College of Science & Engineering Alumni Newsletter

Spring 1999

First System of Multiple Planets Found Around A Sun-Like Star

A team of San Francisco State astronomers leads the discovery of the first planetary system around a nearby, Sun-like star, which was already known to have one planetary companion.

On April 15, 1999, astronomers from four research institutions announced that they have discovered strong evidence for a trio of extrasolar planets that orbit the star Upsilon Andromedae. This is the first multiple planet system ever found around a normal star, other than the nine planets in our Solar System. The closest planet in the Upsilon Andromedae system was detected in 1996 by San Francisco State University (SFSU) astronomers Geoffrey Marcy and R. Paul Butler (BS ‘86 Chemistry; BS ’85 & MS ‘89, Physics & Astronomy). Now, after 11 years of telescope observations at Lick Observatory near San Jose, CA, the signals of two additional planets have emerged from the data. Therefore, Upsilon Andromedae harbors the first planetary system that is reminiscent of our own Solar System.

In parallel, astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, MA and the National Center for Atmospheric Research in Boulder, CO have independently found the two outer planets around Upsilon Andromedae. This team has been studying the star for more than four years at the Smithsonian’s Whipple Observatory near Tucson, AZ. This first planetary system, found from a survey of 107 stars, offers the first suggestion that planetary systems like our own are abundant in our Milky Way Galaxy, which contains 200 billion stars. SFSU researcher Debra Fischer (MS ‘91, Physics & Astronomy) said, “It implies that planets can form more easily than we ever imagined, and that our Milky Way is teeming with planetary systems.” No current theory predicted that so many giant worlds would form around a star. “I am mystified at how such a system of Jupiter-like planets might have been created,” said Marcy, SFSU’s Distinguished Professor of Science. “This will shake up the theory of planet formation.” Currently a staff astronomer at the Anglo-Australian Observatory, Butler is the lead author of the paper, submitted to the Astrophysical Journal, announcing the triple planet system. Along with Marcy, Fischer, and Noyes, the authors include Sylvain Korzennik, Peter Nisenson, and Adam Contos of the Harvard-Smithsonian CfA, and Timothy Brown of the HAO. “Both of our groups found essentially the same size and shape for the orbits of the companions,” said Korzennik. The chances of this happening by accident are infinitesimal. Added Fischer, “This is an extraordinary finding and it demands extraordinary evidence. Having two completely independent sets of observations gives us confidence in this detection.”

Marcy and Butler had suspected that there was something strange about Upsilon Andromedae. The velocity variations that revealed the closest planet to the star in 1996 had an unusual amount of scatter.  Not until early this year had enough observations been made of the star to confirm the presence of an additional planet, which explained some of the confusing pattern in the data.  But another object still seemed to be tugging on the star. “We looked at the two planet solution that we had been expecting and there was still too much extra noise,” said Fischer. “We arrived at the conclusion that the extra observed wobble could only be explained by the presence of a third planet.” Both teams of astronomers considered astrophysical effects that could mimic the velocity signature from these planets, but no such effects are viable. A computer simulation by Greg Laughlin of U.C. Berkeley suggests that these three giant planets could co-exist in stable orbits.

The discovery of this multiple planet system suggests a new paradigm for planet formation where many small seed planets known as planetesimals might develop in the disk of matter surrounding a star. Those planets that grow fastest would engage in a gravitational tug of war that weeds out some of the smaller worlds and determines which planets ultimately remain in orbit. “The Upsilon Andromedae system suggests that gravitational interactions between Jupiter-mass planets can play a powerful role in sculpting solar systems,” said Butler.

The technical paper and graphics about the discovery can be viewed at the Planet Search Homepage. Support for this research was provided by NASA, the National Science Foundation, and SUN Microsystems.

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Updated by Lannie Nguyen-Tang on August 3rd, 2000