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Title: Why do planet numbers drop steeply above three Earth radii?
Plain: Planets just under three Earth radii are very common, but slightly larger ones are much rarer. Something must stop envelopes from growing, possibly hydrogen dissolving into molten rock.
Precise: The occurrence rate of close-in planets falls by about an order of magnitude between $R_{p} \sim 3$ and $\sim 4\ R_{\mathrm{earth}}$. Identify the mechanism: hydrogen sequestration into magma oceans that limits envelope growth, runaway gas accretion that depletes the intermediate range, or a formation-mass cutoff, and predict how the cliff location depends on stellar mass and instellation.
Settled by: Cliff location versus host and orbit properties compared with interior-atmosphere evolution models including hydrogen solubility.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: (empty)
Posed since: (empty)