Transiting planets with radii 2-3 $R_bigoplus$ are much more numerous than larger planets. We propose that this drop-off is so abrupt because at $R$ $sim$ 3 $R_bigoplus$, 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.
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