TOI-3884~b is an unusual 6.4~R$_\oplus$ planet orbiting an M4 host, whose transits display large and persistent spot-crossing events. We used the \textit{ 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 $11.020 \pm 0.015$~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, $\psi_\star$. The data are best described by a planet on a misaligned orbit around a highly inclined star ($\psi_\star = { 77.4^\circ} ^{ +2.3^\circ}_{ -2.5^\circ}$; $i_\star = { 22.3^\circ}^{ +1.8^\circ}_{ -1.6^\circ}$) that hosts a large polar starspot ($r_\mathrm{ spot} = { 31.2^\circ}^{ +2.4^\circ}_{ -1.9^\circ}$; $\lambda_\mathrm{ spot} = { 80.5^\circ}\pm1.2^\circ$). 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.
Categories: astro-ph.EP astro-ph.SR