Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis

Transiting planets with radii 2–3 R⊕ are much more numerous than larger planets. We propose that this drop-off is so abrupt because at R ∼ 3 R⊕ base-of-atmosphere pressure is high enough for the atmosphere to readily dissolve into magma, and this sequestration acts as a strong brake on further growth. The viability of this idea is demonstrated using a simple model. Our results support extensive magma–atmosphere equilibration on sub-Neptunes, with numerous implications for sub-Neptune formation and atmospheric chemistry.