When Bright Spots Go Dark

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