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An exoplanet interior is the structure and composition beneath the layers directly sampled by observations. Mass and radius constrain the mean mass density, but a physical model also needs hydrostatic equilibrium, an equation of state, and assumptions about composition and thermal history. Dense material can be concentrated in a core or mixed into surrounding layers.

Solid-planet calculations show how iron-, silicate- and water-rich bodies occupy different broad mass-radius relations, with compression preventing one constant-density scaling from applying to every mass.^[ref] A light gas envelope can substantially alter the observed radius. Exoplanet interior-composition degeneracy therefore remains even when both mass and radius are measured precisely.

The measured mass and radius implyCompared with the supplied composition curves, this is inconsistent with a bare iron-rich or ordinary rock-only planet. A volatile-rich interior and/or an extended low-density gas envelope is needed. A water-rich body with an atmosphere and a rocky core with a modest H/He envelope are different plausible families; mass and radius do not uniquely determine the composition.^[ref]

For the requested H/He estimate, assume a hydrostatic, approximately isothermal terminator at the zero-Bond-albedo, fully redistributed equilibrium temperature. The orbit and temperature areSurface gravity is $g=Gm/R_p^2\simeq 9.58\mathrm{m}{\mathrm{s}}^{-2}$. With molar mass $\mathcal{M}=2.2\times 10^{-3}\mathrm{kg}\mathrm{mo}{\mathrm{l}}^{-1}$, the atmospheric scale height isA spectral feature spanning $N$ scale heights changes the blocked stellar fraction by the atmospheric annulus area:Thus one scale height is about $338$ parts per million in transit depth; a clear strong feature spanning roughly five scale heights is of order $1.7\times 10^{-3}$. The number of scale heights requires the actual opacity ratio; $N\simeq \ln ({\kappa }_{\mathrm{line}}/{\kappa }_{\mathrm{cont}})$ is a useful isothermal estimate, not a supplied measured value. The true terminator temperature need not equal this redistribution estimate.

A nearly flat spectrum can result from a high-mean-molecular-mass atmosphere with a much smaller scale height, or from optically thick clouds/hazes that hide the deeper absorption features. These are the two qualitative possibilities; detailed precision data can discriminate them. In particular, the pre-2015 HST study favored clouds over the clear high-molecular-mass models it tested, so listing both physical mechanisms is not a claim that all such models fit those data equally well.^[ref]

Sara Seager develops physical methods for interpreting exoplanets. With Dimitar Sasselov, she calculated atmospheric transmission during an exoplanet transit: absorption in the planetary limb makes the measured transit depth depend on wavelength. Their work identified strong atomic features and cloud height as observationally useful diagnostics.^[ref]

Her models of solid planets also connect measured mass and radius to bulk composition through an equation of state. They showed how such measurements can distinguish broad iron-rich, silicate-rich and water-rich families while leaving finer compositional ambiguities. A measured mean mass density therefore constrains a family of interior models rather than uniquely revealing an interior.^[ref]