Resolving the flat-spectrum conundrum: clumpy aerosol distributions in sub-Neptune atmospheres

ABSTRACT Transmission spectroscopy of sub-Neptunes was expected to reveal their compositions and hence origins, yet many show flat near- to mid-infrared spectra. Such spectra can be explained either by metal-dominated atmospheres or by high-altitude grey aerosols. Observations of escaping hydrogen and helium from several of these planets rule out metal-dominated atmospheres, while homogeneous distributions of small aerosols cannot produce flat spectra and large particles require unphysically high production rates. We investigate the role of heterogeneous ‘clumpy’ aerosol distributions in shaping transmission spectra. Modestly optically thick clumps at high altitudes can produce flat spectra even with small particles and physically realistic production rates. Clumping increases the effective photon mean-free path while reducing wavelength dependence, allowing the aerosol distribution to behave as an effective grey absorber. Applying this framework to the sub-Neptune TOI-776c, we show that clumpy aerosols can reconcile the observed flattening of its transmission spectrum with a primordial H/He-dominated atmosphere. We further discuss implications for emission spectra, where enhanced stellar radiation penetration and altered scattering in a clumpy medium could produce observable signatures. These results suggest that clumpy aerosol distributions naturally resolve the tension between flat spectra and low-metallicity atmospheres and may be a common feature of sub-Neptune exoplanets. More broadly, our results highlight the need to consider aerosol heterogeneity when interpreting high-altitude microphysics and the spectral appearance of exoplanet atmospheres with James Webb Space Telescope, and motivate theoretical work to identify the physical mechanisms capable of generating clumpy aerosol distributions.