TOI-880 is an Aligned, Coplanar, Multiplanet 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 multitransiting 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. We found that the host star is a K-type star (T_eff = 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 days orbit; and planet d (TOI-880.03) has a radius of 3.40^+0.22_−0.21 R ⊕ and a period of 14.3 days. By modeling the Rossiter-McLaughlin effect, we found the sky-projected obliquity to be ∣ λ c ∣ = 7.4^+6.8°_−7.2, consistent with a prograde, well-aligned orbit. The lack of detectable rotational modulation of the flux of the host star and a low v sin i ⋆ (1.6 km s⁻¹) 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, multitransiting systems. Additionally, TOI-880 c is a promising target for James Webb Space Telescope follow-up, with a transmission spectroscopy metric 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.