NCAR Scientist's Observations Aid In Discovery Of Multiple Planets Orbiting A Sun-Like Star

A team of astronomers including Timothy Brown of NCAR has beenobserving the star, known as Upsilon Andromedae, since 1996, whenits first planet was discovered. When the CfA/NCAR observationswere added to those of that planet's discoverers, Geoffrey Marcy andPaul Butler of San Francisco State University, the scientists confirmedthat our solar system isn't the only one with more than one planet.The NCAR observations were funded by the National ScienceFoundation, NCAR's primary sponsor.

Butler (now at the Anglo-Australian Observatory) and Marcy havesubmitted a paper to the Astrophysical Journal, with Brown as one ofthe coauthors, describing the evidence that two additional planetsare orbiting Upsilon Andromedae.

The discovery of multiple planets "has something to say about theway solar systems can form," Brown says. "There's not just one bigobject slurping up all the mass" in a planetary system, as some havetheorized. It's not clear whether Upsilon Andromedae's three planetsformed at their current orbital distances or formed elsewhere andmigrated after some catalytic event, such as a close encounterbetween two planets or the passing of another star.

The scientists analyzed independent observations of the star fromtwo locations: Lick Observatory (operated by the University ofCalifornia) and Whipple Observatory (operated by the SmithsonianAstrophysical Observatory). When the teams eliminated the effectsof the 4.6-day orbital period of the known planet from the data,another period of about 1,200 days sprang into view. Calculationssuggested it was caused by a planet four times the mass of Jupiter,the largest planet in our solar system. This new planet circled at adistance of about 2.5 astronomical units from its star. Anastronomical unit (AU) is the distance from the Earth to the Sun, or93 million miles. The original planet is about three-quarters the massof Jupiter, orbiting at a distance of only 4.7 million miles--muchcloser to its star than Mercury is to the Sun.

"After removing the effects of both the 4-day period and the 1,200-day period, the observations still had bigger-than-expected noise,"Brown says. The scientists then pinpointed a third planet, abouttwice the mass of Jupiter, orbiting in about 250 days at a distanceslightly under 1 AU.

At a meeting last June, team members from NCAR and CfA met withGeoff Marcy and indicated that they thought another planet waspresent. Marcy agreed and suggested joining forces to find it. "Wewere both wrong, in a way," says Brown. "We thought we had foundone new planet, but in fact we had two."

About twenty extrasolar planets have been discovered by observingtheir gravitational pull on their stars. All stars move through space,but a star with a big, close planet also wobbles as the circling planettugs it out of its path. To an observer on Earth, the star's lightappears to waver slightly. "The method is very biased toward findingbig planets close to their stars," Brown says. "If we were on anextrasolar planet observing our own solar system, "we wouldn't evenfind Jupiter--much less the Earth--by our method, because Jupiter isso far from the Sun," he points out.

NCAR is managed by the University Corporation for AtmosphericResearch, a consortium of more than 60 universities offering Ph.D.s inatmospheric and related sciences.

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Writer: Carol Rasmussen