ufo wiki

STEHM: Lower Size Limit for Habitable Exoplanets (Phys.org / Tomaswick, May 2026)

2 min read

Summary

Phys.org (May 11 2026) summarizes a UC Riverside arXiv preprint by michelle-l-hill et al. introducing the Smaller Than Earth Habitability Model (STEHM). STEHM couples atmospheric escape (Jeans escape under XUV irradiation) with interior thermal evolution (stagnant-lid cooling and volcanic outgassing) to set a lower size limit for habitable-zone exoplanets at ~0.8 Earth radii. Planets at or below ~0.7 R⊕ lose their atmospheres on timescales too short for life (~400 Myr at 0.6 R⊕, ~30 Myr at 0.5 R⊕). Planets ≥0.8 R⊕ retain atmospheres for billions of years. Three rare exceptions can rescue smaller worlds: a large initial carbon budget, a low core-radius fraction (large mantle / no core), or a “cold start” that delays outgassing until host-star XUV declines.

Key Claims

  • STEHM threshold: ~0.8 R⊕ is the smallest a planet can be and remain atmospherically habitable for Gyr timescales.
  • Sharp cut-off between 0.7 and 0.8 R⊕; 0.6 R⊕ → ~400 Myr atmosphere lifetime; 0.5 R⊕ → ~30 Myr.
  • Two dominant loss mechanisms: (1) low gravity / low escape velocity drives Jeans escape under stellar XUV; (2) high surface-area-to-volume ratio drives rapid interior cooling, lithospheric thickening, and shutdown of volcanic outgassing.
  • Model assumes stagnant-lid tectonics (no plate tectonics) and a CO₂ atmosphere — a best case for retention because CO₂ is heavy and resists Jeans escape.
  • Three rare escape routes for sub-0.8 R⊕ worlds: large carbon budget, low core-radius fraction, or cold-start mantle that delays outgassing until host-star XUV is benign.
  • Practical implication: target-list pruning for HWO and JWST programs — sub-0.8 R⊕ worlds are likely airless rocks absent unusual composition.
  • Caveat (not in STEHM): magnetic-field shielding is not an ingredient; H/He or lighter atmospheres would erode faster; plate-tectonics worlds could have different outgassing curves.

Notable Quotes

“slightly smaller than Earth is likely the smallest a planet can be and still be viable for life”

“0.8 Earth radii or larger can hold onto an atmosphere for billions of years”

“[smaller planets are] likely just an airless rock drifting through space”

Graph View

Backlinks