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Solid core planet discovered

June 30, 2005

An artist's rendering of the hard core planet moving in front of star HD 149026, leaving a trail of its outer atmosphere as it orbitsIn the race to discover and study the formation of extra-solar planets, Debra Fischer's most recent find carries a lot of weight.

Scientists rely on two competing theories to study how planets are formed. In one, a planet forms when a dense cloud collapses. In the other, a planet starts as a small, rock-ice core that grows as it acquires additional mass through the pull of gravity.

"Without observational evidence, either theory is viable," said Fischer, assistant professor of astronomy at SFSU and leader of Next 2,000 Planets (N2K), a consortium of American, Japanese and Chilean astronomers formed last year to search for extra-solar planets. "But we believe that the large, rocky core of this planet couldn't have formed by cloud collapse. Instead we think it must have grown a core first, then acquired gas."

Modeling of the planet's structure by consortium member Peter Bodenheimer, from the Lick Observatory at University of California, Santa Cruz, showed that it has a solid core that is about 70 times the mass of Earth. Similar in size and mass to Saturn, the planet located 250 light years away from our solar system has the largest core of any extra-solar planet ever observed. The consortium's paper about their discovery was accepted for publication in Astrophysical Journal.

"None of the [previous] models predicted that nature could make a planet like this," said Bun'ei Sato, consortium member and a post-doctoral fellow at Okayama Astrophysical Observatory in Japan.

Since 1995, when the search for extra-solar planets began, more than 150 have been discovered by astronomers through the method of observing "wobbles." Wobbles are changes in the speed of a star as it moves to and away from Earth. The changes in speed are caused by the gravitational pull of the planets, with which the star orbits a common center of mass. While this is an excellent method for detection, it provides little information about the structure of a planet.

"But in extremely rare cases, like this one, the planet passes in front of its star and actually dims the starlight," Fischer said. "When that happens, we are able to calculate the physical size of the planet, whether it has a solid core, and even what the planet atmosphere is like."

"This discovery was similar to an anthropologist's find of ancient hominid fossils," consortium member Greg Laughlin, a theorist at UC Santa Cruz, said. "It distinguished which formation path planets take."

It also raised new questions about the core accretion theory of planet formation. Laughlin said that the consortium "bounced some pretty wild hypotheses back and forth across the Pacific until we hit on several formation scenarios that might work."

In one dramatic, but plausible scenario, the team theorized that the newly discovered planet could have formed from the collision and merger of two conventional protoplanets, each with a mass core 35 times that of the Earth's. After the collision, the resulting orbit would have been highly elongated, but over time, tidal forces would have brought it into its present configuration.

"If this hypothesis is correct then the orbit will likely be tilted with respect to the rotation axis of the star," Laughlin said.

This is a boon to scientists as it would allow them to measure changes in the spectrum of the star's light as the planet transits in front of it. The team plans follow-up observations to test whether the collision hypothesis is correct.

"We can now move from simply trying to find planets to truly understanding them as we learn about their interiors and atmospheres," consortium member Chris McCarthy, a post-doctoral fellow in astronomy and physics at SFSU, said. "Do these planets contain the building blocks of life?" he asked.

For more information visit the NASA Web site or the N2K consortium site.

-- Denize Springer
Image: Lynette Cook


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Last modified June 30, 2005 by University Communications