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Astronomers Discover Nearby Solar System

Science: The fourth-nearest star to our sun appears to be orbited by a large planet and possibly a smaller one.

June 11, 1996|K.C. COLE | TIMES SCIENCE WRITER

Astronomers report that they have discovered a solar system far closer to our sun than any of the previous half-dozen planet discoveries. Moreover, the new planetary system has features strikingly similar to our own.

Evidence presented today at the American Astronomical Society meeting in Madison, Wis., suggests that the fourth-nearest star to our sun has a Jupiter-sized companion orbiting at about the distance of Saturn--and possibly a smaller companion at the distance of the asteroid belt.

If confirmed, the new planet--just over eight light-years from Earth in the direction of the Big Dipper--adds another piece of evidence confirming centuries of speculation that planets are quite common and that astronomers don't have to look much beyond their own backyards to find them. Some of these planets, astronomers hope, might harbor the potential for life.

"It's the beginning of a whole new field," said George Gatewood of the Allegheny Observatory in Pittsburgh, who discovered the planet orbiting a fast-moving star called Lelande 21185. "We've just lifted up the corner of the first page of the book."

After decades of false hopes and dashed promises, astronomers have discovered a string of oddball planetary systems since fall 1995. However, until now, all have had bizarre features: One planet orbited its parent star in just four days; others were much larger than Jupiter--the giant of our solar system--and orbited much faster (and therefore much closer) to their suns. The closest of the previously discovered planets is at least 40 light-years away.

Gatewood's planetary system, in contrast, looks much more like our own. The planet weighs in with Jupiter's mass and appears to circle its star at a similar distance. Both possible planets also orbit in the same pancake-flat plane favored by Earth's planetary companions.

Traveling at light speed (an impossibility), one could make the nearly 50-trillion-mile journey to Lelande 21185 in just over eight years. Prospects for life are dim, however, because all large planets previously discovered are mostly gas, lacking any terra firma. In addition, the star itself is too faint to generate enough heat to support life.

The latest discovery was not the result of space-age technology as much as persistence and patience. The Pittsburgh observatory had been keeping track of Lelande 21185 for more than decades with a 30-inch telescope--small by modern standards.

By plotting the star's course through the heavens, astronomers were able to discern a small wobble in its motion--rather like a car weaving in and out of traffic lanes. Instead of being steered, however, the star was being pulled side to side by the gravitational influence of the planet.

Only recently did the telescope get new optics that allowed Gatewood to see the star with 10 times the previous precision. "That's what gave us the nerve to announce this," he said.

Gatewood has a reputation for careful observing and, in fact, proved several purported planet sightings in the 1960s to be false. "We're the ones who went around and debunked everyone else's planet," he said. Caltech astronomer Charles Beichman vouched for Gatewood's caution. "He's a very careful worker," Beichman said. "It's really easy when you're excited and wanting to find something to think you've got it [when you really don't]."

All previous confirmed planet discoveries were based on a method that analyzes the spectrum of a star's light, searching for telltale stretching and compressing of light waves that mean the star is slowing down or speeding up because of the gravitational tug of an invisible companion.

Lelande's companion, in contrast, was the first seen by a more direct method that tracks the star's motion across the sky, said UC Santa Barbara astronomer Stanton Peale. This method allows astronomers to see the star "actually wobbling," Beichman said. The rub is that seeing a wobble takes a long time.

One wobble, or wave, in the star's path marks one complete orbit of the invisible companion, so tracking such a system can take decades. Gatewood's planet, for example, probably has an orbit of about 35 years. He says he is not entirely sure if he has seen a complete orbit yet or not. The second planet, if confirmed, would take somewhat longer to make a complete turn around the star.

Occasionally, what astronomers think is a planet turns out to be another star; companion stars cause wobbles very similar to those caused by planets. But Lelande 21185 has already been observed closely enough to determine that it has no stellar companion.

The fact that Earth-size planets have not been found doesn't mean they don't exist, astronomers say, merely that current technology isn't powerful enough to see them.

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