Kepler-56: A Case Study of Planetary Cannibalism around an Evolved Star

Abstract: The fate of close-in planets orbiting stars evolving off the main sequence is governed
by the rapid expansion of the stellar envelope and enhanced tidal dis-sipation. Kepler-56, a redgiant star hosting multiple planets on misaligned orbits, provides a rare opportunity to examine
this process in an observed sys-tem. In this work, we investigate the tidal evolution of the inner
planets in Kepler-56 and demonstrate that their present orbital configuration places them in a
regime of runaway orbital decay as the stellar radius increases along the red-giant branch. We
show that tidal torques acting within the deep convec-tive envelope lead inevitably to planetary
engulfment on astrophysically short timescales. The analysis further suggests that planetary
engulfment can con-tribute to the anomalously rapid rotation of the stellar envelope and may
play a role in the observed spin–orbit misalignment between the stellar core and envelope.
Kepler-56 thus serves as a benchmark system for studying plane-tary cannibalism during postmain-sequence stellar evolution and highlights the broader implications of tidal interactions for
the long-term survival of close- in exoplanets. The Kepler-56 system therefore provides a
valuable observational laboratory for studying the late-stage dynamical evolution and eventual
engulf-ment of planetary systems around evolved stars.