Hybrid worlds are the commonest outcome among the M-dwarf sub-Neptunes characterised with JWST

Sub-Neptunes orbiting M dwarfs have typically been sorted into two categories: rocky cores under primordial hydrogen envelopes, or volatile-rich bodies that formed beyond the water-ice line. Bulk density alone cannot separate them, because the two are degenerate in mass and radius once envelope evolution is modelled self-consistently, as shown by Rogers et al. 2023. The atmospheric mean molecular weight μ provides a way past that degeneracy.

To test the classification, the authors compiled the eight M-dwarf sub-Neptunes with published JWST spectra and a constrained μ. They combined μ, graded by the quality of evidence, with the restricted Jeans parameter and with energy-limited escape integrated over each system's lifetime. For the XUV history, they used M-dwarf activity-age relations at each host's spectral type rather than a parameterised prescription, and converted it explicitly to the 1-912 Angstrom band.

Four of the eight planets satisfy neither established class. They carry envelopes with 3.8 < μ ≤ 18 amu, which are neither primordial hydrogen nor the signature of a 50 per cent ice interior; one is a strict-sense water world. None of the eight has a μ retrieved from detected molecular features that places it in the gas-dwarf class.

The authors call these intermediate objects hybrid worlds and find they are the commonest outcome in the sample. The selection function works against that result, because targets chosen for transmission-spectroscopy favourability should over-represent extended, low-μ envelopes. The evidence is also asymmetric in the same direction: the hybrid assignments rest on positive molecular detections, while both end-member classes are populated by provisional detections. The debate is therefore not binary, and its two models do not describe the bulk of the population they were framed to explain.