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Binary Evolution Leads to Two Populations of White Dwarf Companions

Planets and other low-mass binary companions to stars face a variety of potential fates as their host stars move off the main sequence and grow to subgiants and giants. Stellar mass loss tends to make orbits expand, and tidal torques tend to make orbits shrink, sometimes to the point that a companion is directly engulfed by its primary. Furthermore, once engulfed, the ensuing common envelope (CE) phase can result in the companion becoming fully incorporated in the primary's envelope; or, if the companion is massive enough, it can transfer enough energy to eject the envelope and remain parked in a tight orbit around the white dwarf core. Therefore, ordinary binary evolution ought to lead to two predominant populations of planets around white dwarfs: those that have been through a CE phase and are in short-period orbits, and those that have entirely avoided the CE and are in long-period orbits.