaru, and Fairborn to test whether this collision hypothesis is likely to be correct.

The research was supported by the National Aeronautics and Space Administration, the National Science Foundation, and the Research Corporation.