  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.
