SCIENTISTS SQUABBLE OVER SOLAR SYSTEM SIBLINGS
by Mark Perew
American Reporter Science Correspondent
Santa Ana, Calif.
SAN DIEGO, Calif., Jan. 9, 2001 -- The nine planets of our solar sy= stem 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 w= ith some very unusual sleeping arrangements. The two surround the star Gli= ese 876, which is a mere 15 light years from us in the constellation Aquari= us.
Orbiting Gliese 876, a team including Geoffry Marcy and Debra Fisch= er from the University of California at Berkeley, Paul Butler from Washingt= on, D.C.'s Carnegie Institution, Steve Vogt from UC Santa Cruz and Jack Lis= sauer 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 c= ompletes 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 in= to this relationship after they were formed. They also have some other tho= rny questions.
"This discovery is very exciting because of the profound theoretica= l 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 unwieldly designati= on HD168443. One object has a mass that is at least seven times that of Ju= piter and orbits a mere 30 million miles from its sun.
The other object, orbiting 10 times farther away, is at least 17 ti= mes more massive than Jupiter. Dubbed "The Whopper," this object is also pu= zzling 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 g= iant neighbor.
Recent discoveries of "Super Jupiters" have caused astronomers to e= xamine 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 d= euterium, 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 b= ecause you call it a brown dwarf doesn't mean that you understand what's ha= ppening."
"It's a matter of semantics," Lin countered. "It is possible to fo= rm this planet in the way that Jupiter was formed, with a few special condi= tions. As a theoretician, I'm comfortable with both explanations."
"If it's found within the disk, then they are planets," argues Butl= er. "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 the= ory 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 seei= ng 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 pe= ople performing the analysis and collection.
"This team has found 38 planets to date," Marcy said. "I'm confiden= t 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 Assoc= iation of Science Writers and a JPL Solar System Ambassador.