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Title: A Panchromatic JWST Spectrum of a Giant Starspot on the Fully Convective M Dwarf TOI-3884
Authors: C. A. Murray et al.
First Author’s Institution: University of Colorado Boulder
Status: Published in ApJ
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 star’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 stellar 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 following:
- 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, and by proxy how much dimmer
These models are promising, but how do we know that we have chosen the right parameters?
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.
Figure 1: Illustration of the TOI-3884 system. The pole (and rotation axis) of the star is indicated with a black “x,” indicating that we see the star almost pole-on. The polar starspot is indicated in grey, and the transiting planet TOI-3884b is indicated in black. Different dates are shown, illustrating how the system evolves over the span of a few weeks. [Adapted from Mori et al. 2025]
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.
Figure 2: The spectrum of TOI-3884’s starspot. The y-axis in the upper panel, the spot contrast, indicates how much dimmer the starspot is compared to the “normal” stellar surface. Different observations taken with JWST are indicated in different colored crosses, and other previous observations are overplotted in various other shapes. Model starspot spectra are shown in dashed or dotted black lines. The residuals between the model spectra and the JWST observations are shown in the bottom panel. [Adapted from Murray et al. 2026]
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.
Original astrobite edited by Natalie Price.
About the author, Elise Koo:
I’m a PhD student at the University of Amsterdam, working to detect magnetic interactions between stars and their planets using radio and spectroscopic observations. Outside of research, I like to try out a variety of sports.