Newly discovered planets don't behave much like Sol's

Large orbiting bodies are closer to their stars, follow elliptical paths


AUSTIN, Texas -- In a stunning run of discoveries over the past three years, astronomers have observed 17 nearby stars that appear to be orbited by planets more or less the size of Jupiter.

The most recent detection of a planet around a star other than the Sun was described here yesterday.

But while astronomers continue the search for more such objects, they have now seen enough to be puzzled by an emerging pattern: None of these extrasolar planetary systems seems to resemble the Sun's family of planets.

Is this an observational fluke? Or is our solar system a cosmic oddball?

At a meeting of the American Astronomical Society, scientists reviewed the mystifying evidence. All of these objects are found in orbits much closer to their parent stars than Jupiter, the solar system's giant, is to the Sun.

Astrophysicists could only speculate how such large planets, ranging in mass from one-half to 10 times the mass of Jupiter, can orbit so close to a star and survive what must be destabilizing gravitational stresses.

Other studies reveal that many stars orbited by large bodies have two to three times more heavy elements than exist in the Sun.

And except for the extrasolar planets extremely close to their host stars, most of these objects appear to travel in orbits unlike those of planets around the Sun.

"For the first time, we have enough extrasolar planets out there to do some comparative study," said Dr. Geoffrey Marcy of San Francisco State University, the leading planet discoverer.

"We are realizing that most of the Jupiter-like planets far from their stars tool around in elliptical orbits, not circular orbits, which are the rule in our solar system."

Marcy noted that nine of the 17 detected extrasolar planets sweep relatively close to their stars and then swing far out again, giving an oval shape to their orbital paths.

The others travel in more circular orbits, presumably because they are even closer to their stars and are regulated by gravitational tides. One of these is only 4 million miles from its star and takes only 4.2 days to complete its revolution, or "year."

An elliptical orbit was observed for the most recently discovered extrasolar planet, around the star HD 168443 in the constellation Serpens, the Snake.

This new planet, Marcy reported Friday, orbits its star once every 58 days at an average distance that is nearer to the star than Mercury, with an 88-day orbit, is to the Sun.

Theoreticians have proposed several possible explanations for the eccentric orbits. When enough large planets orbit a star in proximity, perhaps they generate a gravitational slingshot that projects the planets into elongated orbits. Or perhaps a passing star has upset the delicate balance of these planetary systems.

In some cases, the planets could be perturbed because their star is part of a binary system, where two stars are locked in gravitational embrace.

If elliptical orbits appear to be common for other planets as massive as Jupiter, astronomers are wondering what different circumstances in the solar system keep mighty Jupiter and Saturn in circular orbits.

It is not a trivial question, for the emergence of life on Earth probably depended on the answer.

Pub Date: 1/10/99

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