Evolution of Closely Orbiting Exoplanets about Helium Burning Stars

Abstract

8 UMi b is a planet closely orbiting the helium-burning star 8 UMi [Hon et al., 2023] [Lee et al., 2015]. If 8 UMi b were to have orbited at its present distance throughout the lifetime of 8 UMi, it would have been orbiting within the star during a portion of 8 UMi’s red giant phase [Hon et al., 2023] [Kunitomo et al., 2011] [Villaver and Livio, 2009] [Villaver et al., 2014]. Within this work, we investigate possible routes by which 8 UMi b and other similar closely orbiting exoplanets might evade destruction during the expansion of the stars they orbit. We primarily consider three possible explanations: that of a tidal evolution inwards of 8 UMi b such that the planet evades destruction, that of an initial system where 8 UMi b is orbiting a binary 8 UMi which undergoes a mass transfer and subsequently merges, skipping to its helium burning phase and thus avoiding the problematic portion of the red giant phase entirely, and that where 8 UMi b is a brown dwarf, rather than a planet, and thus may survive a red giant phase. We then estimate the probability of observing planets that avoid destruction, devising an order-of-magnitude probability for each route, for a characteristic sample of 177 stars taken to be similar to that of the survey in which Lee et al. discovered 8 UMi b [Lee et al., 2015]. We find that the tidal evolution explanation is the least likely by roughly an order of magnitude from the most likely explanation, which we determine to be that proposed by Hon et al.: in which the initial state of the system was that of a close binary, with 8 UMi b orbiting around both stars [Hon et al., 2023].