SAO Guest Contribution

The Upsilon Andromedae System

Dr. Paul Butler is a staff astronomer at the Anglo-Australian Observatory at Siding Spring, New South Wales.  He has held previous positions at San Francisco State University (SFSU) and University of California at Berkeley, and has won numerous awards, including being named in NEWSWEEK's 100 Americans for the Next Century!

Dr. Butler is the leader of the Anglo-Australian Planet Search Program and a primary researcher in the SFSU Planet Search Team.  He is the lead author on the publication announcing the recent discovery of a multiple-planet system around a sun-like star (see press release link from the 'Astronomy Press Releases ...' newsgroup).  In this guest contribution, written especially for Swinburne Online, he takes up the issue which participants have been discussing in the HET602 newsgroups - namely, how they can be sure that they have discovered a set of multiple extra-solar planets.

The Upsilon Andromedae System

All ~20 extrasolar planets discovered over the last four years have been revealed by precision Doppler velocity surveys.  It is not surprising that these discoveries have all been of single planet systems.  While a minimum of seven Doppler velocity measurements are needed to describe the orbit of a single planet, in practice 20 observations spread over more than one orbital period are typically required to reliably determine orbital parameters. The discovery of a system with two or more planets by the precision Doppler technique requires a time span longer than the orbital period of the most distant planet, and many dozens of observations.

Upsilon Andromedae "b", the "51 Peg-like" planet in the 4.6 day orbit, was originally announced by our group in June 1996.  With such a short orbital period, this planet can be detected via the precision Doppler velocity technique within any two week period.  It has been detected by all of the precision Doppler velocity groups that have looked.

The additional longer periods are more difficult to detect.  The data baseline must be longer than the orbital period of the most distant planet.  In our discovery paper of Upsilon Andromedea "b" (Butler, Marcy, et al. 1997, ApJ) we noted, ``evidence for variability'' of the 4.6 day periodicity with a ``timescale of about 2 yr''.

Both the "Lick Observatory" and the "AFOE" groups have been observing Upsilson Andromedae whenever time permits since 1996.  Between the two groups, we have more than 140 observations stretching back over more than 11 years.  Both groups independently see the longer periodicities at 242 and 1269 days.  In particular, both periodicities are visible to the eye in Figure 2 of our new paper, which shows the Lick Observatory measurements of Upsilon Andromedae after accounting for the known 4.6 day periodicity.  Also please see Figures 4b, 4c, and 4d in the new paper.

We have very seriously considered alternative astrophysical explanations for the observed periodicities at 4.6, 242, and 1269 days.  These alternatives include radial and nonradial pulsations, rotational modulation of surface features, and stellar magnetic cycles.  These explanations seem unlikely based on the observed photometric and chromospheric stability of the star.  A detailed explanation is given in our new paper.

This system of multiple Jupiter-mass planet all packed within 2.5 AU of the central star is certainly remarkable.  We are doubly confident that our measurements are correct because the "AFOE" group has independently observed the same "signature".  Our first question about this system was, "Can this be dynamically stable?". We were worried that gravitational interactions between these three Jovian-mass planets would cause orbital instabilities. In fact, both analytic and numeric techniques now demonstrate the stability of this system.  A detailed explanation is given in our new paper.

All of the planets orbiting beyound 0.2 AU are in eccentric orbits. Even the orbit of 47 Ursae Majoris is more than twice as eccentric as Jupiter.  Why are the orbits of Solar System planets so circular, and why are the orbits of most of the extrasolar planets so eccentric? Are systems with multiple Jupiter-mass planets more common than systems with a single dominant Jupiter?  Is the architecture of a system of planets dominated by interactions between proto-planet and planetary-disk, planet and nearby stars, or between multiple Jupiter-mass planets in a single system?  This new system suggests that gravitational interactions between multiple Jupiter-mass planets can play a powerful role in sculpting planetary systems.

More information be found at our Web site: http://exoplanets.org

The "Ups And" page includes a graphic that shows the orbits of the newly discovered planets, and a link to our paper submitted to the Astrophysical Journal.  A link is included to the web page of  our collaborators, the AFOE team, which includes an excellent JAVA animation, and links to other media sources.

Paul Butler

A reminder: if you have any questions about this (or any other) Guest Contribution, please do not contact the author directly.  Instead, as has been arranged with the authors, post the question to the Astronomy News newsgroup, and I will collate the questions into one email and forward it to the author for a reply.
Sarah Maddison