Formation pathway of close-in planets around white dwarfs

Determine the dynamical and evolutionary mechanisms that place giant planets into close-in orbits around white dwarfs at orbital distances that should have resulted in engulfment during the progenitor star’s red-giant phase, reconciling observations such as WD 1856+534 b at a semimajor axis of 0.0204 AU with stellar evolution and migration theories (e.g., common-envelope evolution versus high-eccentricity tidal migration).

Background

WD 1856+534 b is a Jupiter-sized planet on a 1.4-day orbit around a white dwarf, residing in the so-called “forbidden zone” where a planet is expected to be engulfed during the host’s red-giant phase. This challenges standard expectations from stellar evolution.

Proposed explanations include common-envelope (CE) evolution and high-eccentricity tidal migration (HETM). Recent JWST constraints suggest the planet’s mass (≤6 MJ) is likely too small for successful CE ejection, lending weight to migration scenarios but leaving the precise pathway unresolved.