Measured Spin-Orbit Alignment of Ultra-Short Period Super-Earth 55 Cancri e

Abstract We present two transits of 55 Cancri e radial velocities (RVs) 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=10.1(+17.4 -18.6)° with an unprojected angle of ψ=22.7(+14.4 -12.4)°. The best-fit RM model to the EXPRES data has a radial velocity semi-amplitude of just 0.41 (+0.09 -0.10) m s-1; this is the smallest amplitude classical RM measurement to date. A planet's orbital alignment places important constraints on how the planet formed and consequently evolved. The dominant formation pathway of ultra-short period planets (USPs; planets with periods less than one day), like 55 Cnc e, remains an open question. The low stellar obliquity of 55 Cnc e favors dynamically gentle migration theories for USPs, namely tidal dissipation through low-eccentricity planet-planet interactions and/or planetary obliquity tides.