2025MNRAS.538.3263P

The sub-Neptune planets have no solar system analogues, and their low bulk densities suggest thick atmospheres containing degenerate quantities of volatiles and H/He, surrounding cores of unknown sizes. Measurements of their atmospheric composition can help break these degeneracies, but many previous studies at low spectral resolution have largely been hindered by clouds or hazes, returning muted spectra. Here, we present the first comprehensive study of a short-period sub-Neptune using ground-based, high-resolution spectroscopy, which is sensitive to the cores of spectral lines that can extend above potential high altitude aerosol layers. We observe four CRIRES+ K-band transits of the warm sub-Neptune GJ 3090 b (T$_{\text{eq}}$ = 693$\pm$18 K) which orbits an M2V host star. Despite the high quality data and sensitivity to CH$_4$, H$_2$O, NH$_3$, and H$_2$S, we detect no molecular species. Injection-recovery tests are consistent with two degenerate scenarios. First, GJ 3090 b may host a highly metal-enriched atmosphere with > 150 Z$_{\odot }$ and mean molecular weight > 7.1 g mol$^{-1}$, representing a volatile dominated envelope with a H/He mass fraction $x_{\text{H/He}} < 33~{{ \rm per\ cent}}$, and an unconstrained aerosol layer. Second, the data are consistent with a high altitude cloud or haze layer at pressures < 3.3$\times 10^{-5}$ bar, for any metallicity. GJ 3090 b joins the growing evidence to suggest that high metallicity atmospheres and high altitude aerosol layers are common within the warm (500 $<T_{\text{eq}}<$ 800 K) sub-Neptune population. We discuss the observational challenges posed by the M-dwarf host star, and suggest observing strategies for transmission spectroscopy of challenging targets around M-dwarfs for existing and ELT instrumentation.