A planet crossing starspots reveals the detailed architecture of the TOI-3884 system

To capture the spot-crossing transits, the team used the multicolor MuSCAT3 and MuSCAT4 instruments mounted on the Las Cumbres Observatory (LCO) 2-meter telescopes. Between February and March 2024, they observed three transits and successfully detected clear spot-crossing signals. The color dependence of the signal provides critical information about starspot temperature.

Light curve analysis revealed that the starspots are about 200 K cooler than the stellar surface (3150 K) and cover roughly 15% of the visible stellar disk. Also, the three transit light curves show changes in the shape of the spot-crossing signal. Because these variations occurred over a short timescale, they are more likely caused by stellar rotation than by spot evolution.

To confirm this, the team carried out a photometric monitoring campaign using the global network of LCO 1-meter telescopes. From December 2024 to March 2025, they measured the star’s brightness variations several times per night and detected clear periodic fluctuations. This revealed, for the first time, that the stellar rotation period is 11.05 days.

The measured rotation period was consistent with the spot position shifts inferred from the transit observations, enabling the team to obtain a unique solution for the system geometry. They found that the stellar spin axis and the planet’s orbital axis are misaligned by about 62°, revealing that TOI-3884 is a highly tilted planetary system. Such large tilts are typically attributed to past gravitational interactions with massive planets or stellar companions—yet no such companions are known to exist, making this system particularly intriguing.