To study the moments when the planet crossed over starspots, researchers relied on the multicolor MuSCAT3 and MuSCAT4 instruments installed on the Las Cumbres Observatory (LCO) 2-meter telescopes. During February and March 2024, they recorded three separate transits and clearly identified signals produced by the planet passing over these dark regions on the star. The way the signal changed with color offered valuable clues about the temperature of the starspots.
Analysis of the light curves showed that the starspots are approximately 200 K cooler than the surrounding stellar surface (3150 K) and cover about 15% of the visible area of the star. The three transit observations also displayed subtle differences in the shape of the spot-crossing features. Since these changes happened over a relatively short period, they are more consistent with the star rotating rather than the starspots themselves evolving.
Monitoring Brightness to Measure Stellar Rotation
To verify this idea, the team conducted an extended photometric monitoring program using LCO’s network of 1-meter telescopes around the world. From December 2024 through March 2025, they tracked the star’s brightness several times each night and identified regular, repeating variations. These measurements allowed them to determine, for the first time, that the star completes a full rotation in 11.05 days.
A Strongly Tilted Planetary System
The rotation period matched the shifts in starspot position seen in the transit data, allowing the researchers to piece together the three-dimensional layout of the system. Their analysis showed that the star’s rotation axis and the planet’s orbital axis differ by roughly 62°, meaning TOI-3884 hosts a significantly tilted planetary orbit. Such extreme misalignments are usually linked to past interactions with massive planets or stellar companions — yet none have been found here, making TOI-3884 an especially compelling system to study.
Glossary of Key Terms
Transit: A transit occurs when a planet passes in front of its star from our point of view, causing a small dip in the star’s brightness.
Spot-Crossing Signal: A change in the transit light curve that happens when a planet moves across a darker, cooler starspot on the star’s surface.
Starspot: A relatively cool, dark region on a star, similar to sunspots on the Sun.
Light Curve: A graph showing how a star’s brightness changes over time. Scientists study its shape to learn about planets, starspots, and stellar behavior.
MuSCAT3 and MuSCAT4: Specialized multicolor cameras designed to measure changes in starlight across different wavelengths to improve transit and starspot analysis.
Photometric Monitoring: Repeatedly measuring the brightness of a star to detect variations caused by rotation, starspots, or orbiting planets.
Stellar Rotation Period: The time it takes for a star to complete one full spin on its axis.
Orbital Axis: An imaginary line defining the orientation of a planet’s orbit around its star.
Stellar Spin Axis: An imaginary line that describes the direction of a star’s rotation.
Misalignment (Orbital Tilt): When the star’s rotation axis and the planet’s orbital axis do not line up. A large misalignment can provide clues about the system’s past.
LCO (Las Cumbres Observatory): A global network of telescopes used for continuous sky monitoring and time-sensitive observations such as transits.
