Spot-crossing Variations Confirm a Misaligned Orbit for a Planet Transiting an M Dwarf

TOI-3884 b is an unusual 6.4 R ⊕ planet orbiting an M4 host, whose transits display large and persistent spot-crossing events. We used the Tierras Observatory to monitor both the long-term photometric variability of TOI-3884 and changes in the spot-crossing events across multiple transits of the planet. We show that the star rotates with a period of 11.020 ± 0.015 days. We simultaneously model the rotational modulation of the star and variations in transit shapes that arise due to rotation of the spot, allowing us to determine the true stellar obliquity, ψ ⋆ . The data are best described by a planet on a misaligned orbit around a highly inclined star ( ψ ⋆ = 77 . ° 4 − 2 . ° 5 + 2 . ° 3 ; i ⋆ = 22 . ° 3 − 1 . ° 6 + 1 . ° 8 ) that hosts a large polar starspot ( r spot = 31 . 2 ∘ − 1 . 9 ∘ + 2 . 4 ∘ ; λ spot = 80 . ° 5 ± 1 . ° 2). Archival photometry from the Zwicky Transient Facility suggests that this polar spot has persisted on TOI-3884 for at least seven years. The TOI-3884 system provides a benchmark for studying the evolution of a polar spot on an M dwarf.