= Solution
The measured mass and radius illustrate the <exoplanet interior-composition degeneracy>. Three plausible structures are:
* a predominantly rocky iron-silicate interior carrying a small hydrogen-helium envelope whose low density inflates the radius;
* a water-rich differentiated planet with rock and metal below high-pressure ice, supercritical water, and a steam atmosphere;
* an iron-poor silicate-rich interior with a high-mean-molecular-mass secondary atmosphere, possibly including water and carbon dioxide.
An <exoplanet transmission spectrum> can distinguish a large-scale-height hydrogen atmosphere from compact steam or carbon-dioxide atmospheres. Molecular abundances, clouds, atmospheric mean molecular mass, and atmospheric escape constrain the envelope mass and evolution. An <exoplanet emission spectrum> constrains temperature, albedo, and internal cooling. A detected hydrogen-rich atmosphere favors the first scenario, a water-dominated high-molecular-weight spectrum supports the second, and a thin secondary atmosphere supports the third, although clouds can preserve degeneracies.
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