TOI-880 is an Aligned, Coplanar, Multi-planet System

Abstract

Although many cases of stellar spin-orbit misalignment are known, it is usually unclear whether a single planet's orbit was tilted or if the entire protoplanetary disk was misaligned. Measuring stellar obliquities in multi-transiting planetary systems helps to distinguish these possibilities. Here, we present a measurement of the sky-projected spin-orbit angle for TOI-880 c (TOI-880.01), a member of a system of three transiting planets, using the Keck Planet Finder (KPF). We found that the host star is a K-type star ($T_{\mathrm{eff}}=5050\pm 100$ K). Planet b (TOI-880.02) has a radius of $2.19\pm 0.11{\mathrm{R}}_{\oplus }$ and an orbital period of $2.6$ days; planet c (TOI-880.01) is a Neptune-sized planet with $4.95\pm 0.20{\mathrm{R}}_{\oplus }$ on a $6.4$-day orbit; and planet d (TOI-880.03) has a radius of $3.40_{-0.21}^{+0.22}{\mathrm{R}}_{\oplus }$ and a period of $14.3$ days. By modeling the Rossiter-McLaughlin (RM) effect, we found the sky-projected obliquity to be $|\hat{I}{»}_c|=7.4_{-7.2}^{+6.8}$${}^{\circ }$, consistent with a prograde, well-aligned orbit. The lack of detectable rotational modulation of the flux of the host star and a low $\mathrm{v}\sin {\mathrm{i}}_{\star }$ (1.6~km/s) imply slow rotation and correspondingly slow nodal precession of the planetary orbits and the expectation that the system will remain in this coplanar configuration. TOI-880 joins a growing sample of well-aligned, coplanar, multi-transiting systems. Additionally, TOI-880 c is a promising target for JWST follow-up, with a transmission spectroscopy metric (TSM) of $\sim 170$. We could not detect clear signs of atmospheric erosion in the H$\hat{I}\pm$ line from TOI-880 c, as photoevaporation might have diminished for this mature planet.