When astronomers observe a planet passing in front of its star, the slight dip in starlight reveals a wealth of information.
This method, known as a transit, has become the cornerstone of exoplanet discovery and atmospheric study.
But a new international study shows that transits can also reveal something else—dark, cool blemishes on stars called starspots.
Researchers from the Astrobiology Center in Tokyo have combined ground-based observations to explore the planetary system TOI-3884 in extraordinary detail.
Their findings, published in The Astronomical Journal, not only describe the characteristics of the star’s spots but also uncover the surprising geometry of the system itself.
TOI-3884 is a small red dwarf star about 140 light-years away. It hosts TOI-3884b, a “super-Neptune” exoplanet roughly six times Earth’s radius. What makes this system special is that the planet’s transits consistently show the planet crossing directly in front of a starspot.
These rare spot-crossing events provide astronomers with a unique opportunity to study both the star and the planet’s orbit at the same time.
The timing of this research is especially important. NASA’s James Webb Space Telescope (JWST) can now measure the depths of transits with breathtaking precision—down to differences of just 0.01%.
But this level of sensitivity also means that even small distortions from starspots can mimic or hide the chemical signals astronomers are trying to detect in a planet’s atmosphere. To properly interpret data from TOI-3884b, and similar planets, scientists must understand the influence of starspots.
The Bristol team focused on three transits of TOI-3884b observed between February and March 2024 with the MuSCAT3 and MuSCAT4 instruments at the Las Cumbres Observatory.
By capturing light curves in multiple colors, they were able to determine that the starspots are about 200 degrees cooler than the star’s surface temperature of 3150 Kelvin and cover around 15% of the visible disk. Changes in the transit signals suggested that these features are not quickly fading spots but instead are moving with the star’s rotation.
To confirm this, the team monitored the star’s brightness from December 2024 to March 2025 using the Las Cumbres Observatory’s global network of telescopes.
They found that the star’s brightness varied on an 11.05-day cycle, revealing the star’s rotation period for the first time. When they compared this result with the shifts in the transit spot-crossing signals, the data fit together perfectly.
The analysis led to a striking conclusion: the orbit of TOI-3884b is tilted by about 62 degrees relative to the star’s spin axis.
This means the planet’s path does not align with the star’s rotation—a surprising discovery, since such extreme tilts are usually explained by the gravitational pull of a nearby giant planet or a second star. Yet no such companions have been found around TOI-3884, making the system a mystery.
These insights carry wider implications. Large polar starspots are often linked to rapid stellar rotation, but TOI-3884 rotates at a moderate pace and still hosts a huge polar spot. This finding hints that such features may be common among red dwarfs, challenging current ideas about stellar magnetic activity.
TOI-3884b is already considered one of the best candidates for atmospheric study with JWST and other telescopes.
Thanks to this new work, scientists now have a clearer picture of the star’s magnetic quirks and the planet’s tilted orbit—information that will be essential for interpreting the atmospheric data to come.
