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Published online by Cambridge University Press: 24 September 2020
The high energy X-ray and UV radiation fields of host stars play a crucial role in determining the atmospheric conditions and habitability of potentially-habitable exoplanets. This paper focuses on the major surveys of the UV/X-ray emissions of M- and K-type exoplanet hosts that have been undertaken by the MUSCLES and MegaMUSCLES Hubble Space Telescope (HST) Treasury programs and associated contemporaneous X-ray and ground-based observations. The quiescent and flaring radiation (both photons and implied particles) were observed from this extensive sample of relatively old, low mass, exoplanet host stars and show that, from the viewpoint of a habitable-zone exoplanet, there is no such thing as an “inactive” M dwarf star. The resulting implications are significant for planetary habitability. Extensive monitoring of the X-ray/UV emission from a representative younger M dwarf is also presented and the direct stellar effects that influence exoplanets during the earlier phases of their formation and evolution discussed.