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

TL;DR

This study measures the orbital alignment of the ultra-short period Super-Earth 55 Cancri e, providing insights into its formation and migration history through precise radial velocity observations and the Rossiter-McLaughlin effect.

Contribution

First measurement of the sky-projected spin-orbit alignment of 55 Cancri e using EXPRES, revealing a near-aligned orbit that supports gentle migration theories.

Findings

Spin-orbit angle λ ≈ 10° with uncertainties

Radial velocity semi-amplitude of 0.41 m/s

Supports low-eccentricity migration scenarios

Abstract

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 $\lambda=10\substack{+17\\ -20}^{\circ}$ with an unprojected angle of $\psi=23\substack{+14\\ -12}^{\circ}$. The best-fit RM model to the EXPRES data has a radial velocity semi-amplitude of just $0.41\substack{+0.09\\ -0.10} m s^{-1}$. 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.