ASTRO
In the literature: open
Why do planet numbers drop steeply above three Earth radii?
In plain words
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 statement
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.
What would settle it
Cliff location versus host and orbit properties compared with interior-atmosphere evolution models including hydrogen solubility.