Presentation #401.02 in the session Plenary 6.
Multi-planet systems offer insight into a host star’s influence on its planets’ atmospheres. K2-3 is an M0 star (effective temperature of 3880 K) orbited by three small transiting planets, K2-3b, c, and d (2.0, 1.6, and 1.4 Earth radii, respectively). We combine stellar spectral information with self-consistently derived stellar and planetary properties to investigate the K2-3 system. We measure the UV flux of K2-3 with HST/COS and place an upper limit on its X-ray flux with XMM-Newton. We use empirical scaling techniques and a differential emission measure to fill in gaps in the high energy spectrum. We constrain the K2-3 system properties by simultaneously fitting transit photometry, radial velocity, and spectral energy density information. The radii of all three planets are constrained to 2% and the masses of the inner two planets, K2-3b and c, to 10%, while the outermost planet, K2-3d, is not detected in the RV measurements. K2-3b and c are enhanced in volatiles and consistent with sub-Neptune-like compositions, while K2-3d is likely terrestrial due to its small radius. The compositions of sub-Neptune planets are highly degenerate, and indeed we find that the radii and masses of K2-3b and c allow for solar composition envelopes making up 0.30% and 0.05% of their masses, respectively, as well as water steam atmospheres making up for 50% and 13% of their masses. However, the solar composition scenarios are unlikely given the high energy output of K2-3; the timescale for atmospheric escape (1-2 Gyr) is short compared to the estimated age of the system (3-12 Gyr). Additionally, assuming that K2-3d is terrestrial, we find that the architecture of the K2-3 planets is inconsistent with photoevaporation as the sole mechanism for sculpting their atmospheres. Rather we must invoke more stochastic processes such as gas-depleted planet formation or giant impacts to account for the observed system architecture. Finally, we present model transmission spectra that include photochemical reactions and disequilibrium chemistry for a range of atmospheric cases: for K2-3 b and c we model the aforementioned solar composition and steam atmosphere cases, while for K2-3d we model early Venus, modern Venus, and modern Earth cases. Discriminating between these atmospheric cases will require spectral information about the atmospheres of the K2-3 planets, something that is achievable with current observatories. This work serves as a template for future investigations into the atmospheres of small exoplanets in multi-planet systems in the context of their host stars.