Smaller Than Earth Habitability Model (STEHM) — 2026 framework introduced by michelle-l-hill et al. (UC Riverside) that couples atmospheric escape (Jeans escape under XUV irradiation) with interior thermal evolution (stagnant-lid cooling and volcanic outgassing) to set a ~0.8 Earth-radii lower size limit for habitable-zone exoplanets.
Key numbers
- ≥0.8 R⊕ → atmosphere retained for billions of years.
- ~0.7 R⊕ → sharp drop-off in retention.
- 0.6 R⊕ → ~400 Myr atmosphere lifetime.
- 0.5 R⊕ → ~30 Myr.
Mechanisms
- Atmospheric escape — low gravity / low escape velocity drives Jeans escape under stellar XUV.
- Interior cooling — high surface-area-to-volume ratio drives rapid cooling, lithospheric thickening, and shutdown of volcanic outgassing.
Assumptions and exceptions
- Assumes stagnant-lid tectonics (no plate tectonics) and a CO₂-dominant atmosphere — a best case for retention.
- Three rare exception channels for sub-0.8 R⊕ worlds: (1) large initial carbon budget; (2) low core-radius fraction (large mantle / no core); (3) “cold start” mantle that delays outgassing until host-star XUV declines.
- Does not include magnetic-field shielding as an ingredient.
Implications
- Target-list pruning for HWO and JWST programs.
- Complements Interplanetary Habitable Zone framework (Scharf 2026): IHZ filters by system-scale civilizational sustainability, STEHM by per-planet atmospheric retention.
- Reinforces M-dwarf pessimism alongside src-oxygen-false-positive-biosignatures-2026-03 photochemical caveats.