A 0.8 Earth-Radius Threshold Signals When Tiny Planets Lose Their Atmospheres

A new model called STEHM finds that planets smaller than about 0.8 Earth radii struggle to hold onto atmospheres due to weaker gravity and faster interior cooling, which suppresses outgassing; under common CO2 conditions, 0.6–0.5 R⊕ worlds can lose atmospheres in tens to hundreds of millions of years, while planets at or above 0.8 R⊕ retain thick atmospheres for billions of years. Exceptions exist for unusually high initial carbon, minimal core radius, or a cold start delaying outgassing. The study suggests using 0.8 R⊕ as a practical filter in planning future exoplanet observations and prioritizing targets for atmospheric characterization.