Existence of a past stellar flyby or ejected planetary perturber in WD 1856+534

Ascertain whether the WD 1856+534 system experienced a stellar flyby or once hosted a companion planet that was subsequently ejected, either of which could have provided the high-eccentricity excitation necessary for WD 1856+534 b’s inward migration, by developing observational or indirect diagnostics capable of confirming or ruling out these scenarios.

Background

High-eccentricity tidal migration requires an external perturber to excite the planet’s eccentricity. Candidates include additional planets, distant stellar companions, or a stellar flyby.

This study finds no compelling evidence for a bound planetary perturber via transit timing variations, leaving other possibilities such as a past flyby or ejected companion, which the authors note are hard to test directly.

References

It is not possible to confirm or rule out the possibility of a stellar flyby, and we are unlikely to find a companion planet if it was ejected from the system in the course of the interaction that excited WD\,1856\,b onto an eccentric orbit.

— Transit Timing of the White Dwarf-Cold Jupiter System WD 1856+534  (2511.21611 - Gendreau-Distler et al., 26 Nov 2025) in Section 5, Conclusions