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K2-25b is a Neptune-sized exoplanet (3.45 Earth radii) that orbits its M4.5host with a period of 3.48 days. Due to its membership in the Hyades Cluster,the system has a known age (727 +/- 75 Myr). K2-25b's youth and itssimilarities with Gl 436b suggested that K2-25b could be undergoing strongatmospheric escape. We observed two transits of K2-25b at Lyman-alpha usingHST/STIS in order to search for escaping neutral hydrogen. We were unable todetect an exospheric signature, but placed an upper limit of (R_p/R_s) < 0.56at 95% confidence by fitting the light curve of the Lyman-alpha red-wing, or <1.20 in the blue-wing. We reconstructed the intrinsic Lyman-alpha profile ofK2-25 to determine its Lyman-alpha flux, and analyzed XMM-Newton observationsto determined its X-ray flux. Based on the total X-ray and extreme ultravioletirradiation of the planet (8763 +/- 1049 erg/s/cm^2), we estimated the maximumenergy-limited mass loss rate of K2-25b to be 10.6 x 10^10 g/s (0.56 Earthmasses per 1 Gyr), five times larger than the similarly estimated mass lossrate of Gl 436b (2.2 x 10^10 g/s). The photoionization time is about 3 hours,significantly shorter than Gl 436b's 14 hours. A non-detection of a Lyman-alphatransit could suggest K2-25b is not significantly losing its atmosphere, orfactors of the system are resulting in the mass loss being unobservable (e.g.,atmosphere composition or the system's large high energy flux). Furtherobservations could provide more stringent constraints.

A Lyman-alpha transit left undetected: the environment and atmospheric behavior of K2-25b