Title: A Panchromatic JWST Spectrum of a Giant Starspot on the Fully Convective M-dwarf TOI-3884
Authors: C. A. Murray, L. Garcia, B. V. Rackham, Z. Berta-Thompson, et al.
First Author’s Institution: Department of Astrophysical and Planetary Sciences, University of Colorado Boulder, Boulder, CO 80309, USA
Status: Submitted [open access on arXiv]
We often picture stars as smooth, glowing spheres, as if they’ve been run through an Instagram filter. But real stars have spots: cooler, darker regions on a star’s surface, caused by strong magnetic fields.
Annoyingly, these spots can seriously interfere with how we study exoplanet atmospheres.
How to probe a planetary atmosphere
Planetary atmospheres are often probed through transmission spectroscopy. When a planet transits its star, some starlight passes through the planet’s atmosphere before reaching us. We can distinguish the light that has passed through the planet’s atmosphere by comparing the planet’s light in transit versus out of transit. In doing so, we can isolate the atmospheric spectrum and look for absorption features of species like water, or oxygen.
Easy, right?
How starspots get in the way
Things get more complicated once we stop thinking of the star as if it’s been smoothed out by an Instagram filter. Starspots are cooler than the surrounding surface, which means they emit a different spectrum. They are also constantly changing: new spots can form, old ones can disappear, and the star’s rotation carries them in and out of view. This changes things.
First of all, your star’s spectrum changes over time, which could mean that the star’s spectrum out-of-transit is not the same as the star’s spectrum during transit. On the other hand, the planet may transit across a starspot instead of a “normal” stellar region. In this case, the light we measure during the transit is affected by the spot, and it is no longer accurate to directly compare it with the overall stellar light outside the transit.
Ultimately, absorption features that we thought were from the planetary atmosphere could actually be coming from starspot contamination instead.
Starspot models: our solution?
To deal with this, astronomers build starspot models, where they vary the spots in terms of:
- The surface covering fraction; this parameter tells us how much of the stellar surface is covered in spots
- The temperature contrast; this parameter tells us how much cooler (in ratio) the spot is compared to the rest of the stellar surface temperature, and by proxy how much dimmer
These models are promising, but how do we know that we have chosen the right parameters?
Figure adapted from M. Mori et al (2025).
A unique laboratory: TOI-3884
The TOI-3884 system is a unique laboratory for testing our starspot models. As seen in Figure 1, it has very convenient starspot geometry, with a large starspot located close to its pole. On top of that, we observe this star almost completely pole-on, which means that we always see this starspot, no matter how much the star rotates. To top this all off, the star hosts a close-in planet, which orbits the star from pole-to-pole.
The planet transits the star, and as it does so, it always passes over the polar starspot. This gives us a rare opportunity to probe the spectra of starspots. Similar to how we can use normal in/out of transit observations to probe a planetary atmosphere, we can now compare the observations during/after a “starspot transit”, to probe the starspot region. Today’s authors do exactly this, with six different transit observations from the James Webb Space Telescope (JWST).
So… how good are our models?
From the JWST observations, the authors extract a starspot spectrum for the first time. Figure 2 shows this as the spot contrast (how much dimmer the spot is than the surrounding stellar surface) plotted across different wavelengths. To see how well our models are doing, they compare this observed spectrum to two commonly used starspot models. The difference between the data and the models (the residual) is plotted in the lower panel.
Adapted from Figure 5 of the paper.
At wavelengths longer than about 1 micron, in the near-infrared regime, things look good! The residuals stay small, meaning the models do a solid job reproducing the observations. But move left into shorter wavelengths, the optical regime, and the agreement quickly falls apart. The residuals grow, and it becomes clear that the models are missing something.
There’s still work to be done before we can confidently probe planetary atmospheres at optical wavelengths without worrying about stellar contamination, and this observed starspot spectrum provides a unique benchmark to test future starspot models. For now, the near-infrared remains a safer and more reliable window for planet atmosphere studies.
Astrobite edited by Natalie Price
Featured image credit: adapted from Mayuko Mori
