Fundamental nature of sub-Neptune exoplanets

Determine whether sub-Neptune exoplanets (temperate exoplanets with sizes between Earth and Neptune) are more similar to (i) terrestrial planets with thin atmospheres and potentially habitable surfaces, (ii) mini-Neptunes that are less massive analogs of Solar System ice giants with deep hydrogen–helium atmospheres lacking a habitable surface, or (iii) intermediate volatile-rich configurations such as water worlds with substantial H2O interiors.

Background

Sub-Neptune exoplanets constitute a large fraction of known exoplanets but exhibit strong mass–radius degeneracies, allowing multiple interior structures to fit current observations. This degeneracy leads to uncertainty about whether these worlds are rocky with thin atmospheres, Neptune-like with deep H/He envelopes, or volatile-rich water worlds.

K2-18 b, a canonical temperate sub-Neptune, has recent JWST detections of CH4 and CO but a nondetection of CO, intensifying interest in using atmospheric composition to constrain interior structure. However, despite these advances, the overarching classification of sub-Neptunes as terrestrial-like, mini-Neptune-like, or water-world-like remains unresolved.

References

Because a range of interior compositions can explain the observed masses and radii of sub-Neptunes, it is unknown whether these planets are closer to (1) terrestrial planets with thin atmospheres and potentially habitable surfaces, (2) “mini-Neptunes” which are less massive versions of our solar system’s ice giants, or (3) “in-between state” planets, such as “water worlds” with volatile-rich interiors.

— Volatile-rich Sub-Neptunes as Hydrothermal Worlds: The Case of K2-18 b  (2409.06258 - Luu et al., 2024) in Section 1 (Introduction)

For all planet samples, there are multiple volatile scenarios that can reasonably reproduce the observed density distributions and are not rejected by the KS and AD tests. This indicates that current data do not provide sufficient statistical leverage to distinguish whether the low-density population is dominated by water-rich planets, gas-rich planets, or a mixture of both.

— A Stellar-Type Dependence in the Rocky and Volatile Composition of Small Exoplanets  (2609.09319 - Wang et al., 8 Sep 2026) in Section Conclusions, item 3 (Degeneracy of volatile composition)

While our helium absorption nondetections cast doubt on the helium-dominated upper atmosphere interpretation of LHS 1140 b, it does not constrain the relative likelihood of LHS 1140 b being a mini-Neptune versus a water world. We leave this to future, panchromatic analyses of LHS 1140 b's transmission spectrum, coupled with emission photometry results from the Rocky Worlds DDT program for this target.

— No Helium Detected in LHS 1140 b from Four JWST NIRISS/SOSS Transits  (2608.13473 - Bennett et al., 13 Aug 2026) in Discussion and Conclusion, Section 6