Reanalysis of HD 3167 b Transits Favors -66 Degree Obliquity, Undermining Perpendicular Orbit Claims

The study combines three new Keck Planet Finder transit observations with two archival ESPRESSO datasets to remeasure the Rossiter-McLaughlin effect during HD 3167 b transits. Previous claims of near-zero obliquity rested on a single marginal detection; the expanded dataset instead prefers a moderately misaligned orbit while noting that the best-fit stellar rotation velocity exceeds expectations from spectral type. This tension suggests either underestimated systematics or an incomplete model of the stellar surface.

Context from the broader sample of compact multi-planet systems shows that high mutual inclinations are rare when both planets transit, yet dynamical excitation by distant companions or disk warping can produce such configurations. HD 3167 c's near-polar orbit, if confirmed, would require a mechanism capable of tilting only the outer planet after the inner one formed, a scenario not easily accommodated by standard migration theory.

The geometry remains unsettled because omitting the most discrepant transit inflates the obliquity uncertainty substantially. Higher-precision spectroscopy or Doppler tomography on future transits could resolve whether the apparent misalignment is physical or an artifact of correlated noise.

Next steps include joint modeling with the outer planet's orbit and continued monitoring to test whether the system truly deviates from the coplanar norm observed in most Kepler multis.