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

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 (Teff=5050 ± 100 K). Planet b (TOI-880.02) has a radius of 2.19±0.11R⊕ and an orbital period of $2.6$ days; planet c (TOI-880.01) is a Neptune-sized planet with 4.95±0.20R⊕ on a $6.4$-day orbit; and planet d (TOI-880.03) has a radius of 3.40-0.21+0.22R⊕ and a period of $14.3$ days. By modeling the Rossiter-McLaughlin (RM) effect, we found the sky-projected obliquity to be |λc| = 7.4-7.2+6.8∘$, consistent with a prograde, well-aligned orbit. The lack of detectable rotational modulation of the flux of the host star and a low $ vsin{i_}$ (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 ~ 170. We could not detect clear signs of atmospheric erosion in the H$α$ line from TOI-880 c, as photoevaporation might have diminished for this mature planet.