First system of multiple planets found around a sun-like star

A team of astronomers from Australia and the USA have discovered the first planetary system around a nearby, Sun-like star.

Astronomers 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. Well-known planet hunters Geoffrey Marcy and R. Paul Butler detected the closest planet in the Upsilon Andromedae system in 1996.  Now, after 11 years of telescope observations, evidence 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. 

This first planetary system, found from a survey of 107 stars carried out at Lick Observatory in California, offers the first suggestion that systems of multiple planets like our own are abundant in our Milky Way Galaxy, which contains 200 billion stars.  Over the last 4 years, single planets have been found around 20 nearby Sun-like stars.

This makes the current planetary search programs even more relevant.  The team led by Marcy and Butler are using the worldÂ’s largest telescope, the 10-m Keck in Hawaii, and the 3-m telescope at Lick Observatory to study the northern skies. The companion Anglo-Australian Planet Search Program is led by Anglo-Australian Observatory staff astronomers Paul Butler and Chris Tinney. They are using Australia's largest optical telescope, the Anglo-Australian Telescope to survey the 200 nearest and brightest Sun-like stars in the southern skies over the next decade.

The innermost (and previously known) of the three planets orbiting Upsilon Andromedae 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 it 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 which is characteristic of the nine other extrasolar planets in distant orbits around their stars. 

Anglo-Australian Observatory staff astronomer Paul Butler is the lead author of the paper, submitted to the Astrophysical Journal, announcing the triple planet system.  Along with Marcy and Dr Debra Fischer from San Francisco State University, the authors include an independent team of researchers from High Altitude Observatory in Boulder Colorado and the Harvard-Smithsonian Center for Astrophysics in  Cambridge, Massachusetts. This second team independently found the two outer planets around Upsilon Andromedae.  "This is an extraordinary finding and it demands extraordinary evidence," said Butler.  “Having two completely independent sets of observations gives us confidence in this detection." 

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, "This will shake up the theory of planet formation."

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. 

One big question left to answer is how such a solar system arose.  Previous theories hold 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.  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.

A bright star visible to the naked eye during Australian winter, 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.

The technical paper and graphics about the discovery can be viewed at: