Kozai Migration Naturally Explains the White Dwarf Planet WD1856 b

The Jovian-sized object WD 1856 b transits a white dwarf (WD) in a compact 1.4 day orbit. Unlikely to have endured stellar evolution in its current orbit, WD 1856 b is thought to have migrated from much wider separations. Because the WD is old, and a member of a well-characterized hierarchical multiple, the well-known Kozai mechanism provides an effective migration channel for WD 1856 b. The tidal dissipation that makes this mechanism possible is sensitive to the mass of WD 1856 b, which remains unconstrained by observations. Moreover, the lack of tides in the star allows us to directly connect the current semimajor axis to the pre-migration one, from which we can infer the initial conditions of the system. By further requiring that planets must survive all previous phases of stellar evolution before migrating, we are able to constrain the main-sequence semimajor axis of WD 1856 b to have been ∼2.5 au, and its mass to be ≃0.7–3MJ. These mass limits put WD 1856 b firmly within the planet category. Furthermore, our inferred values imply that WD 1856 b was born a typical gas giant. We further estimate the occurrence rate of Kozai-migrated planets around WDs to be ${ \mathcal O }({10}^{-3}-{10}^{-4})$, suggesting that WD 1856 b is the only one in the TESS sample, but implying ${ \mathcal O }({10}^{2})$ future detections by the LSST survey. In a sense, WD 1856 b was an ordinary Jovian planet that underwent an extraordinary dynamical history.