Obliquity Bimodality Is a Binary Signature

01The evidence

• High-eccentricity migration with disk-induced spin-orbit misalignment predicts a preference for perpendicular hot Jupiters, which is a specific and unusual signature rather than a generic tilt.Astrophysical Journal Letters
• The measured obliquity distribution of hot Jupiters around cool stars is broad and structured, with a population near alignment and a substantial tail extending to perpendicular and retrograde.Obliquity distribution

02The tension

Two channels, disk migration and high-eccentricity migration, are invoked to explain the aligned and misaligned populations. The trouble is that the channels are properties of the planetary system, so the prediction is a mixture whose proportions are free parameters, fitted after the fact.

03The theory

A stellar companion at the right separation truncates the protoplanetary disk, removing the gas that would damp inclination and circularise the orbit. The planet is left on an eccentric, inclined path and its obliquity is excited by Kozai-Lidov cycles or by chaotic evolution.

Without a companion, the disk survives, migration is smooth, and the star and orbit stay aligned. The binary is therefore the switch, and the bimodality is a bimodality in binarity rather than in migration physics.

The prediction is observational and cheap: aligned hot Jupiters should have fewer close companions than misaligned ones, and within the misaligned group the obliquity should correlate with companion separation through the Kozai timescale.

This also predicts that the bimodality should weaken in a sample selected against binaries, which is a way to check whether the structure is real or an artefact of heterogeneous detection.

04What this is not

05Predictions

• P1Aligned hot Jupiters should have a lower close-binary fraction than misaligned ones.
• P2Among misaligned systems, obliquity should correlate with companion separation through the Kozai timescale.
• P3The bimodality should disappear, or invert, in samples selected to exclude binaries.

06What would refute it

No correlation between obliquity and binarity, in a sample with well-characterised companions, would leave the migration mixture as the better description.

07What it would change

It would turn an exotic dynamical taxonomy into a stellar multiplicity census, and make binarity a required covariate in exoplanet obliquity surveys.