A Preponderance of Perpendicular Planets

Observing the Rossiter–McLaughlin effect during a planetary transit allows the determination of the angle λ between the sky projections of the star's spin axis and the planet's orbital axis. Such observations have revealed a large population of well-aligned systems and a smaller population of misaligned systems, with values of λ ranging up to 180°. For a subset of 57 systems, we can now go beyond the sky projection and determine the 3D obliquity ψ by combining the Rossiter–McLaughlin data with constraints on the line-of-sight inclination of the spin axis. Here we show that the misaligned systems do not span the full range of obliquities; they show a preference for nearly perpendicular orbits (ψ = 80°–125°) that seems unlikely to be a statistical fluke. If confirmed by further observations, this pile-up of polar orbits is a clue about the unknown processes of obliquity excitation and evolution.