Astronomers' findings raise many questions

November 09, 1999|By Michael Carroll

Armed with data collected over the past 11 years, a team of crack astronomers has determined that our solar system is not unique: There is at least one more sun-like star with several objects in its planetary family. This star and its trio of companions are changing our fundamental view of how planetary systems form.

Upsilon Andromedae lies 44 light-years from us, a mere stone's throw in the cosmos.

It is very much like our own sun, only slightly larger and more massive, and it glows three times as brilliantly. But what of its mysterious planets?

The innermost world is smaller than Jupiter and circles its parent sun at a sizzling distance, eight times as close as Mercury orbits our sun. A "year" on the newly discovered planet - the time it takes to make one circuit - is less than five days.

Upsilon Andromedae's other two planets are several times as massive as Jupiter, the king of worlds in our own system. The orbits of these alien planets are not circular, but elliptically "eccentric."

One travels around its sun every 250 days in a looping orbit that brings it as close to Upsilon Andromedae as Venus is to our sun, and as far away as the Earth-sun distance. The other planet takes a leisurely 1,200 days to orbit, about the same distance from its sun as the asteroid belt lies from ours.

Because of the new planets' sheer size, researchers believe them to be gas giants - worlds similar to Jupiter, Saturn, Uranus and Neptune.

In fact, the density of the innermost planet lies somewhere between Jupiter and Saturn. However, the outer titans are thought to be unlike any planet we have yet seen, as massive as two and four Jupiters, respectively. Their mass puts them in a class far larger than planets in our own system.

Astronomers Geoffrey Marcy of San Francisco State University and Paul Butler of the Anglo-Australian Observatory could tell that something was strange about Upsilon Andromedae.

The two researchers have an excellent track record - they have successfully confirmed the existence of more than a dozen planets circling other stars.

They use a technique that detects telltale anomalies in a star's motion caused by the gravitational pull of a large planet.

This motion, when painstakingly tracked, reveals the size and distance of the planet, even though it is too faint to be seen with current telescopes.

But a one-planet model failed to fit the wobble in Upsilon Andromedae observed by Mr. Marcy and Mr. Butler. As a result, they surmised that another unseen object was tugging on the star.

If true, Upsilon Andromedae would join our own sun as the only other sun-like star with confirmed multiple planets circling it. (Another multiple-planet system was discovered in 1994, but its star is a deadly pulsar, and the environs of this system are nothing like our own.)

"Finding the third planet was certainly a surprise," recalls Peter Nisenson of the Harvard-Smithsonian group. "We were dubious at first, but that gave way to excitement as we realized what we had."

With the multi-planet discovery, scientists are left with new puzzles. The location and orbital paths of the three planets contradict most existing theories of solar system formation and evolution.

"The most peculiar thing," says Timothy Brown ofthe National Center for Atmospheric Research "is that somehow this system created three massive planets from the disk of proto-star material. It's hard to explain how there was enough material."

Planetary mystery

Another mystery involves the orbits of the planets. Current theory holds that massive planets develop in zones far away from their parent star. It is thought that these icy regions give birth to planetary cores as the solar system coalesces from a disk of gas and dust, and that the outer planets migrate inward over time. But all three of Upsilon Andromedae's planets orbit far closer to their sun than classic models of planetary formation suggest they should.

With 20-some Jupiter-class planets now known to be in close orbits around their stars, Mr. Brown asks, "Why is our Jupiter so far out? It tells us that we still don't understand much about how solar systems form.

Michael Carroll wrote this for Popular Science magazine. This article was distributed by the Los Angeles Times Syndicate.

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