The Planet Around Gamma Cephei
Tuesday, February 13, 2007
Science Update - A look at CfA discoveries from recent journals

The star Gamma Cephei is about thirty-eight light-years from Earth, readily visible in the sky roughly between Polaris (the North Star) and the "chair" in Cassiopeia. Astronomer's have suspected for nearly twenty years that Gamma Cep had a planetary companion. In fact, Gamma Cep was one of the first stars to be be studied with new instruments capable of measuring the slight wobble in the star's motion that signals the presence of an orbiting companion. Today the field of extra-solar planetology is booming, with over 200 confirmed examples, and every example helps scientists studying the Earth's formation and evolution to refine their models. Gamma Cep, however, remains one of the pioneering discoveries of the field.

The story of scientific progress can often make for dramatic reading, and the case of Gamma Cep is a good example. After discovering what appeared to be wobbles in the star's orbit, astronomers began to monitor the star very closely. After a few years they reported that they had been in error. They decided that tiny spectral variations they observed, corresponding to a stellar wobble of only about ninety kilometers per hour (for comparison, the Earth's velocity around the Sun is about 100,000 kilometer per hour) were instead due to periodic flaring activity in the star's photosphere. To cap it off, the astronomers concluded that Gamma Cep was in a binary system, with a companion star orbiting every 30 years or so -- the companion's period and stellar type were very poorly known -- potentially disrupting the orbits of any planets (if there were any). Not everyone was convinced about the retraction, however, and astronomers have continued to try to refine the observations to sort out this puzzle.

SAO astronomer Guillermo Torres, writing in last month's Astrophysical Journal, finally presents a convincing case. By using data from the Hipparcos astrometry satellite, and reviewing many of the ground-based observations as well, he finds that indeed Gamma Cep does have a planetary companion. The planet has a mass of at least 1.4 Jupiter-masses (but less than ten times this amount), and it orbits the star at approximately the distance that Mars orbits our Sun. The companion star, he calculates, is a dwarf star of mass about 0.4 solar-masses, and it orbits the star only ten times further away than the planet; it is the closest companion known for any planet-hosting star. The companion is so faint and so close to Gamma Cep that it has never been directly seen, and it is itself of great interest to astronomers who wonder how such a small, nearby companion can long survive ... and how its gravity might disrupt the path of planets orbiting closer to the primary star. The new paper is a fine example of how careful research can resolve a long-standing mystery while simultaneously offering a peek into more subtle puzzles.