Measured spin-orbit alignment of ultra-short-period super-Earth 55 Cancri e

A planet's orbital alignment places important constraints on how a planet formed and consequently evolved. The dominant formation pathway of ultra-short-period planets (P < 1 day) is particularly mysterious as such planets most likely formed further out, and it is not well understood what drove their migration inwards to their current positions. Measuring the orbital alignment is difficult for smaller super-Earth/sub-Neptune planets, which give rise to smaller amplitude signals. Here we present radial velocities across two transits of 55 Cancri (Cnc) e, an ultra-short-period super-Earth, observed with the Extreme Precision Spectrograph. Using the classical Rossiter-McLaughlin method, we measure 55 Cnc e's sky-projected stellar spin-orbit alignment (that is, the projected angle between the planet's orbital axis and its host star's spin axis) to be λ =10_-20°^+17° with an unprojected angle of ψ =23_-12°^+14°. The best-fit Rossiter-McLaughlin model to the Extreme Precision Spectrograph data has a radial velocity semi-amplitude of just 0.41_-0.10^+0.09 m s⁻¹. The spin-orbit alignment of 55 Cnc e favours dynamically gentle migration theories for ultra-short-period planets, namely tidal dissipation through low-eccentricity planet-planet interactions and/or planetary obliquity tides.