Exoplanet Orbit Reassessment
New transits weaken the case for perpendicular planets at HD 3167
Keck and ESPRESSO measurements no longer support an aligned inner planet, but the authors say the system's geometry remains unsettled.
Briefed September 18, 2026 · Aliens in the Clouds
Accepted Manuscript · Geometry Unsettled
HD 3167 became notable after its two transiting planets were reported to follow nearly perpendicular paths: the inner planet aligned with the star's equator and the outer planet nearly polar. That picture depended heavily on a difficult Rossiter-McLaughlin measurement from one transit of the inner world.
Three new Keck Planet Finder transits, analyzed with two archival ESPRESSO datasets, do not confirm the earlier low obliquity. The combined fit favors a projected angle of minus 66 degrees, which could make the two planetary orbits broadly coplanar. The stellar rotation estimate is unexpectedly high, however, and removing the most discrepant dataset greatly enlarges the uncertainty, so the accepted paper explicitly leaves the architecture unresolved.
Why it matters
This is evidence correction working properly: a visually memorable planetary arrangement becomes less certain when a fragile single-transit result meets additional measurements.
What the source establishes
Accepted ApJ Letters manuscript based on five transit datasets. The earlier perpendicular-orbit interpretation is weakened, but neither coplanarity nor the exact system geometry is securely established.
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