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

April 15, 1999

BOULDER--Two more planets orbiting a Sun-like star, in addition to one found three years ago, have emerged from data analyzed by three teams of scientists at San Francisco State University, the Harvard-Smithsonian Center for Astrophysics (CfA), and the National Center for Atmospheric Research (NCAR). This is the first time that multiple planets have been discovered around a Sun-like star.

A team of astronomers including Timothy Brown of NCAR has been observing the star, known as Upsilon Andromedae, since 1996, when its first planet was discovered. When the CfA/NCAR observations were added to those of that planet's discoverers, Geoffrey Marcy and Paul Butler of San Francisco State University, the scientists confirmed that our solar system isn't the only one with more than one planet. The NCAR observations were funded by the National Science Foundation, NCAR's primary sponsor.

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

The discovery of multiple planets "has something to say about the way solar systems can form," Brown says. "There's not just one big object slurping up all the mass" in a planetary system, as some have theorized. It's not clear whether Upsilon Andromedae's three planets formed at their current orbital distances or formed elsewhere and migrated after some catalytic event, such as a close encounter between two planets or the passing of another star.

The scientists analyzed independent observations of the star from two locations: Lick Observatory (operated by the University of California) and Whipple Observatory (operated by the Smithsonian Astrophysical Observatory). When the teams eliminated the effects of the 4.6-day orbital period of the known planet from the data, another period of about 1,200 days sprang into view. Calculations suggested it was caused by a planet four times the mass of Jupiter, the largest planet in our solar system. This new planet circled at a distance of about 2.5 astronomical units from its star. An astronomical unit (AU) is the distance from the Earth to the Sun, or 93 million miles. The original planet is about three-quarters the mass of Jupiter, orbiting at a distance of only 4.7 million miles--much closer 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, about twice the mass of Jupiter, orbiting in about 250 days at a distance slightly under 1 AU.

At a meeting last June, team members from NCAR and CfA met with Geoff Marcy and indicated that they thought another planet was present. Marcy agreed and suggested joining forces to find it. "We were both wrong, in a way," says Brown. "We thought we had found one new planet, but in fact we had two."

About twenty extrasolar planets have been discovered by observing their gravitational pull on their stars. All stars move through space, but a star with a big, close planet also wobbles as the circling planet tugs it out of its path. To an observer on Earth, the star's light appears to waver slightly. "The method is very biased toward finding big planets close to their stars," Brown says. "If we were on an extrasolar planet observing our own solar system, "we wouldn't even find Jupiter--much less the Earth--by our method, because Jupiter is so far from the Sun," he points out.
NCAR is managed by the University Corporation for Atmospheric Research, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences.

National Center for Atmospheric Research/University Corporation for Atmospheric Research

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