Paul Butler

Upsilon Andromedae and Multiple-Planet Systems

The rich diversity of planets in the solar system display an astonishing range of environments, from surface temperatures high enough to melt lead on Venus to the frozen rings of Saturn. Yet to first order, one can accurately describe the solar system as consisting of the Sun, Jupiter, and some leftover debris. If alien astronomers are looking at the solar system with technologies similar to our own, they will detect only the Sun and Jupiter. It is therefore not surprising that the first rush of extrasolar planet discoveries have all been of single-planet systems.

Over the last four years, about 30 planets have been discovered orbiting nearby Sun-like stars (Fig. 5). My collaborator Geoffrey Marcy, of the University of California at Berkeley, and I have found two-thirds of these using the "precision Doppler technique." An orbiting Jupiter-mass planet will gravitationally tug its parent star in a small counter orbit. This stellar wobble can be detected via the Doppler effect acting on the star's light. The planets discovered to date have orbital periods of a few days to a few years, and most of these planets reside in eccentric orbits. None of these planets reminds us of our own solar system, with giant planets in elegant circular orbits with periods of a decade or longer. And none of the systems found prior to 1999 has yielded more than one planet.

The discovery of a true multiple-planet system with current state-of-the-art technology requires a combination of measurement precision, patience, and luck. Reliable detection of an extrasolar planet by the precision Doppler technique requires that observations span a time covering at least two orbits. Since the longest-running precision Doppler program is only 12 years old, systems in which the outer planet has a period longer than 6 years would not yet be detectable. With this limitation, only those systems of multiple Jupiter-mass planets orbiting within 4 AU (1 astronomical unit = Earth-Sun distance = 93 million miles) of the parent star can be detected.

Upsilon Andromedae, the only confirmed system of multiple extrasolar planets, meets these conditions. Marcy and I have observed Upsilon Andromedae at Lick Observatory for the past 12 years. In June of 1996, we announced the presence of a planet in a scorching hot, 4.6-day orbit with a minimum mass of 0.7 Jupiter-masses (Fig. 6). Like the handful of other Ò51 Peg-likeÓ planets with orbital periods of less than 5 days, the orbit is circular. The discovery paper noted evidence for additional longer-period planets.

Because of these suspicions, the Lick group and the Advanced Fiber Optic Echelle (AFOE) group (led by Robert Noyes at Harvard) independently monitored the Doppler velocity of Upsilon Andromedae as often as possible. By early 1999, more than 140 observations had been collected, allowing the complicated pattern of Doppler velocities to be untangled into two additional planets (Fig. 7). The middle planet in this system has 2 Jupiter-masses in a 240-day orbit; the outer planet has 4 Jupiter-masses in a 3.5-year orbit. Both the outer two planets are in eccentric orbits, as are all of the extrasolar planets that orbit more than 0.2 AU from their host stars.

With the possible exception of the companion to 47 Ursae Majoris, all of the extrasolar planets found to date are either "51 Peg-like" or eccentric. In the Upsilon Andromedae system we have both types of planets, perhaps making this a Rosetta stone that will allow us to understand the formation and evolution of these alien planetary systems. As the existing Doppler survey programs improve their precision, time baseline, and sample size, many more multiple-planet systems will be found.

Over the next 10 years, the precision Doppler technique will continue to provide the bulk of the extrasolar planet discoveries. Within the past three years, Marcy and I have increased the size of our survey from 100 stars to 1,100 stars using telescopes in both the northern and southern hemispheres, including the biggest telescope in the world -- the 10-m Keck. By 2010 these surveys will provide the first hints about the fraction of planetary systems similar to the solar system, with giant planets orbiting out to 5 AU and beyond. These surveys will ultimately point the way to advanced technology missions (such as the proposed NASA Terrestrial Planet Finder) that will directly image Jupiter-like and Earth-like extrasolar planets, and ultimately take their spectra.

Fig. 5. Known extrasolar planets are shown as a function of orbital distance.

Fig. 6. These are the discovery data for the inner planet with a 4.6-day period in the Upsilon Andromedae system, from December 1995 to January 1996.

Fig. 7. The discovery data for the outer two planets in the Upsilon Andromedae are shown here, with the Doppler velocities of the inner planet removed.