Red Stars, Green Planets: Evaluating the Habitability of Planets via M-Dwarf Superflares

Even though thousands of exoplanets are known, unambiguous evidence of habitable worlds remains elusive. M-dwarfs are the majority of stars in our Galaxy and the majority of known rocky exoplanet hosts, but these stars frequently emit high-energy flares; superflares, with energies of at least 10³³ erg, are not uncommon. Repeated superflares can render planets uninhabitable, but at the same time, M-dwarfs do not emit the ultraviolet radiation that Earth-like life would need to catalyze prebiotic chemistry necessary for RNA synthesis except during powerful flares. Life around M-dwarfs may require that the host star’s flare activity be “just right,” with high-energy flares frequent enough to support prebiotic chemistry but not frequent enough to irreversibly damage planetary atmospheres. M-dwarf flares may even facilitate the detection of habitable planetary atmospheres: if the largest flares are followed by large coronal mass ejections (CMEs), an Earth-like atmosphere struck by such a CME would respond with a correspondingly intense auroral display. The most extreme exoaurorae could be visible from ground-based facilities with high-resolution spectroscopy targeting the brightest visible oxygen auroral emission line at 5577 Å. My research combines multiple observatories, disciplines, and observational techniques to investigate planetary habitability through M-dwarf superflares. Using Kepler and Evryscope data, I evaluated whether TRAPPIST-1’s superflares deplete ozone or facilitate abiogenesis on its planets, finding that its superflare rate is consistent with neither possibility. My ongoing work uses high-resolution spectroscopy to search for exoauroral emission in the aftermath of Evryscope-detected superflares from M-dwarf stars. This work will provide direct evidence of oxygen in a terrestrial exoplanet’s atmosphere, or, in the absence of an unambiguous detection, yield the best yet constraints on exoauroral emission from Earth-like planets orbiting M-dwarf stars.

Amy Glazier​ ​University of North Carolina, Chapel Hill