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 ₄₅₅=5050 100 K). Planet b (TOI-880. 02) has a radius of 2. 190. 11R_ and an orbital period of 2. 6 days; planet c (TOI-880. 01) is a Neptune-sized planet with 4. 950. 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 vi_ (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 Hα line from TOI-880 c, as photoevaporation might have diminished for this mature planet.
Zhang et al. (Tue,) studied this question.