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 e, an ultra-short period Super-Earth, observed with the
Extreme Precision Spectrograph (EXPRES). Using the classical
Rossiter-McLaughlin (RM) method, we measure 55 Cnc e's sky-projected stellar
spin-orbit alignment (i.e., the projected angle between the planet's orbital
axis and its host star's spin axis) to be λ=10+17−20​∘ with an unprojected angle of ψ=23+14−12​∘. The best-fit RM model to the EXPRES data has a radial velocity
semi-amplitude of just 0.41+0.09−0.10​ms−1. The spin-orbit
alignment of 55 Cnc e favors dynamically gentle migration theories for
ultra-short period planets, namely tidal dissipation through low-eccentricity
planet-planet interactions and/or planetary obliquity tides.Comment: 12 pages, 4 figures, published in Nature Astronom