NEID reveals that the young warm Neptune TOI-2076 b has a low obliquity

TOI-2076 b is a sub-Neptune-sized planet (R = 2.39 ± 0.10 R _⊕) that transits a young (204 ± 50 MYr) bright (V = 9.2) K-dwarf hosting a system of three transiting planets. Using spectroscopic observations obtained with the NEID spectrograph on the WIYN 3.5 m Telescope, we model the Rossiter-McLaughlin effect of TOI-2076 b, and derive a sky-projected obliquity of λ = − 3⁺¹⁶°_-15. Using the size of the star (R = 0.775 ± 0.015 R _⊙), and the stellar rotation period (P _rot = 7.27 ± 0.23 days), we estimate an obliquity of ψ = 18^{+10 °}_-9 (ψ < 34° at 95% confidence), demonstrating that TOI-2076 b is in a well-aligned orbit. Simultaneous diffuser-assisted photometry from the 3.5 m telescope at Apache Point Observatory rules out flares during the transit. TOI-2076 b joins a small but growing sample of young planets in compact multi-planet systems with well-aligned orbits, and is the fourth planet with an age ≲300 Myr in a multi-transiting system with an obliquity measurement. The low obliquity of TOI-2076 b and the presence of transit timing variations in the system suggest the TOI-2076 system likely formed via convergent disk migration in an initially well-aligned disk.