Kavli Affiliate: Andrew Vanderburg
| First 5 Authors: Emily K Pass, David Charbonneau, Andrew Vanderburg, ,
| Summary:
Detecting and characterizing the atmospheres of terrestrial exoplanets is a
key goal of exoplanetary astronomy, one that may now be within reach given the
upcoming campaign to conduct a large-scale survey of rocky M-dwarf worlds with
the James Webb Space Telescope. It is imperative that we understand where known
planets sit relative to the cosmic shoreline, the boundary between planets that
have retained atmospheres and those that have not. Previous works modeled the
historic XUV radiation received by mid-to-late M-dwarf planets using a scaling
relation calibrated using more massive stars, but fully convective M dwarfs
display unique rotation/activity histories that differ from Sun-like stars and
early M dwarfs. We synthesize observations of the active lifetimes of
mid-to-late M dwarfs to present an updated estimate of their historic XUV
fluence. For known planets of inactive, mid-to-late M dwarfs, we calculate a
historic XUV fluence that is 2.1-3.1 times the canonical XUV scaling relation
on average, with the larger value including corrections for the
pre-main-sequence phase and energetic flares. We find that only the largest
terrestrial planets known to orbit mid-to-late M-dwarfs are likely to have
retained atmospheres within the cosmic shoreline paradigm. Our calculations may
help to guide the selection of targets for JWST and may prove useful in
interpreting the results; to this end, we define a novel Atmosphere Retention
Metric (ARM) that indicates the distance between a planet and the cosmic
shoreline, and tabulate the ARM for known mid-to-late M-dwarf planets.
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