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

TL;DR

The paper tackles the puzzle of flat JWST transmission spectra in sub-Neptunes, which are inconsistent with simple homogeneous haze models and hydrogen-dominated exospheres. It introduces a clumpy aerosol framework based on a mega-grain radiative-transfer approach, showing that moderately optically thick clumps at high altitudes can produce grey-like spectra even with small particles and realistic haze production rates. Applying the model to TOI-776c, the authors demonstrate that clumpy aerosols can reproduce a flat 1–5 µm transmission spectrum with solar metallicity, and reveal a degeneracy between clump properties and atmospheric metallicity in transmission, while potentially imprinting distinctive features in emission spectra. The study suggests aerosol heterogeneity as a natural resolution to the flat-spectrum/high-altitude cloud problem, motivates exploration of physical clumping mechanisms, and emphasizes the need to incorporate clumpy aerosols in high-altitude microphysics interpretations for JWST data.

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 JWST, and motivate theoretical work to identify the physical mechanisms capable of generating clumpy aerosol distributions.

Figures (10)

• Figure 1: The left panel shows the fraction of transmitted flux through the simple box as a function of wavelength for our homogeneous model and simplistic “banded” model. As the optical depth through a band at 5 $\mu$m increases, the wavelength dependence of the transmitted flux decreases. The clumpy nature of the medium results in weaker wavelength dependence but higher transmitted flux due to the aerosol-free regions. The right-hand plots show how the filling factor ($N_{\rm bands}\ell/L$) and the total aerosol mass in the box (scaled to the homogeneous case) evolve with the band optical depth. As the optical depth through a single band increases, the total mass in aerosols in the box must increase to compensate. With a moderate increase in aerosol mass, a clumpy aerosol distribution can give rise to grey transmission, even with small particles.
• Figure 2: Schematic cartoon showing the difference between the patchy and clumpy formalisms, with a zoom in to the transmission geometry (where the star is to the left, and the observer to the right). In the patchy case, photons are either attenuated in the aerosol layers, or transmit freely (other than gas absorption) through the aerosol free regions. In the clumpy formalism, photons that do not interact with a clump pass freely through, uneffected (other than gas absorption). Photons that interact with a clump are generally scattered off their surfaces or completely absorbed, relatively few photons make it through a clump (in the case of optically thick clumps). The volume filling factor ($V_{\rm ff}$) of the clumps is the fractional volume of the region that contains aerosols particles.
• Figure 3: The Rayleigh index for 0.1 $\mu$m particle sizes as a function of clump optical depth at 5 $\mu$m for different aerosol compositions. The range of Rayleigh indices that would yield a flat aerosol spectrum for a TOI-776c-like planet at 20 ppm precision is shown as the blue shaded area.
• Figure 4: The left panels show the model transmission spectra of TOI-776c, where the thick coloured lines represent different production rates (indicated by the coloured points in the right panels). The thin grey line shows the aerosol-free transmission spectrum and the points show the observed G395H transmission spectrum from Teske2025. The right panels show the $\chi^2/N$ goodness of fit metric as a function of the aerosol production rate, with the equivalent value for a flat line shown as the thin dotted horizontal line. The top row shows small aerosol particles, while the bottom row shows large aerosol particles. Small particles do not provide a good fit to the data at any production rate due to their strong wavelength dependence when $\lambda>a$. Large particles can provide a flat spectrum, but only at unphysically large aerosol production rates $\dot{\Sigma}_p \gtrsim10^{-8}~{\rm g~cm^{-2}}~{\rm s}^{-1}$. Therefore, it is challenging to explain sub-Neptunes' flat spectra with a homogeneous aerosol distribution in a primordial-dominated atmosphere.
• Figure 5: The left panels show the model transmission spectra of TOI-776c, where the thick coloured lines represent different clump optical depths at 5 $\mu$m (indicated by the coloured points in the right panels). The thin grey line shows the aerosol-free transmission spectrum and the points show the observed G395H transmission spectrum from Teske2025. The right panels show the $\chi^2/N$ goodness-of-fit metric as a function of the clump optical depth, with the equivalent value for a flat line shown as the thin dotted horizontal line. The top row shows 0.01 $\mu$m particles, while the bottom row shows 0.1 $\mu$m particles. The production rate is $10^{-10}$ g cm$^{-2}$ s$^{-1}$ in all models. These plots show that, provided the clumps are at least marginally optically thick at 5 $\mu$m, one can explain flat spectra with small particles and physical haze production rates.