Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

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.

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 spectrum of a starspot. 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 using six different transit observations from 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.

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, in 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.

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.

The post The First Starspot Spectrum Revealed by JWST appeared first on AAS Nova.

Which of the nearly 6,000 known exoplanets have atmospheres? New research suggests that small, rocky planets around the smallest and coolest stars are even less likely to hang on to their atmospheres than suggested by previous research. This result will help to guide the selection of target exoplanets for atmospheric characterization.

Approaching a Milestone

Today, the Exoplanet Archive reports that humanity has discovered 5,921 exoplanets, bringing us only a handful of detections away from the remarkable milestone of 6,000 known exoplanets. Though just a small fraction of the hundreds of billions to trillions of worlds estimated to occupy our galaxy, this planet sample is ripe for the search for atmospheres, habitable surfaces, and even life. But where to look?

When seeking planets with atmospheres, researchers must consider how a planet has fared in its lifelong battle between gravity and radiation. Massive planets orbiting calm stars are more likely to have atmospheres than lightweight planets around active stars. Astronomers define the cosmic shoreline as the dividing line between planets with atmospheres and planets without, in terms of escape velocity and the extreme-ultraviolet flux of the star.

Charting the Cosmic Shoreline

To predict on which side of the cosmic shoreline a particular planet lies, it’s not enough to know the current extreme-ultraviolet flux of its host star. A star’s extreme-ultraviolet flux changes over time, and integrating these changes over the lifetime of a planet yields its position along the shoreline. In a new research article, Emily Pass (Massachusetts Institute of Technology) and collaborators have suggested that this integration hasn’t been performed correctly for certain planets.

As stars age, they spin more slowly, and this slowdown is accompanied by a decrease in atmosphere-stealing stellar activity. For all stars down through early type M dwarfs, the slowdown is uniform, but research has shown that for mid-to-late M dwarfs, the transition from fast to slow rotation is sudden. Pass and coauthors suspected that this difference could impact how much radiation the planets of mid-to-late M dwarfs receive over their lifetimes.

Crossing the Line

Pass’s team estimated the flux of mid-to-late M dwarfs during two phases of their lives: the “saturated” phase, during which the stars rotate rapidly and their high-energy flux does not scale with their rotation rate, and the “unsaturated” phase, during which the high-energy flux declines as the rotation rate slows. The team consulted the literature to find the X-ray flux of M dwarfs during the two phases, then used a statistical approach to estimate how long the stars spend in each of these phases. The team also considered the impact of stellar flares as well as the stars’ lengthy pre-main-sequence phase, during which their overall luminosity is higher than it is once the star has fully contracted and begun its main-sequence lifetime.

Ultimately, these factors mean that it’s more challenging than previously predicted for planets around mid-to-late M dwarfs to hold on to their atmospheres. This recalculation even pushed some planets from one side of the cosmic shoreline to the other. The team also noted that certain planets still tagged as potentially having atmospheres may in reality be gaseous rather than terrestrial, highlighting the need for more work to characterize these worlds and guide the selection of target exoplanets for JWST and other sensitive telescopes.

Citation

“The Receding Cosmic Shoreline of Mid-to-Late M Dwarfs: Measurements of Active Lifetimes Worsen Challenges for Atmosphere Retention by Rocky Exoplanets,” Emily K. Pass et al 2025 ApJL 986 L3. doi:10.3847/2041-8213/adda39

The post Planets Around Small Stars Even Less Likely to Have Atmospheres Than Previously Thought appeared first on AAS Nova.