Inflated planets and their low-mass companions

Various mechanisms have been proposed to explain the inflated size of HD 209458b after it became clear that it has no companions capable of producing a stellar reflex velocity greater than around 5 m/s. Had there been such a companion, the hypothesis that it forces the eccentricity of the inflated planet thereby tidally heating it may have been readily accepted. Here we summarize a paper by the author which shows that companion planets with masses as low as a fraction of an Earth mass are capable of sustaining a non-zero eccentricity in the observed planet for at least the age of the system. While such companions produce stellar reflex velocities which are fractions of a meter per second and hence are below the stellar jitter limit, they are consistent with recent theoretical work which suggests that the planet migration process often produces low-mass companions to short-period giants.