A Brief History of the upsilon Andromedae planetary system

The star upsilon Andromedae was discovered to be hosting a short period planet in 1996. In this case, the orbital period was 4.6 days. Two years later, it was revealed that this star is also orbited by two further planets, with orbital periods of 58 days and 437 days respectively. So the upsilon Andromedae system is one of the growing number of multiple planet systems known around other stars.

The star is located in the northern constellation Andromeda, not far (in angular extent!) from the famous Andromeda Galaxy (the nearest large galaxy to the Milky Way). It is located about 44 light years from earth, and is one of the brightest stars known to host a planet. It has a low mass stellar companion about 750 astronomical units away (so far enough away to not have a significant gravitational influence on the planetary orbits today).

The upsilon Andromeda system has been monitored enough that we know that the innermost planet does not cross the face of the star as viewed from Earth, i.e. it does not transit. Thus, we know that the planet orbit is not aligned exactly edge-on with our line of sight. There are also indications that the outer two planets are not orbiting with orbits that lie face-on to the line of sight. This comes from the fact that radial velocity measurements do not measure the mass directly, but only the mass divided by a function of the inclination angle. If the outer planet orbits were almost face-on, the measured quantities would imply that the true planet masses were quite a bit larger. They cannot be too large, because then the gravitational interactions between the planets would make the planetary system unstable and we would not see the configuration we see today. We do not know for certain if the innermost planet (the one we study here) is aligned with the outer two, but that is the expectation based on the idea that planets form from disks of gas left over from the formation of the star. Thus, the most likely orientation for the planet orbit lies somewhere in between the two extremes of face-on and edge-on.

There is one interesting piece of evidence for some level of star-planet interaction in this system. Scholnik and collaborators report evidence that some of the lines in the stellar chromosphere vary with the orbital period of the innermost planet. They interpret this as evidence for starspot activity induced on the star by interaction with the planet. We considered whether this mechanism was responsible for our observation too, but it seems unlikely, because the energy required to explain our observations is a lot larger. As a result, if one were to invoke the same mechanism, the planet would have spiralled into the star a long time ago. The reason it doesn't do so in our proposed model is that the energy that powers the variation we see comes from the radiation of the star, whereas it comes from the energy of the planet orbit in the case of the starspot model.

The full specifications of the system can be found here

A picture of the ups Andromedae field can be found here .

Brad Hansen