Contact: Robert Irion (408) 459-2495; irion@ua.ucsc.edu

ASTROPHYSICISTS CREATE MODEL FOR THE BIRTH AND STRANGE LIFE OF THE FIRST NEW PLANET FOUND ORBITING AN ORDINARY STAR

* This news release is EMBARGOED until 6 p.m. EDT Wednesday, April 17, 1996. The authors will publish their model in the April 18 issue of the journal Nature.

SANTA CRUZ, CA--Delight zipped around planet Earth last October when astronomers discovered signs of a new planet in the cosmos, orbiting a star much like our own Sun. But fantasies about life on this distant body went up in flames when the details became clear: The planet is so close to its star that temperatures on its sunny side soar to about 2,000 degrees.

Indeed, the planet's existence seemed so marginal that it strained credulity. Its size--about as big as Jupiter--implies that it is mostly made of gas, yet its blast-furnace orbit surely would have boiled off such an atmosphere. Further, astronomers had thought that large planets develop only in colder parts of the gaseous disks that swathe baby stars. A few suggested the evidence might point to something else entirely--remnants of a brown dwarf, or even vibrations of the star itself.

To paraphrase former vice presidential candidate James Stockdale, the key questions about the planet were: What is it? How did it get there? Now, three researchers at the University of California, Santa Cruz, and the University of Toronto think they have the answers.

In a paper in the April 18 issue of the journal Nature, the team proposes that the planet, called 51 Pegasi B, is indeed a gas giant that coalesced at a more reasonable distance from its star, perhaps 100 times further away than it is today. Then, in a million-year gravitational tug-of-war among the star, the planet, and gas and dust in the rest of the disk, the planet spiraled slowly but relentlessly toward the star. Finally, inward and outward forces on the planet's orbit canceled each other out just before the star would have consumed the planet.

"We have said for years that planets can migrate toward their stars early in a solar system's history," says lead author Douglas N. C. Lin, professor of astronomy and astrophysics at UCSC. "The new part of the theory is that this migration could stop. It is possible that 51 Pegasi B is the last of a series of planets, all but one of which have perished in their parent star."

Lin's coauthors are Peter Bodenheimer, a professor of astronomy and astrophysics at UCSC, and Derek Richardson, a researcher at the Canadian Institute for Theoretical Astrophysics, University of Toronto.

51 Pegasi B whirls around its star once every four days at a distance just one-twentieth that of Earth from our Sun. The team cites two reasons that a large planet could not have formed at that spot. First, gas giants require massive cores of ice and rock, but temperatures close to the star were too hot even for small particles of ice and rock to exist. Second, if a planet had started to take shape, it would have evaporated rapidly in the intense conditions.

The team's calculations reveal that a mature and compact gas giant would _not_ evaporate in such an orbit, however. An infant planet is bloated and has a weak gravitational field at its surface, but a mature planet has much stronger self-gravity. Even under the onslaught of scorching heat and a fierce solar wind, 51 Pegasi B could have kept most of its atmosphere for billions of years, the researchers maintain.

The real test lay in figuring out how to move the planet toward its star and how to "park it," in Lin's words, so tantalizingly close to its demise. The first part, it turns out, is relatively simple. When a giant planet forms, it sweeps up all of the particles and gas in its neighborhood, clearing a "gap" in the disk around the star. Physical laws of gravity and the conservation of angular momentum then take over. In essence, the planet tugs on the material in the disk between it and the star, slowing down the inner part of the disk and making it spiral inward. At the same time, the planet loses momentum to material in the outer part of the disk, forcing the planet inward as well. The pace of this migration depends on the size of the planet and the amount of gas and dust in the disk.

"The migration of a young planet actually can occur in either direction," Bodenheimer notes. "Whether the planet moves inward or outward depends on the distribution of mass within the solar nebula. With 51 Pegasi B, the conditions were right for the planet to migrate inward."

To stop the planet just a few million miles out, the team invokes circumstances that seem special but, Lin feels, are plausible. Again, gravity is the key player, this time in the guise of tidal forces between planet and star. Just as our Moon raises tides on the Earth, 51 Pegasi B raises tides on the surface of its star. And, just as tides in the Earth-Moon system have the curious effect of nudging the Moon to greater distances (a few inches every year), tides on the rapidly spinning young star may have imparted an outward kick to 51 Pegasi B. This, the model states, was just enough to counteract the inward migration of the planet--and to hold it in place since then.

If the young star was not spinning quickly enough, tides would not have done the trick. So, the team offers a second alternative. Strong magnetic fields may have cleared gas and dust from the innermost part of the disk, much as an eggbeater hollows out the middle of a bowl of whipped cream. Once the planet crept into this clear zone, the model states, the disk no longer would have compelled the planet to continue spiraling inward.

Other gas giants may have arisen with 51 Pegasi B, Lin says. But if they were closer to their parent star at birth, the inward forces--stubbornly strong until the gaseous disk begins to dissipate--would have forced them into a fiery plunge.

"It's as if you have several people in a row standing on a slippery slope," Lin says. "People in front get pushed off by those behind them until there's only one left. The privileged position is last in line."

Regardless of whether their model is correct, Lin and his colleagues believe astronomers should gear up to try to comprehend a puzzling panoply of planets and orbits. Lin is leading an effort to model the histories of two more planets (around the stars 70 Virginis and 47 Ursae Majoris) announced in January by San Francisco State University and UC Berkeley astronomers Geoffrey Marcy and Paul Butler. Already, it is evident that the models will be radically different than the one for 51 Pegasi B.

"When you look at our solar system, the conditions from Earth to Jupiter to Neptune are very different," Lin says. "Nature, from star to star, may have created even greater differences. Diversity among planets and planetary systems appears to be the rule, rather than the exception."

Editor's note: Call (408) 459-2495 for a copy of the paper. You may reach the authors as follows: Douglas Lin: (408) 459-2732 or lin@ucolick.org Peter Bodenheimer: (408) 459-2064 or peter@ucolick.org Derek Richardson: (416) 978-6879 or richards@cita.utoronto.ca

This release is also available on the World Wide Web at UCSC's "Services for Journalists" site (http://www.ucsc.edu/news/journalist.html) or via modem from UC NewsWire (209/244-6971).

Press Releases Home | Search Press Releases | Press Release Archive | Services for Journalists

Maintained by:pioweb@cats.ucsc.edu