Planet hunting
Astronomers report discovery of three new extrasolar planets, plus hints of multi-planet systems

By Robert Sanders, Public Affairs

16 AUGUST 00 | As they add another three to the list of 41 known planets outside our solar system, a team of Berkeley astronomers is beginning to see patterns, including hints that many extrasolar planets may have siblings.

To date, only one Sun-like star has been found with multiple planets: Upsilon Andromedae, around which the same team discovered three planets last year.

However, Debra Fischer, a post-doctoral fellow at Berkeley, looked more closely at data for 12 stars her team has been observing long enough to betray the existence of long-period planets, if any, and around which they had already discovered one planet. She found that five -- nearly half -- exhibit unexplained wobbles that could result from the tug of a companion, whether another planet, an unseen star or something in between.

"This is the first time anyone has noticed that such a high percentage of stars with one known planet show evidence of a second companion," Fischer said.

All five planets with a possible second planetary companion are large gas giants very near their central star, so close that most astronomers think such planets must have formed farther out and later migrated inward.

"It's important to know if there are other companions out there," Fischer said, "because anything else in the system will affect the dynamics and theories of how the planets moved in and parked in their current orbits."

The analysis, plus data on the three new extrasolar planets, was presented Aug. 7 by team leaders Geoffrey Marcy, professor of astronomy at Berkeley, and Paul Butler, an astronomer at the Carnegie Institution of Washington, at the International Astronomical Union meeting in Manchester, England. A paper by Fischer, Marcy, Butler and their colleagues, describing the results, will appear soon in the Astrophysical Journal.

The three new planets discovered around the stars HD12661, HD 92788 and HD 38529 are large gas giants similar to the planet Jupiter. All are in highly eccentric orbits that alternately bring them close to the planet and carry them far away. This dance alternately drags the star toward and away from Earth by a distance about half the radius of the star, causing a Doppler shift in the star's light that astronomers can detect.

One of the stars, HD 38529, has a planet that whips around in 14.3 days. This planet alone cannot explain the observed wobble in the star, however, leading Fisher and her colleagues to speculate that it has a second companion at a much greater distance from the star.

"We see indication of a second companion, but we can't be sure because we only see a portion of the orbit," Fischer said. "The companion could be a dim star, a brown dwarf or another planet."

Brown dwarfs are failed stars -- large objects just shy of the mass necessary to sustain hydrogen burning in their core.

More data on another star, 55 Cancri, around which the team found a planet in 1996, indicates it may have a second companion too, Fischer noted. Such a planet, if real, would be at least 3-4 Jupiter masses and have a period of 4,715 days -- about 13 Earth years -- in a highly eccentric orbit at an average distance of 5.5 AU, similar to Jupiter's orbit around our Sun. One astronomical unit, or AU, is 93 million miles, the distance from the Earth to the Sun.

"This is still speculative," she cautioned. Still, it led her to look more closely at the dozen stars with known planets for which her team had at least two years of data, enough to show evidence of a second, more distant companion orbiting with a long period.

Five of these planets turned out to have "residual velocities" that could not be explained by a single planet, suggesting the presence of a second companion. The team must collect data for a complete orbit before concluding that another companion exists.

The authors noted that detecting a second companion requires an accurate knowledge of the wobbles caused by the already known planet, which is possible only when that planet is near the star and has a short period. Thus, stars with more distant planets might well have other companions that would not be detectable using the Doppler method.

Interestingly, all three of the new stars are rich in the heaviest atoms, like iron, meaning the stars were formed from dust that had already been cycled through at least one other star. This continues a trend of high metallicity among stars with known planets.

Marcy, Butler, Fischer and their colleagues are monitoring about 900 stars in the northern sky using the Lick Observatory in California and the Keck Observatories in Hawaii, plus another 200 in the southern sky with the Anglo-Australian Telescope. They look for wobbles that indicate a planet may be circling the distant sun.

To date they have discovered 30 extrasolar planets, including the three reported today. In all, there are now 44 known stars with planets or planetary systems, excluding our own.

"We're now at a stage where we are finding planets faster than we can investigate them and write up the results," Marcy said. "It's wonderful. Planet-hunting has morphed from the marvelous to the mundane."



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