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Physicists see violent beginnings for recently discovered planets

The dozen or so new planets discovered within the past year probably had violent beginnings, mainly because they were born in solar systems with two or more massive planets the size of Jupiter, according to MIT astrophysicists.

"The properties of these new planets are completely different from those of the planets in our own solar system," said Frederic A. Rasio, an assistant professor in the Department of Physics. Professor Rasio and Eric B. Ford, a sophomore in physics, have suggested that the properties of the new planets, all of which are themselves Jupiter-sized, may be the result of instabilities that developed when they formed. These instabilities, in turn, were caused by the planets' close proximity to one or more other Jupiter-sized planets in their respective planetary systems.

One implication of the work "is that a system like our solar system, which has one dominant massive planet (Jupiter) and is very stable over long time scales (several billions of years), may be very rare," Professor Rasio said. "The only reason why we happen to be in such a rare system is that long-term stability may be necessary for the development of intelligent life."

The work, which involves super-computer simulations to try to determine how the new planets formed, was reported in the November 8 issue of Science.

Professor Rasio said a new era in astronomy began about a year ago when Swiss astronomers detected a planet in orbit around a star named 51 Pegasi. The announcement marked the first confirmed discovery of a planet in orbit around another Sun-like star. Since then, more than 10 other detections have been reported by several groups around the world. The new planets are all Jupiter-sized objects in orbit around Sun-like stars, but they have very unusual orbits that have surprised astronomers and left theorists to puzzle over how they might have formed.

In current physical models for planetary system formation, the minimum distance at which a giant gaseous planet like Jupiter can form around a star like the Sun is thought to be several astronomical units (an astronomical unit, or AU, is the mean radius of the Earth's orbit around the Sun). The minimum distance for the formation of a rocky planet like Earth or Mars is a few tenths of an AU, comparable to Mercury's distance from the Sun.

The models also say that all planets should be on nearly circular orbits, which is the case in our solar system. But it is not the case with the new planets, Professor Rasio said. Most of the new planets have masses comparable to that of Jupiter, but they are orbiting at distances much smaller than 1 AU from the central star and some have very eccentric (i.e., noncircular) orbits.

The computer simulations by Professor Rasio and Mr. Ford suggest that this is because, in many cases, an orbital instability develops when two or more Jupiter-sized planets are formed in the same system. This leads to a strong gravitational interaction between two of the planets, resulting in the ejection of one while the other is left in a smaller, eccentric orbit.

Physical collisions between the planets also can occur as a result of the instability, leading to mergers and to a build-up of the mass of the planet that eventually remains in the system.

The physicists also report in Science that "other, less massive planets [like Earth or Mars] that may have formed in the same system are likely to be lost as a result of the. instability." When they included inner terrestrial planets in their simulations, Professor Rasio and Mr. Ford found that "in all cases, large eccentricities are induced in the orbits of these. planets, eventually causing them to escape from the system or to collide with the central star."

Professor Rasio said the implications of this work are that many-perhaps most-planetary systems that form around other stars may go through a type of dynamical evolution that is far more violent than was ever imagined for our own solar system.

In addition, he said, the long-term stability of our solar system may be a result of the presence of one single dominant planet, Jupiter, and that stability may be a necessary condition for the development of intelligent life.

"If it really turns out that Earth is in fact in a very unusual kind of planetary system, then it is likely that the new planetary systems, which we are now discovering in rapidly increasing numbers, do not contain intelligent life," Professor Rasio said.

He added that he and Mr. Ford are continuing their examination of the consequences of dynamical interactions in newly formed planetary systems. They plan to perform about 10,000 more numerical calculations over the next year.

The supercomputer simulations were done at the Cornell Theory Center, which receives major funding from the NSF and the State of New York, with additional support from the Defense Advanced Research Projects Agency (DARPA), the National Center for Research Resources at the NIH, IBM Corp., and other members of the Center's Corporate Partnership Program.

A version of this article appeared in MIT Tech Talk on November 13, 1996.

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