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

TL;DR

This study reveals that the planet TOI-3884~b orbits an M dwarf star with a highly inclined, misaligned orbit, confirmed through spot-crossing variations and stellar rotation analysis, highlighting a persistent polar starspot over years.

Contribution

We demonstrate a novel method combining transit spot-crossing events and stellar rotation modeling to measure the true obliquity of a planet around an M dwarf star.

Findings

The star's rotation period is 11.02 days.

The planet's orbit is highly misaligned with a stellar obliquity of 77.4°.

A large, persistent polar starspot was identified.

Abstract

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.