A Sanity Check for Planets around Evolved Stars

We present the radius-period plot for exoplanet candidates around giant stars. The diagram contains two distinct regions. While planets of giants with radii smaller than 21 R_⊙ exhibit a wide range of orbital periods, there is evidently a lack of both relatively short-period (≤300 days) and long-period (≥800 days) planets around bigger stars. In other words, planets around K giants all have similar orbital periods above a certain stellar radius, presumably pointing out a new phenomenon which preferably occurs in stars with radii larger than ~21 R_⊙. So far, it is speculative if we are seeing rotational modulation due to some kind of surface structure or an unprecedented form of nonradial stellar oscillations. Consequently, the radius is the second key parameter for giants apart from the stellar mass. Thus, we propose the radius-period plot as a tool to check the plausibility of planetary companions around more challenging host stars by taking into account their stellar identity (e.g., stellar radius and metallicity) to exclude intrinsic stellar variability.