Exoplanets, -moons, -comets

Resolving The Flat-spectrum Conundrum: Clumpy Aerosol Distributions In Sub-Neptune Atmospheres

By Keith Cowing

Status Report

astro-ph.EP

November 30, 2025

Filed under astro-ph.EP, astronomy, atmosphere, exoplanet, JWST, spectorscopy, sub-Neptune, TOI-776 c

Resolving The Flat-spectrum Conundrum: Clumpy Aerosol Distributions In Sub-Neptune Atmospheres — 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 (𝑉ff) of the clumps is the fractional volume of the region that contains aerosols particles. — astro-ph.EP

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.

James E. Owen, James Kirk

Comments: Accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2511.19013 [astro-ph.EP] (or arXiv:2511.19013v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2511.19013
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Related DOI:
https://doi.org/10.1093/mnras/staf2066
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Submission history
From: James Owen
[v1] Mon, 24 Nov 2025 11:43:29 UTC (5,180 KB)
https://arxiv.org/abs/2511.19013

Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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