Presentation #106.03 in the session Stars and Planets 1.
Recent work has demonstrated that high levels of X-ray and UV activity on young M dwarfs may drive rapid atmospheric escape on temperate, terrestrial planets orbiting within the habitable zone. However, secondary atmospheres on planets orbiting older, less active M dwarfs may be stable and present more promising candidates for biomarker searches. In order to evaluate the potential habitability of Earth-like planets around old, inactive M dwarfs, the Mega-MUSCLES program acquired HST and Chandra observations of Barnard’s Star (GJ 699), a 10 Gyr old M3.5 dwarf. Despite the old age and long rotation period of Barnard’s star, we observed two large FUV flares and one X-ray flare, and estimate a high-energy flare duty cycle of approximately 25 %.
In this talk, we present the stellar observations and evaluate the atmospheric stability of a hypothetical, unmagnetized terrestrial planet in the habitable zone around Barnard’s star. Both thermal and non-thermal escape modeling indicate the quiescent stellar XUV flux does not lead to strong atmospheric escape — atmospheric heating rates are comparable to periods of high solar activity on modern Earth. However, the flare environment could drive the atmosphere into a hydrodynamic loss regime at the observed flare duty cycle — sustained exposure to the flare environment of GJ 699 results in the loss of approximately 90 earth atmospheres per Gyr through thermal and non-thermal escape processes. These results suggest that the UV flare duty cycle may be the controlling stellar parameter for the stability of Earth-like atmospheres around old M stars.