Exoplanetology: Exoplanets & Exomoons

The Receding Cosmic Shoreline of Mid-to-Late M Dwarfs: Measurements of Active Lifetimes Worsen Challenges for Atmosphere Retention by Rocky Exoplanets

By Keith Cowing
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astro-ph.EP
April 3, 2025
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The Receding Cosmic Shoreline of Mid-to-Late M Dwarfs: Measurements of Active Lifetimes Worsen Challenges for Atmosphere Retention by Rocky Exoplanets
The sample of known transiting terrestrial planets that orbit mid-to-late M dwarfs within 50pc. Our estimates are noted by solid circles, including PMS and flare corrections. For comparison, the predictions from the Zahnle & Catling (2017) scaling law are shown as open circles. We find that our revised estimates result in historic XUV fluences that are 2.1 times the scaling relation on average, or 3.1 times after considering the PMS and flare corrections. The dashed line indicates the cosmic shoreline as defined in Zahnle & Catling (2017): IXUV ∝ v4esc, with the constant of proportionality defined such that Mars sits on the shoreline. For IXUV in Earth units and vesc in kms−1 , this equation can be written as IXUV = 6.9 × 10−4 v4 esc. The underlying heatmap is colored by our Atmosphere Retention Metric (ARM), indicating the log-space distance from the cosmic shoreline. For legibility, only stars within 15pc are labeled by name; however, all values are given in Table 2. Stars that are still rapidly rotating at the present day are noted with an asterisk; our estimates may be inaccurate in these cases, as the correct answer will be dependent on the age of the star and reliable age estimates for M dwarfs are generally unavailable. — astro-ph.EP

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.

Emily K Pass, David Charbonneau, Andrew Vanderburg

Comments: Submitted to AAS Journals; 11 pages, 3 figures, 2 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2504.01182 [astro-ph.EP] (or arXiv:2504.01182v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2504.01182
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Submission history
From: Emily Pass
[v1] Tue, 1 Apr 2025 20:49:14 UTC (283 KB)
https://arxiv.org/abs/2504.01182
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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