Probing the atmosphere of young Neptune-sized exoplanet AU Mic b with Subaru/IRD

The characterization of young exoplanetary atmospheres represents one of the most exciting frontiers in modern astronomy, offering a unique window into the early stages of planetary evolution. As these planets are still in their formative years, their atmospheric composition, thermal profiles, and dynamical structures encapsulate the critical fingerprints of their birth and subsequent migration history. In this study, we present a high-resolution transmission spectroscopy of AU Mic b, a young(~22Myr) Neptune-sized planet orbiting a M dwarf, obtained using the InfraRed Doppler (IRD) spectrograph (0.97 – 1.73 $\mu$m) on the Subaru Telescope.

Our analysis focuses on searching for $\mathrm{H_2O}$ and $\mathrm{CH_4}$, which are expected to exhibit strong absorption features given AU Mic b’s equilibrium temperature and the observed wavelengths. Recent JWST studies of young planets have reported low metallicity and relatively cloud/haze-free atmospheres (Thao et al. 2024 & Barat et al. 2025). Based on these results, we employed cross-correlation techniques, assuming a cloud/haze-free, H/He-dominated atmosphere in chemical equilibrium. While our injection-recovery tests confirmed that such signals could be successfully recovered, analysis of the actual data yielded no robust detection. This lack of a definitive signal allows us to place meaningful constraints on the atmospheric properties of AU Mic b. We interpret this non detection as a potential indication of a gray opacity source, such as high-altitude clouds or photochemical hazes, which effectively mask the underlying spectral lines.

Furthermore, we discuss these findings in the broader context of young planetary systems. By comparing AU Mic b with other young exoplanets of similar ages such as HIP 67522b or V 1298 Tau b, our results highlight a significant atmospheric diversity and suggest that even planets formed in similar environments may undergo divergent evolutionary paths depending on their host star.