Wet Removal and Cloud Enhancement: The Microphysics of Cloud–Haze Interactions on Sub-Neptunes
- 1University of Chicago
- 2Max Planck Institute for Astronomy
- 3Carnegie Institution for Science
Abstract
Aerosols are a near-ubiquitous feature of sub-Neptune atmospheres, yet their microphysical nature remains poorly understood. Both condensate clouds and photochemical hazes have been proposed to explain observations, but have largely been studied in isolation. Here we present a new bin-scheme microphysical model, adapted from the Community Aerosol and Radiation Model for Atmospheres ( CARMA ), which explicitly couples cloud and haze formation through heterogeneous nucleation—the dominant mode of cloud formation on Earth and throughout the solar system—in which haze particles act as cloud condensation nuclei (CCN). Applying this model to KCl clouds on GJ 1214 b–like warm sub-Neptunes, we find that the microphysical contact angle θ between cloud and haze particles governs distinct regimes of aerosol behavior: at moderate contact angles (25° ⪅ θ ⪅70°), hazes are efficiently removed from the upper atmosphere through “wet removal” as they seed gravitationally settling clouds; at small contact angles ( θ ⪅ 25°), heterogeneous nucleation instead produces an enhanced population of mixed cloud-haze particles at high altitudes, dramatically increasing aerosol optical depth (“cloud enhancement”). These structural changes produce differences of up to four scale heights in transmission spectra, with strong effects at optical and near-infrared wavelengths relevant to instruments such as JWST/NIRISS/SOSS, while wavelengths beyond ∼3 μ m remain comparatively unaffected. We map these effects across orders of magnitude in metallicity, haze production rate, and vertical mixing strength, establishing their generality across sub-Neptune parameter space. Because heterogeneous nucleation is a universal phase-change process, this framework extends naturally to other types of exoplanet atmospheres and potentially any astrophysical environments where condensation onto foreign substrates may occur, including protoplanetary disks and stellar outflows.
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BibTeX
@article{nagpal2026removal,
title = {{Wet Removal and Cloud Enhancement: The Microphysics of Cloud–Haze Interactions on Sub-Neptunes}},
author = {Vighnesh Nagpal and Maria E. Steinrueck and Diana Powell and Peter Gao and Dominic Samra and Wolf Cukier},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/ae9a84},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/ae9a84},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Wet Removal and Cloud Enhancement: The Microphysics of Cloud–Haze Interactions on Sub-Neptunes
AU - Vighnesh Nagpal
AU - Maria E. Steinrueck
AU - Diana Powell
AU - Peter Gao
AU - Dominic Samra
AU - Wolf Cukier
PY - 2026
DA - 2026/10/13
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/ae9a84
UR - https://iopscience.iop.org/article/10.3847/1538-4357/ae9a84
AB - Aerosols are a near-ubiquitous feature of sub-Neptune atmospheres, yet their microphysical nature remains poorly understood. Both condensate clouds and photochemical hazes have been proposed to explain observations, but have largely been studied in isolation. Here we present a new bin-scheme microphysical model, adapted from the Community Aerosol and Radiation Model for Atmospheres ( CARMA ), which explicitly couples cloud and haze formation through heterogeneous nucleation—the dominant mode of cloud formation on Earth and throughout the solar system—in which haze particles act as cloud condensation nuclei (CCN). Applying this model to KCl clouds on GJ 1214 b–like warm sub-Neptunes, we find that the microphysical contact angle θ between cloud and haze particles governs distinct regimes of aerosol behavior: at moderate contact angles (25° ⪅ θ ⪅70°), hazes are efficiently removed from the upper atmosphere through “wet removal” as they seed gravitationally settling clouds; at small contact angles ( θ ⪅ 25°), heterogeneous nucleation instead produces an enhanced population of mixed cloud-haze particles at high altitudes, dramatically increasing aerosol optical depth (“cloud enhancement”). These structural changes produce differences of up to four scale heights in transmission spectra, with strong effects at optical and near-infrared wavelengths relevant to instruments such as JWST/NIRISS/SOSS, while wavelengths beyond ∼3 μ m remain comparatively unaffected. We map these effects across orders of magnitude in metallicity, haze production rate, and vertical mixing strength, establishing their generality across sub-Neptune parameter space. Because heterogeneous nucleation is a universal phase-change process, this framework extends naturally to other types of exoplanet atmospheres and potentially any astrophysical environments where condensation onto foreign substrates may occur, including protoplanetary disks and stellar outflows.
SN - 0004-637X
SN - 1538-4357
ER -