Seeking Multiple Planetary Systems

In my opinion it is not enough just to discover new planets. It is critically important to do follow-up studies. One of the most obvious types of follow-up study is to continue to monitor the systems where a first planet has been discovered, to see if any additional modulations in the Doppler velocities emerge as you move to higher precision and longer time spans. Additional periodic modulations would then be interpreted as second or even third planets; a planetary system. This work is important because we want to know more about the formation and evolution of planetary systems, not just single planets. The problem is that there is little hope that the Doppler technique can ever attain the precision needed to detect planets like the earth, primarly because the tiny orbital velocity induced by an earth would be swamped by velocity jitter due to other astrophysical phenomena, such as star spots coupled with rotation, or macroscopic motions in the atmosphere of the star. But, if we can learn more about how systems form, then maybe we can invoke some theoretical arguments about where and when earth-like planets are likely to form, even though we can not detect them observationally.

As noted above, the first reliable detection of a system of three planets orbiting a solar-type star, upsilon Andromedae, was announced April 15, 1999. The innermost planet, with a period of only 4.6 days, was the first to be discovered in this system. The residuals of the observed Doppler velocities from the orbital solution for this planet were not as good as expected, and the disagreement grew more serious as time passed and more observations were accumulated. Eventually it became clear that there were at least two additional periodic modulations, corresponding to a second and a third planet in wider orbits, with periods of 240 and 1267 days. There are two interesting patterns in the characteristics of these planets. The orbits grow progressively more elliptical with increasing period, and the minimum masses grow progressively larger. At first glance these patterns appear to support theories of planet migration, where giant planets form initially in the cool outer regions of a circumstellar disk, in a region where there is plenty of material for planet building, and then gravitational interactions move one or more of the planets into the inner regions of the system while others move outward, in some cases escaping from the system altogether. In this scenario the three planets orbiting upsilon Andromedae would all be gas giants similar to Jupiter, and unsuitable for life as we know it. However, if the analogy between these planets and Jupiter holds true, they will be orbited by rocky moons, just as Jupiter is, and maybe there is a chance of finding a habitable world among the moons.

Contributed by: Dr. David Latham