Light and dark regions speckle the Sun and other stars, complicating the interpretation of exoplanet transmission spectra. New modeling suggests that faculae — bright areas on the visible surface of the Sun — appear dark on cooler stars, adding a new wrinkle to this challenge.

Dark Spot, Bright Spot

It’s an adage as old as exoplanet science itself: know thy star, know thy planet. This recommendation is key when using transmission spectroscopy to tease out the components of an exoplanet’s atmosphere; the backdrop to this process is a rotating star dotted with ever-changing active regions, and the better the star’s behavior is understood, the more reliable the extracted spectrum becomes.

The Sun is freckled with dark spots — sunspots — as well as bright regions called faculae. Sunspots form where the Sun’s magnetic field bubbles up, creating pockets of cooler, less luminous plasma. Solar faculae form in regions of concentrated magnetic field, creating bright spots that line the dark crevasses between convective cells.

Other stars also have starspots and faculae, but thus far, astronomers have no direct observations of faculae on any star except the Sun. Out of necessity, researchers interpreting exoplanet spectra assume that faculae on other stars are similar to those on the Sun — but are they actually?

Would a Facula on Any Other Star Look as Bright?

To explore what faculae might look like on stars other than the Sun, Alexander Shapiro (University of Graz; Max Planck Institute for Solar System Research) and collaborators turned to three-dimensional radiative magnetohydrodynamics simulations and radiative transfer modeling. Their aim was to simulate the appearance and spectra of faculae on stars of spectral type G2, K2, M0, and M4 — from Sun-like to substantially smaller and cooler than the Sun.

The modeled faculae on the Sun-like G2 star were bright, as expected. On the K2 and M0 star, these features were fainter, with the faculae on the M0 star being roughly as bright as the surrounding stellar surface. On the M4 star, the faculae were darker than surrounding areas, upending the assumption that faculae on all stars are brighter than the stellar surface.

Exoplanetary Effects

The transition from bright faculae on Sun-like stars to dark faculae on M4-class stars directly affects interpretations of exoplanet transmission spectra. The existence of dark faculae increases the derived exoplanet radius, whereas bright faculae on Sun-like stars decrease the derived radius. The contamination due to the faculae is wavelength dependent, with the largest contamination occurring at shorter wavelengths. This contamination, if not fully accounted for, can mimic spectral features from an exoplanet’s atmosphere.

What determines whether faculae appear brighter or darker than their surroundings? Using a qualitative framework, Shapiro and coauthors found that the appearance of faculae can be traced to a few competing factors: the strong magnetic fields within faculae prevent the incursion of hotter material (making the spots appear darker), while the magnetic field lowers the opacity of the gas in that region, allowing us to peer deeper and glimpse hotter material (making the spots appear brighter). On cooler stars, effects related to higher surface gas pressure tilt the balance in favor of darker faculae.

Citation

“The Curious Case of Dark Faculae on M Dwarf Stars,” A. I. Shapiro et al 2026 ApJ 1008 24. doi:10.3847/1538-4357/ae7105

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This image shows the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA), a recently upgraded collection of 352 antennas spanning 2.4 kilometers east of the Sierra Nevada mountain range in California. A research team led by Ivey Davis (California Institute of Technology) recently outlined their survey that uses OVRO-LWA to study space weather on young Sun-like stars. The Study of Space Weather Around Young Suns (SWAYS) uses both OVRO-LWA and a 0.5-meter optical telescope to observe white-light stellar flares and any accompanying low-frequency radio emission from five stars with temperatures of 5000–6000K and ages of 100–800 million years. Though detections of stellar flares are common, the bursts of radio emission associated with flares and coronal mass ejections have yet to be identified conclusively on other stars. Previous studies have focused on M-dwarf stars, whose strong magnetic fields and plasma environments might limit the production of radio bursts, and have lacked the dedicated monitoring needed to detect transient bursts — problems that SWAYS has solved. To learn more about the instrumentation and design behind SWAYS, be sure to check out the research article linked below.

Citation

“A Dedicated System for Coordinated Radio and Optical Monitoring of the Space Weather of Young, Solar-Type Stars,” Ivey Davis et al 2025 ApJ 993 82. doi:10.3847/1538-4357/adfbe9

The post Featured Image: Studying the Space Weather of Young Suns appeared first on AAS Nova.