A Family of Giants
Astronomers from four institutions are announcing today
their discovery of the first system of planets found orbiting a sun-like star
April 15, 1999
SAN FRANCISCO, April 15, 1999 Astronomers from four research institutions 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. 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 High Altitude Observatory (HAO) in Boulder, CO have independently found the two outer planets around Upsilon Andromedae. This tea'as 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 said, "It implies that planets can form more easily than we ever imagined, and that our Milky Way is teeming with planetary systems."
The innermost (and previously known) of the three planets contains at least three-quarters of the mass of Jupiter and orbits only 0.06 AU from the star. (One "AU" equals the distance from the Earth to the Sun). It traverses a circular orbit every 4.6 days. The middle planet contains at least twice the mass of Jupiter and takes 242 days to orbit the star once. It resides approximately 0.83 AU from the star, similar to the orbital distance of Venus. The outermost planet has a mass of at least four Jupiters and completes one orbit every 3.5 to 4 years, placing it 2.5 AU from the star. The two outer planets are both new discoveries and have elliptical (oval) orbits, a characteristic of the nine other extrasolar planets in distant orbits around their stars.
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." Robert Noyes, a professor of astronomy at Harvard-Smithsonian CfA and a member of the CfA-HAO team, said, "A nagging question was whether the massive bodies orbiting in apparent isolation around stars really are planets, but now that we see three around the same star, it is hard to imagine anything else."
Currently a staff astronomer at the Anglo-Australian Observatory, Butler, an American, 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 three giant planets could co-exist in stable orbits.
One big question left to answer is how such a solar system arose. "The usual picture is that gas giant planets can only form at least four AU away from a star, where temperatures are low enough for ice to condense and begin the process of planet formation," said Brown. "But all three giant planets around Upsilon Andromedae now reside inside this theoretical ice boundary." The planets may have formed close to the host star, or, like balls on a billiard table, the planets may have scattered off of each other, migrating into their current orbits from a more distant place of origin.
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.
If these Jupiter-mass planets are like our own Jupiter, they would not be expected to have solid Earth-like surfaces. But, Nisenson noted, "Our observations can't rule out Earth-sized planets as well in this planetary system, because their gravity would be too weak for them to be detectable with present instruments."
A bright star visible to the naked eye starting this June, Upsilon Andromedae is 44 light-years away from Earth, and it is roughly 3 billion years old, two-thirds the age of the Sun. This star should make an ideal target for NASA's upcoming Space Interferometry Mission (SIM). Expected to launch in 2005, SIM will spend five years probing nearby stars for Earth-sized planets and will test technology slated for future planet-searching telescopes. The ongoing ground-based planet search will enable SIM to home in on those stars most likely to harbor small planets.
San Francisco State University is a highly diverse community of 27,000 students and 3,500 faculty and staff. It is one of the largest campuses in the nationally recognized 23-campus California State University system. Founded in 1899, the University is celebrating its 100th year of service to San Francisco, the Bay Area and beyond. The Harvard-Smithsonian Center for Astrophysics, located in Cambridge, MA, is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory.