Nitrogen is a critical ingredient of complex biological molecules 1. Molecular nitrogen, however, which was outgassed into the Earth's early atmosphere 2 , is relatively chemically inert and nitrogen fixation into more chemically reactive compounds requires high temperatures. Possible mechanisms of nitrogen fixation include lightning, atmospheric shock heating by meteorites, and solar ultraviolet radiation 3,4. Here we show that nitrogen fixation in the early terrestrial atmosphere can be explained by frequent and powerful coronal mass ejection events from the young Sun-so-called superflares. Using magnetohydrodynamic simulations constrained by Kepler Space Telescope observations, we find that successive superflare ejections produce shocks that accelerate energetic particles, which would have compressed the early Earth's magnetosphere. The resulting extended polar cap openings provide pathways for energetic particles to penetrate into the atmosphere and, according to our atmospheric chemistry simulations, initiate reactions converting molecular nitrogen, carbon dioxide and methane to the potent greenhouse gas nitrous oxide as well as hydrogen cyanide, an essential compound for life. Furthermore, the destruction of N 2 , CO 2 and CH 4 suggests that these greenhouse gases cannot explain the stability of liquid water on the early Earth. Instead, we propose that the e cient formation of nitrous oxide could explain a warm early Earth. Here we develop a new concept for the rise of prebiotic chemistry on early Earth that suggests abiotic nitrogen fixation mediated by the energy flux from palaeo-solar eruptive events. The flare statistics of Kepler data suggest that the frequency of occurrence of superflares with energies >5 × 10 34 erg observed on G-type dwarfs follows a power-law distribution with a spectral index between α = −2.0 and −2.3, which is comparable to those observed on active M-type red-dwarf stars and the Sun 5,6. If the occurrence rate of superflares on young solar-like stars is ∼0.1 events per day 6 , then the frequency of super Carrington-type flare events with energy

The presence of an atmosphere over sufficiently long timescales is widely perceived as one of the most prominent criteria associated with planetary surface habitability. We address the crucial question of whether the seven Earth-sized planets transiting the recently discovered ultracool dwarf star TRAPPIST-1 are capable of retaining their atmospheres. To this effect, we carry out numerical simulations to characterize the stellar wind of TRAPPIST-1 and the atmospheric ion escape rates for all of the seven planets. We also estimate the escape rates analytically and demonstrate that they are in good agreement with the numerical results. We conclude that the outer planets of the TRAPPIST-1 system are capable of retaining their atmospheres over billion-year timescales. The consequences arising from our results are also explored in the context of abiogenesis, biodiversity, and searches for future exoplanets. In light of the many unknowns and assumptions involved, we recommend that these conclusions must be interpreted with due caution.

Stellar winds from active solar-type stars can play a crucial role in removal of stellar angular momentum and erosion of planetary atmospheres. However, major wind properties except for mass loss rates cannot be directly derived from observations. We employed a three-dimensional magnetohydrodynamic Alfvén wave driven solar wind model, ALF3D, to reconstruct the solar wind parameters including the mass loss rate, terminal velocity and wind temperature at 0.7, 2 and 4.65 Gyr. Our model treats the wind thermal electrons, protons and pickup protons as separate fluids and incorporates turbulence transport, eddy viscosity, turbulent resistivity, and turbulent heating to properly describe proton and electron temperatures of the solar wind. To study the evolution of the solar wind, we specified three input model parameters, the plasma density, Alfvén wave amplitude and the strength of the dipole magnetic field at the wind base for each of three solar wind evolution models that are consistent with observational constrains. Our model results show that the velocity of the paleo solar wind was twice as fast, ~ 50 times denser and 2 times hotter at 1 AU in the Sun's early history at 0.7 Gyr. The theoretical calculations of mass loss rate appear to be in agreement with the empirically derived values for stars of various ages. These results can provide realistic constraints for wind dynamic pressures on magnetospheres of (exo)planets around the young Sun and other active stars, which is crucial in realistic assessment of the Joule heating of their ionospheres and corresponding effects of atmospheric erosion.

Wm present the numerical simulations of winds from evolved giantntazo using u fully nnu-Uuear, time dependent 2.5-dimensional o (MBD) code. This ou/dv extends our pomimm fully non-linear MBD wind simulations to include mbmadbmnd frequency spectrum of &1fv6u p/uvou that drive winds from red giant stars. We calculated four Alfvtu wind models that cover the vvbo)u range of&|Mu vvuvo frequency upcuknmo to characterize the role of freely propagated and reflected &lFv6n waves in the gravitationally aomi6od atmosphere of late-type giant star. Our simulations demonstrate that, unlike linear /\|{v6n vvuvc-ddvo/ wind oondo|e ` a stellar wind model based on plasma acceleration due to broadband non-linear Alfv6n waves, can cuumioreu1k/ reproduce the wide range of observed radial velocity profiles of the winds, their terminal me|ucidca and the observed mass |oox rates. Comparison of the cadoulatoduaua loss rates with the oogpidcuUy dcieoniucd couxa loss rate for u Tau suggests an uois000pin and timedependent nature of stellar winds from evolved giants.