Context. Exoplanet characterization is one of the main foci of current exoplanetary science. For super-Earths and sub-Neptunes, wemostly rely on mass and radius measurements, which allow to derive the body’s mean density and give a rough estimate of the planet’sbulk composition. However, the determination of planetary interiors is a very challenging task. In addition to the uncertainty in theobserved fundamental parameters, theoretical models are limited due to the degeneracy in determining the planetary composition.
Aims. We aim to study several aspects that affect internal characterization of super-Earths and sub-Neptunes: observational uncertain-ties, location on the M-R diagram, impact of additional constraints as bulk abundances or irradiation, and model assumptions.
Methods. We use a full probabilistic Bayesian inference analysis that accounts for observational and model uncertainties. We employa Nested Sampling scheme to efficiently produce the posterior probability distributions for all the planetary structural parameter ofinterest. We include a structural model based on self-consistent thermodynamics of core, mantle, high-pressure ice, liquid water, and H-He envelope.
Results. Regarding the effect of mass and radius uncertainties on the determination of the internal structure, we find three differentregimes: below the Earth-like composition line and above the pure-water composition line smaller observational uncertainties lead tobetter determination of the core and atmosphere mass respectively, and between them internal structure characterization only weaklydepends on the observational uncertainties. We also find that using the stellar Fe/Si and Mg/Si abundances as a proxy for the bulkplanetary abundances does not always provide additional constraints on the internal structure. Finally we show that small variationsin the temperature or entropy profiles lead to radius variations that are comparable to the observational uncertainty. This suggests thatuncertainties linked to model assumptions can eventually become more relevant to determine the internal structure than observationaluncertainties.
Authors: J.F. Otegi, C. Dorn, R. Helled, F. Bouchy, J. Haldemann, and Y. Alibert