by Mark Perew
American Reporter Science Correspondent
Santa Ana, Calif.
January 9, 2001
SAN DIEGO, Calif., Jan. 9, 2001 -- The nine planets of our solar system have gained at least a couple of relatives in the neighborhood, but in a friendly family squabble, scientists say they're not so sure about some others.
At the winter meeting of the American Astronomical Society convened here this week, astronomers were puzzled over discoveries of two planets with some very unusual sleeping arrangements. The two surround the star Gliese 876, which is a mere 15 light years from us in the constellation Aquarius.
Orbiting Gliese 876, a team including Geoffry Marcy and Debra Fischer from the University of California at Berkeley, Paul Butler from Washington, D.C.'s Carnegie Institution, Steve Vogt from UC Santa Cruz and Jack Lissauer of NASA's Ames Research Center found two planets with unusual orbits. The innermost planet has a minimum mass of one-half that of Jupiter and completes its orbit every 30.1 days.
The outer planet is at least 1.8 times the mass of Jupiter with an orbit of 60 days. This 2:1 resonant orbit -- the inner planet orbits twice for every one orbit of the outer -- is causing some head scratching among astronomers who specialize in the theory of planet formation. Theoreticians don't know if the planets formed in this configuration or if they moved into this relationship after they were formed. They also have some other thorny questions.
"This discovery is very exciting because of the profound theoretical implications," said Douglas N. C. Lin of UC Santa Cruz. Lin is a highly regarded expert in the theory of planetary system formation.
A second pair of objects orbits a star with the unwieldy designation HD168443. One object has a mass that is at least seven times that of Jupiter and orbits a mere 30 million miles from its sun.
The other object, orbiting 10 times farther away, is at least 17 times more massive than Jupiter. Dubbed "The Whopper," this object is also puzzling the researchers. While its weight is enormous compared to Jupiter's, the physics of gaseous planets suggest it is about the same size as our giant neighbor.
Recent discoveries of "Super Jupiters" have caused astronomers to examine the distinction between a planet and a star. Many researchers have agreed that anything more than 13 times the size of Jupiter would be a kind of failed star named a "brown dwarf." While objects above this size would not shine as regular stars do, they do produce heat through the fusion of deuterium, the heavy form of the hydrogen molecule.
Butler, however, balks at this label.
"That's just hiding the physics under the table," he says. "Just because you call it a brown dwarf doesn't mean that you understand what's happening."
"It's a matter of semantics," Lin countered. "It is possible to form this planet in the way that Jupiter was formed, with a few special conditions. As a theoretician, I'm comfortable with both explanations."
"If it's found within the disk, then they are planets," argues Butler. "The disk" being the swirling mass of dust and gas left over from the formation of the central star. If it's a planet, then it can't be a brown dwarf star. If it's a star, then why is it so close to the central star?
This jovial argument is typical of researchers trying to find a theory to explain the truth they see in observations.
"Perhaps our friends in the theoretical community will accept this as a mystery to be solved," Butler concludes.
Not all researchers, though, are satisfied that all the conditions are understood. David Black of the Lunar & Planetary Institute in Houston suggests that the researchers need to take another look at the data.
"Keep in mind," he admonished, "that these mass estimates are minimums. These objects could be much larger." He also offers that, "If I were seeing these ratios and resonances, I'd be concerned about errors in the data." Marcy, the project leader, isn't concerned about the data or the people performing the analysis and collection.
"This team has found 38 planets to date," Marcy said. "I'm confident in our results."
Additional details about these and other extrasolar planets can be found at http://www.exoplanets.com.
Mark Perew is a freelance science writer, a member of the National Association of Science Writers and a JPL Solar System Ambassador.