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Multi-planet systems orbiting M dwarfs provide valuable tests of theories ofsmall planet formation and evolution. K2-3 is an early M dwarf hosting threesmall exoplanets (1.5-2.0 Earth radii) at distances of 0.07-0.20 AU. We measurethe high-energy spectrum of K2-3 with HST/COS and XMM-Newton, and useempirically-driven estimates of Ly-alpha and extreme ultraviolet flux. We useEXOFASTv2 to jointly fit radial velocity, transit, and SED data. Thisconstrains the K2-3 planet radii to 4% uncertainty and the masses of K2-3b andc to 13% and 30%, respectively; K2-3d is not detected in RV measurements. K2-3band c are consistent with rocky cores surrounded by solar composition envelopes(mass fractions of 0.36% and 0.07%), H2O envelopes (55% and 16%), or a mixtureof both. However, based on the high-energy output and estimated age of K2-3, itis unlikely that K2-3b and c retain solar composition atmospheres. We pass theplanet parameters and high-energy stellar spectrum to atmospheric models.Dialing the high-energy spectrum up and down by a factor of 10 producessignificant changes in trace molecule abundances, but not at a level detectablewith transmission spectroscopy. Though the K2-3 planets span the small planetradius valley, the observed system architecture cannot be readily explained byphotoevaporation or core-powered mass loss. We instead propose 1) the K2-3planets are all volatile-rich, with K2-3d having a lower density than typicalof super-Earths, and/or 2) the K2-3 planet architecture results from morestochastic processes such as planet formation, planet migration, and impacterosion.

The K2-3 system revisited: testing photoevaporation and core-powered mass loss with three small planets spanning the radius valley