More than 6000 exoplanets have been discovered to this day. Among them, small planets (radius < 4 R_earth), also known as sub-Neptunes, represent the largest population by number. The most intriguing property of sub-Neptunes is the bimodal distribution of their radii: a small-size population (~1.5 Re) suggesting a rocky structure, super-Earths, and a larger population (~2.4 Re) suggesting the presence of a thick envelope of volatile material, mini-Neptunes. This peak at ~2.4 Re is especially intriguing as it could represent the radius of a planet with a rocky core and a small H/He envelope, like a scaled-down Jupiter, or it could represent a planet with a rocky core and a thick water envelope making 50% of the planet’s mass, like a scaled-up version of icy moons of the solar system. I will present the theoretical approach to determine the composition of sub-Neptunes, with a focus on interior structure modeling of these planets. This approach consists in modeling the variety and richness of physical processes at play in planetary interior and their atmospheres. I will also present how new theoretical developments were guided by the recent measurements with Gaia and JWST, as well as observational challenges that remain to be overcome. Understanding the true nature of sub-Neptunes is one of the most important questions in exoplanet science. The possibility that sub-Neptunes could be water worlds makes them prime objects of interest for astrobiology research.
