The HD 191939 Exoplanet System is Well-Aligned and Flat

TL;DR

This study reveals that the HD 191939 exoplanet system is dynamically cold, flat, and well-aligned, with detailed measurements of the spin-orbit angle and orbital parameters of its planets, indicating a formation from a well-aligned protoplanetary disk.

Contribution

First measurement of the spin-orbit angle for HD 191939 b, and new constraints on the orbits of distant planets, demonstrating the system's overall coplanarity and flatness.

Findings

HD 191939 b's orbit is well-aligned with the star.

The distant super-Jupiter planet f has a mass of 2.88 M_J and a 2898-day orbit.

The inner planets are nearly coplanar with mutual inclinations less than 12 degrees.

Abstract

We report the sky-projected spin-orbit angle $\lambda$ for HD 191939 b, the innermost planet in a 6 planet system, using Keck/KPF to detect the Rossiter-McLaughlin (RM) effect. Planet b is a sub-Neptune with radius 3.4 $\pm$ 0.8 R$_{\oplus}$ and mass 10.0 $\pm$ 0.7 M$_{\oplus}$ with an RM amplitude $<$1 ms$^{-1}$. We find the planet is consistent with a well-aligned orbit, measuring $\lambda= \, $ 3.7 $\pm$ 5.0 degrees. Additionally, we place new constraints on the mass and period of the distant super-Jupiter, planet f, finding it to be 2.88 $\pm$ 0.26 $M_J$ on a 2898 $\pm$ 152 day orbit. With these new orbital parameters, we perform a dynamical analysis of the system and constrain the mutual inclination of the non-transiting planet e to be smaller than 12 degrees relative to the plane shared by the inner three transiting planets. Additionally, the further planet f is inclined off this shared plane, the greater the amplitude of precession for the entire inner system, making it increasingly unlikely to measure an aligned orbit for planet b. Through this analysis, we show that this system's wide variety of planets are all well-aligned with the star and nearly co-planar, suggesting that the system formed dynamically cold and flat out of a well-aligned proto-planetary disk, similar to our own solar system.