The Evaporation History of Three Planets around K2-136

I present an X-ray evaporation study of the three transiting planets orbiting K2-136, which span the period-radius valley. The K-dwarf host star is relatively young and active, being a member of the Hyades. This presents an opportunity to test different prescriptions of planetary evaporation, as the three planets will have experienced the same X-ray irradiation history.

I compare mass-loss histories of the three planets for different atmospheric escape models and conclude that K2-136d must have a mass at the upper end of the range allowed by HARPS-N radial velocity measurements in order to be consistent with mass loss histories of the other planets. I also present an XMM-Newton X-ray observation of K2-136, which show the star to be fainter than suggested by activity relations. This could be the result of either X-ray variability or intrinsically low X-ray levels.

The three planets (K2-136 b, c, d) have radii 1.0, 3.0, and 1.5 Earth radii (in order of orbital separation) and all are within 0.15 AU from their host star. I find that K2-136b is most likely rocky and its past envelope (if any) was evaporated within tens of Myr. Furthermore, K2-136c has a stable envelope that is unlikely to be lost within its lifetime, thanks to its high gravitational potential. Finally, I explore a set of scenarios for K2-136d, with masses ranging from 1.3 to 3.0 Earth masses, the 68% and 95% confidence upper mass limits of the HARPS-N measurements. I find that the heaviest case corresponds with an entirely rocky core, whereas the low-mass case warrants the presence of an envelope that should have already been evaporated. The simulations indicate that a mass of at least 2.0 to 2.5 Earth masses is required for the survival of an envelope.