Characterising small exoplanets

Abstract

It was only thirty years ago that the first extrasolar planet, or exoplanet, orbiting a Sun-like star was discovered. Since then (as of October 2025), 6,022 have been confirmed across 4,490 planetary systems, 1,013 of which host multiple planets. Whilst these exoplanets have been discovered through a range of methods, transit photometry and radial velocity measurements have proven the most effective, accounting for∼96% of confirmed exoplanet discoveries. Through these two techniques, planetary radius and mass can be constrained to high precision. From these two parameters, planet density can be derived, enabling estimates of both atmospheric and internal compositions. Characterising small (<4 R⊕) exoplanets in this way is crucial for inferring the frequency of true Earth-analogues and assessing the uniqueness of our own planet.

However, there are several compositional trends for small exoplanets that remain poorly understood. The first is the ‘radius valley’ that separates super-Earths and sub-Neptunes, which has been consistently observed from ∼1.5–2 R⊕, and is largely without planets. Debate currently surrounds the origin of this gap, with proposed scenarios including core-powered mass-loss, photoevaporation, or that these planets are primordially rocky. Interpretations differ on the physical mechanism of atmospheric mass-loss, but the result is the same – primordially accreted atmospheres are removed in such a way that different planets are affected in different ways over different timescales, resulting in a ‘valley’ that separates a population of stripped-core planets (super-Earths) from those that have retained their H/He envelopes (sub-Neptunes). Secondly, the internal structure of sub- Neptunes is not just limited to that of a rocky core surrounded by a gaseous atmosphere, it has been theorised that these planets might hold significant fractions of ices or liquid water. It has been suggested that the radii of planets hotter than 900 K and with masses below 20 M⊕ can be reproduced assuming ice-dominated compositions without significant gaseous envelopes. However, it has also been argued that the existence of small planets with hydrogen atmospheres is consistent with the data, once thermal evolution and mass-loss are properly accounted for. This means that there is a strong degeneracy between water-world and silicate/iron-hydrogen models, and that the characterisation of larger sub-Neptunes in this region of the mass–radius diagram can be used to determine planetary evolution and formation pathways.

With our understanding still limited regarding the origins of these compositional trends, taking steps towards improving characterisation methods of bodies and systems in this size range is vital. Improving our understanding of the origins of the radius valley and the diverse pathways of planetary development will finally help us to ascertain the uniqueness of our own solar system and planets, which is a question that humanity has attempted to answer since the beginning of time.