False Planets around Giant Stars: A Case Study of Sanders 364 in M67

Abstract

Discovering planets in sparsely populated regions of parameter space is crucial to improving our understanding of planetary formation and evolution. One such region is the subset of planets that orbit giant, evolved stars. However, some of these evolved stars are known to exhibit long-period quasiperiodic radial velocity signals, which can masquerade as signals from orbital motion due to planetary companions. In this paper, we investigate the case of Sanders 364, a K giant star in the old open cluster M67. A paper by Brucalassi et al. reports the discovery of a giant planet with a period of 121 days orbiting Sanders 364. From our analysis of a large set of independent radial velocities, we find no convincing evidence for the giant planet reported by Brucalassi et al. We did identify six long-period radial velocity signals of unclear origin, including the 121 days signal reported by Brucalassi et al., but based on our analysis, we speculate that these are quasiperiodic signals that arise from nonplanetary origins, such as stellar variability or aliasing. The results from our study of Sanders 364 suggest that the detection of true orbital motion from a long-period planetary companion requires extra care when the host star is highly evolved. We conclude by offering recommendations for future study of planetary companions around evolved host stars.