Bizarre new planets puzzle astronomers
BY JEFF FOUST
Posted: January 10, 2001
A team of astronomers, led by veteran planet hunter Geoffrey Marcy of the University of California Berkeley, reported their discoveries -- one solar system with an improbably large planet and another where the planets orbit their parent star in lockstep -- at a conference of the American Astronomical Society in San Diego on Tuesday.
One of the new planets is the largest extrasolar planet yet discovered with an estimated mass 17 times that of Jupiter, our solar system's largest world. It lies an average of 430 million kilometers from the star HD 168443, 123 light-years from the Earth, completing one elliptical orbit around the Sun-like star every 4.85 years.
Objects as large as this are usually not classified as planets: at masses about 13 times that of Jupiter, gravitational pressures grow high enough in the core of such bodies to initiate fusion of deuterium atoms, the widely accepted dividing line between planets and more massive brown dwarfs, or failed stars. However, the unusually close proximity of this object to its parent star, as well as the existence of a previously-discovered planet 7.7 times the mass of Jupiter in an even closer orbit lead Marcy and his colleagues to reconsider that classification.
"This massive planetary object defies our expectations for the largest planets," said Paul Butler, a staff scientist at the Carnegie Institution of Washington and a member of Marcy's team. "But it's orbiting right there next to another planet. We never expected Nature would make such gargantuan planets, and indeed maybe they aren't planets at all."
"How did this form so close in orbit around the star, and in close association with another planet?" asked Steve Vogt, a University of California Santa Cruz astronomer and another member of the discovery team. "This is going to bug astronomers."
Another new planetary system that may bug astronomers is around the star Gliese 876, a small M-type star 15 light-years from Earth. In addition to a previously known planet orbiting the star every 60 days, astronomers said Tuesday they discovered another, smaller planet orbiting the star in exactly half the time. The planets appeared to be locked in a 2:1 gravitational resonance, the first such resonance seen outside the solar system.
Such resonances have been found within our own solar system. Neptune and Pluto are locked in a 3:2 resonance, where Neptune orbits the Sun three times for every two circuits by Pluto. In addition, three of Jupiter's largest moons -- Io, Europa, and Ganymede -- orbit the planet in a 4:2:1 resonance.
"The resonance between the two orbiting planets is among the most exciting planet detection discoveries to date," said Jack Lissauer, a planetary dynamicist at NASA's Ames Research Center. "This is the first extra-solar planetary system to show a strong resonance. It also is the smallest star known to have any orbiting planets, much less two."
Scientists hope the discovery of a similar resonance in another solar system may provide insights into their formation. "Questions about planetary migration and gravitational influence are still very much unsolved," Lissauer noted.
With the latest discoveries astronomers have now cataloged 55 extrasolar planets orbiting nearby, mostly Sun-like stars in the last five years. Most of those planets have been found by Marcy's group and another team led by Michel Mayor of Switzerland's Geneva Observatory. Both teams use similar techniques to indirectly detect the planets, looking for small periodic variations in the Doppler shifts of spectra lines from the stars. Such shifts are caused when the star wobbles under the gravitational influence of an orbiting planet or planets.
Astronomers plan to continue their searches for years to come: Marcy's group is in the midst of a multi-year project to look for planets around 1,100 stars within 300 light-years of Earth.