The Survival Paradox of Close-In Planets Orbiting Red Clump Giants

Abstract

Close-in planets orbiting core-helium-burning red-clump (RC) giants directly challenge tidal-engulfment theory, which predicts that planets within about 1 au of Sun-like stars should be engulfed during red-giant-branch (RGB) expansion. The existence of such survivors would imply missing physical processes in the late stages of stellar and planetary evolution. Yet only one such "forbidden" planet had been identified to date. Here we report three systems-24 Boo b, HD 12648 b, and TYC 4282-605-1 b-whose host stars are confirmed red-clump giants through asteroseismology, showing that each has already undergone dramatic expansion and contraction following helium ignition. Their planets now reside at orbital separations smaller than the maximum stellar radii attained during the RGB phase, implying that they should not have survived. Among them, 24 Boo b orbits at only 0.183 au with a 30-day period, making it the closest planet known around a red-clump giant. These discoveries demonstrate that this paradox is not an isolated anomaly but signals the emergence of a new class of evolved planetary systems. Their survival defies current models of stellar-planetary coevolution and points to unexplored physics shaping planetary architectures around post-RGB stars.

arXiv comment10 pages, 3 figures, 1 table

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