Early JWST observations of TRAPPIST-1 have revealed an unexpected puzzle: energetic white-light flares (E>1030 erg) reach temperatures of only ∼3500--4000\,K, nearly three times cooler than typical solar flares, which peak around 9000--10000\,K. Here we explain this difference by identifying the physical mechanism that regulates flare temperatures on late M-dwarfs. The key factor is that in the cool, dense atmosphere of TRAPPIST-1, magnetic heating is strongly moderated by the dissociation of molecular hydrogen (H2) into atomic hydrogen. This "H2 dissociation thermostat" acts as an efficient energy sink, preventing flare regions from heating above ∼4000\,K. Our chemical equilibrium and heat capacity calculations show that this effect depends sensitively on stellar atmospheric pressure and the local abundance of H2. In hotter stars, from early M dwarfs to solar-type stars, the scarcity of molecular hydrogen renders this mechanism ineffective; instead, atomic hydrogen ionization limits flare temperatures near ∼9000\,K.
@article{arxiv.2601.00386,
title = {Molecular hydrogen controls the temperatures of flares on TRAPPIST-1},
author = {Alexander I. Shapiro Nadiia Kostogryz Sara Seager Veronika Witzke Julien de Wit Valeriy Vasilyev Astrid M. Veronig Robert Cameron Hardi Peter Sami K. Solanki},
journal= {arXiv preprint arXiv:2601.00386},
year = {2026}
}