= Solution
At fixed <mass>, three broad controls of a super-Earth-sized body's observed <radius> are:
* Bulk composition and material compressibility. A larger iron-core fraction usually makes an <exoplanet interior> denser and smaller, whereas silicate or water-rich interiors are larger. The <planetary mass-radius relation> therefore differs among compositions; mass and radius alone retain an <exoplanet interior-composition degeneracy>.
* A gaseous envelope and its retention. Even a modest <hydrogen>-<helium> mass fraction can substantially increase <radius> through the <transit-radius contribution of a gaseous envelope>. Its <mean molecular weight>, total atmospheric mass and <opacity> determine how far the slant-optical-depth surface lies above the condensed interior. <Atmospheric escape> can strip that envelope, producing a much smaller object without a comparable loss of core mass.
* Thermal and irradiation history. Higher interior <entropy>, youth and stellar heating tend to expand an envelope; <Kelvin-Helmholtz contraction> reduces its size over time. Stellar high-energy exposure affects the envelope through <atmospheric escape>. <Exoplanet clouds> and wavelength-dependent <opacity> shift the measured transit radius even when the deep interior is unchanged.
\b[Bulk composition, envelope fraction/composition, and thermal/irradiation history are three independent controls.] If mass is not fixed, the <mass> itself is an additional major variable. The label <super-Earth> does not ensure a rocky composition or an Earth-like atmosphere, and a radius alone does not determine which of these effects dominates.
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