Not so long ago, the idea of studying the atmosphere of a planet orbiting a distant star was the stuff of science fiction. Today, it is a daily reality for astronomers armed with instruments like NASA’s James Webb Space Telescope (JWST). With its extraordinary sensitivity, JWST can measure how much starlight filters through a planet’s atmosphere during a transit—when the planet passes in front of its star—and identify the fingerprints of atoms and molecules floating in alien skies.
We are now entering an age where differences as subtle as 0.01% in the depth of a transit can be detected. That level of precision is astonishing; it means we can distinguish between a world rich in water vapor and one dominated by methane, or identify whether hazes and clouds blanket its skies. Yet, this very precision introduces new challenges. Small imperfections on the host star—features once lost in the noise—suddenly matter. Among these, starspots play a particularly disruptive role.
The Hidden Influence of Starspots
Starspots are cooler, darker patches on a star’s surface caused by magnetic activity. On our Sun, they appear as sunspots, sometimes visible even to small telescopes. But on other stars, especially active red dwarfs, they can grow to enormous scales, covering large fractions of the stellar disk.
Why does this matter for exoplanet science? Imagine observing a planet transit across a star. If the planet happens to cross over a starspot, the star appears momentarily brighter than expected—because the planet is blocking a dimmer region, not the brighter stellar surface. Conversely, if large spots exist elsewhere on the star but remain un-crossed, the star appears dimmer overall, making the planet’s transit look deeper.
Both scenarios can mimic or obscure signals from a planet’s atmosphere. An absorption feature that looks like water vapor could, in part, be an illusion created by the spotted star itself. Correcting for these effects isn’t just important—it’s essential if we hope to extract the true story from the light curves we so carefully record.
A Rare and Fortunate Opportunity
This brings us to a particularly intriguing system: TOI-3884, a red dwarf star about 140 light-years away in the constellation of Pisces. Its planet, TOI-3884b, is a so-called super-Neptune—a world roughly six times the size of Earth.
What makes this system stand out is that every time the planet transits, astronomers observe a persistent spot-crossing signal. This means that TOI-3884b consistently passes in front of starspots during its orbit. Such systems are exceptionally rare. Most of the time, the alignment of spots and orbits is not so cooperative. But in TOI-3884, nature has given us a laboratory where we can study both the exoplanet and its star’s magnetic scars at the same time.
A Puzzle in the System’s Geometry
Earlier studies of TOI-3884 offered conflicting results. Was the star spinning rapidly or slowly? Was the planet’s orbit neatly aligned with the star’s equator, or tilted at a strange angle? Without clarity on these questions, interpreting atmospheric signals from TOI-3884b would be fraught with uncertainty.
That is where the latest international effort, led by scientists at the Astrobiology Center in Tokyo, comes in. Their work, published in The Astronomical Journal, combines ground-based precision photometry with clever monitoring campaigns to resolve the puzzle.
Watching Planets Cross Starspots
Between February and March 2024, the team used the MuSCAT3 and MuSCAT4 instruments, mounted on the Las Cumbres Observatory’s 2-meter telescopes, to capture three of TOI-3884b’s transits. These instruments are designed for simultaneous observations in multiple colors, a powerful way to probe spot properties.
The data were clear and striking: the planet passed over large starspots during each transit. By analyzing how the spot-crossing signals varied with color, the team determined that these spots are about 200 Kelvin cooler than the surrounding stellar surface (which itself burns at around 3150 K). In terms of coverage, spots occupy a staggering 15% of the visible stellar disk.
Even more fascinating, the shape of the spot-crossing signals changed over the course of just a few weeks. Were the spots themselves evolving so quickly, or was something else at play? The researchers suspected stellar rotation.
Pinning Down the Star’s Spin
To test this idea, the team conducted a photometric monitoring campaign between December 2024 and March 2025, using the global network of LCO 1-meter telescopes. By measuring the star’s brightness several times each night, they detected periodic fluctuations—a telltale signature of spots rotating in and out of view.
The result was definitive: TOI-3884 rotates once every 11.05 days. This finding fit perfectly with the shifting positions of the spots inferred from the transits. For the first time, the system’s rotational rhythm was pinned down with confidence.
A Dramatic Misalignment
With this information in hand, the team could finally solve the system’s geometry. The verdict was surprising: the star’s spin axis and the planet’s orbital axis are tilted relative to each other by about 62 degrees.
This kind of misalignment is not unheard of, but it raises intriguing questions. Typically, large orbital tilts are thought to arise from past gravitational interactions—perhaps with a massive outer planet or a nearby companion star. But in TOI-3884, no such companions have been detected. The planet’s unusual orbit remains a mystery, whispering of a past dynamical upheaval that we have yet to understand.
Why This Matters for Exoplanet Atmospheres
TOI-3884b is already considered a prime candidate for detailed atmospheric studies with JWST. Its large size, frequent transits, and relatively bright host star make it a natural target. But without accounting for the starspots, atmospheric interpretations could go badly astray.
Now, thanks to this study, astronomers know the starspot coverage, temperature contrast, and rotational geometry. These details will allow upcoming JWST observations to be corrected with far greater precision, ensuring that any detected molecules truly belong to the planet and not to the star’s mottled surface.
In other words, TOI-3884 is no longer just a target—it is a carefully calibrated laboratory for exoplanet science.
Beyond a Single Star
The implications reach beyond this one system. Large polar starspots—spots concentrated near a star’s poles—were once thought to exist mainly on rapidly spinning stars with strong magnetic fields. Yet TOI-3884 rotates modestly and still hosts a giant polar spot. This hints that polar spots may be common among red dwarfs, the very stars that make up the majority of our galaxy’s population.
If that is true, then starspot corrections will be essential for most exoplanet studies in the years ahead. The universe may not give us smooth, featureless stars to work with. Instead, it offers us stars scarred with magnetic storms—reminders that they are alive, dynamic, and restless.
A Universe Written in Shadows
Astronomy often feels like a conversation with shadows. We cannot touch exoplanets or scoop samples from their skies. Instead, we measure delicate dips of starlight and interpret them through the lens of physics. Starspots complicate that conversation, sometimes shouting over the planetary whispers we are trying to hear.
Yet, as the TOI-3884 study shows, even these obstacles can become opportunities. A dark blemish on a distant star transforms into a treasure trove of information—not only about the star’s own magnetic heart but also about the delicate alignment and history of its planetary system.
Every shadow has a story. And with telescopes like JWST and patient ground-based campaigns, we are learning to read those stories with ever greater clarity. TOI-3884b reminds us that the road to understanding alien worlds is not straightforward. It twists through the complexities of stellar activity, geometry, and magnetic storms. But it is precisely this complexity that makes the journey so endlessly fascinating.
More information: Mayuko Mori et al, Multiband, Multiepoch Photometry of the Spot-crossing System TOI-3884: Refined System Geometry and Spot Properties, The Astronomical Journal (2025). DOI: 10.3847/1538-3881/ade2df
