Bold claim: a planet is orbiting a star at a wildly tilted angle, defying easy explanation. And this is where the story gets even more intriguing: the clues come from watching the planet cross dark starspots and measuring how those signals vary with color. Researchers used the multicolor MuSCAT3 and MuSCAT4 instruments mounted on Las Cumbres Observatory’s 2-meter telescopes to capture three separate transits in February and March 2024. By analyzing how the transit signals changed across different wavelengths, they gained valuable insights into the temperature and distribution of the starspots.
What the data showed: the starspots are about 200 K cooler than the surrounding stellar surface, which sits at roughly 3150 K, and they cover about 15% of the star’s visible surface. The three observed spot-crossing events also exhibited subtle differences in their shapes. Because these changes occurred over a short timescale, they’re interpreted as the star rotating rather than the spots evolving significantly during the observations.
Tracking rotation to unlock geometry: to test the rotation hypothesis, the team launched an extended photometric monitoring campaign using LCO’s worldwide network of 1-meter telescopes. From December 2024 through March 2025, they measured the star’s brightness multiple times each night, detecting regular, repeating brightness variations. This enabled them to determine, for the first time, that the star completes a full rotation every 11.05 days.
A highly tilted system: the rotation period aligned with the observed shifts in starspot positions during transit, allowing reconstruction of the system’s three-dimensional arrangement. The analysis revealed that the star’s rotation axis and the planet’s orbital axis differ by about 62°, indicating a markedly tilted planetary orbit around TOI-3884. Such extreme misalignment is often linked to historical gravitational interactions with massive planets or stellar companions, yet no such companions have been found here, making TOI-3884 a particularly compelling case for study.
Glossary of key terms
- Transit: When a planet passes in front of its star from our viewpoint, causing a small dip in the star’s brightness.
- Spot-crossing signal: A feature in the transit light curve created when the planet traverses a cooler, darker starspot.
- Starspot: A relatively cool, dark region on a star, similar to sunspots on the Sun.
- Light curve: A plot showing how a star’s brightness changes over time; shapes in the curve help reveal planets, starspots, and stellar activity.
- MuSCAT3 and MuSCAT4: Multicolor cameras designed to measure how starlight changes across wavelengths, improving transit and starspot analyses.
- Photometric monitoring: Repeated brightness measurements to detect rotation, starspots, or orbiting planets.
- Stellar rotation period: The time a star takes to complete one full spin on its axis.
- Orbital axis: The imaginary line that defines a planet’s orbit orientation around its star.
- Stellar spin axis: The direction of the star’s rotation.
- Misalignment (orbital tilt): When the star’s rotation axis and the planet’s orbital axis are not aligned; large misalignments offer clues about a system’s history.
- LCO (Las Cumbres Observatory): A global telescope network used for continuous sky monitoring and time-sensitive observations such as transits.
