A Readily Implemented Atmosphere Sustainability Constraint for Terrestrial Exoplanets Orbiting Magnetically Active Stars

Abstract

With more than 4,300 confirmed exoplanets and counting, the next milestone in exoplanet research is to determine which of these newly found worlds could harbor life. Coronal Mass Ejections (CMEs), spawn by magnetically active, superflare-triggering dwarf stars, pose a direct threat to the habitability of terrestrial exoplanets as they can deprive them from their atmospheres. Here we develop a readily implementable atmosphere sustainability constraint for terrestrial exoplanets orbiting active dwarfs, relying on the magnetospheric compression caused by CME impacts. Our constraint focuses on a systems understanding of CMEs in our own heliosphere that, applying to a given exoplanet, requires as key input the observed bolometric energy of flares emitted by its host star. Application of our constraint to six famous exoplanets, (Kepler-438b, Proxima-Centauri b, and Trappist-1d, -1e, -1f and -1g), within or in the immediate proximity of their stellar host's habitable zones, showed that only for Kepler-438b might atmospheric sustainability against stellar CMEs be likely. This seems to align with some recent studies that, however, may require far more demanding computational resources and observational inputs. Our physically intuitive constraint can be readily and en masse applied, as is or generalized, to large-scale exoplanet surveys to detect planets that could be sieved for atmospheres and, perhaps, possible biosignatures at higher priority by current and future instrumentation.

Interior Heating of Rocky Exoplanets from Stellar Flares with Application to TRAPPIST-1

“…Hence while intrinsic planetary magnetic fields mitigate the impact of plasma particles on planetary atmospheres, at the same time they amplify external magnetic field variations, which leads to larger amounts of heat produced in the interior. Induced magnetospheres can also form around planets without dynamo fields, as with Mars and Venus (Ramstad & Barabash 2021), but their amplifying effect, if any, will be much smaller compared to intrinsic magnetospheres, and the peak magnetic field variations exerted on a planet will mostly be driven by magnetic energy carried within an ICME (Samara et al 2021). Presently there is no information on the existence or strength of an intrinsic magnetic field for any TRAPPIST-1 planets.…”

“…Randomly generated flares of a given energy are assumed to be associated with CME events. These events are propagated to the planet and the ICME magnetic field strength, M ICME , is calculated following the flux-rope model of Samara et al (2021;Appendix C). The peak external magnetic field strength at a planet is then obtained as…”

“…where r p is the distance from the center of a star, M å [T] is the magnetic field strength at the distance 10R å , and γ is the magnetic field decay rate. The model in Equation (C4) is adopted from Samara et al (2021), and factor M å , representing the magnetic field strength in the near-star zone, is derived from their mean model (Appendix A in Samara et al 2021). Following the analysis of extensive observational and MHD simulation data (Samara et al 2021), the decay rate γ was estimated to lie in the range [1.2, 2.0], with the most probable value of 1.6 for our solar system.…”

“…The model in Equation (C4) is adopted from Samara et al (2021), and factor M å , representing the magnetic field strength in the near-star zone, is derived from their mean model (Appendix A in Samara et al 2021). Following the analysis of extensive observational and MHD simulation data (Samara et al 2021), the decay rate γ was estimated to lie in the range [1.2, 2.0], with the most probable value of 1.6 for our solar system. We have verified this model independently by comparing it with the geoeffective solar CME events from the Large Angle and Spectrometric COronagraph catalog (Gopalswamy et al 2009) and found a good agreement.…”