When an exoplanet passes in front of its star, irregularities in the light curve can betray the presence of dark regions on the star's surface, providing clues about magnetic activity and rotation. These phenomena, although rare, allow astronomers to study details of distant planetary systems.
An international team focused its work on the TOI-3884 system, a red dwarf located about 140 light-years from Earth. Their research, published in
The Astronomical Journal, highlights starspots observed during transits of the planet TOI-3884b, a super-Neptune.
Artist's impression of the TOI-3884 system: the super-Neptune TOI-3884b transiting in front of the red dwarf star TOI-3884, which has a large starspot. (using generative AI tools and image editing).
Credit: Mayuko Mori, Astrobiology Center
To capture these events, scientists used the MuSCAT3 and MuSCAT4 instruments installed on the 2-meter telescopes at Las Cumbres Observatory. Between February and March 2024, three transits were observed, with clear signals of spot crossings detected in different wavelength bands. Analysis of the light curves showed that the spots are about 200 kelvins cooler than the stellar surface at 3150 K and cover nearly 15% of the visible disk, providing important data about their nature.
Variations in signal shape over a short period indicate that the star's rotation is the main cause, rather than a transformation of the spots. To verify this, an observation campaign was conducted with the global network of 1-meter telescopes at Las Cumbres Observatory. From December 2024 to March 2025, regular brightness measurements showed periodic fluctuations, establishing for the first time that TOI-3884's rotation period is 11.05 days, which matches the spot movements.
The consistency between the rotation period and spot positions helped understand the system's geometry, revealing a misalignment of about 62 degrees between the stellar rotation axis and the planet's orbital axis. This marked inclination, often linked to past gravitational interactions, makes TOI-3884 particularly interesting, as no massive companion has been detected. Such misalignments can affect the interpretation of atmospheric data and the understanding of planetary formation.
Light curves (bottom) of three transits with spot crossings of TOI-3884b observed with MuSCAT3 and MuSCAT4, with models (top) showing the relative positions of the planet and the starspot during each event. From left to right, the panels correspond to three different transit epochs. The four colors represent observations in four wavelength bands (g, r, i, z).
Credit: Mayuko Mori, Astrobiology Center
These results have direct consequences for atmospheric studies with instruments like the James Webb Space Telescope (JWST). Accurate characterization of spots and the orbit will help prevent errors in the analysis of exoplanetary atmospheres. Furthermore, the presence of a large polar spot on a moderately rotating star suggests that such structures could be common in red dwarfs, offering opportunities for future research on stellar magnetic activity.
The study of TOI-3884 shows how transit observations can provide details about starspots and alignments. In the future, comparable methods could be applied to other systems, enabling a broader understanding of planetary diversity and stellar processes.