Planetary Albedo Is Limited By The Above-cloud Atmosphere: Implications For sub-Neptune Climate (K2-18b)

Energy limits that delineate the habitable zone’ for exoplanets depend on a given exoplanet’s net planetary albedo (or Bond albedo’). We here demonstrate that the planetary albedo of an observed exoplanet is limited by the above-cloud atmosphere – the region of the atmosphere that is probed in remote observation.

We derive an analytic model to explore how the maximum planetary albedo depends on the above-cloud optical depth and scattering versus absorbing properties, even in the limit of a perfectly reflective grey cloud layer. We apply this framework to sub-Neptune K2-18b, for which a high planetary albedo has recently been invoked to argue for the possibility of maintaining a liquid water ocean surface, despite K2-18b receiving an energy flux from its host star that places it inside of its estimated `habitable zone’ inner edge.

We use a numerical multiple-scattering line-by-line radiative transfer model to retrieve the albedo of K2-18b based on the observational constraints from the above-cloud atmosphere. Our results demonstrate that K2-18b’s observed transmission spectrum already restricts its possible planetary albedo to values below the threshold required to be potentially habitable, with the data favouring a median planetary albedo of 0.17-0.18. Our results thus reveal that currently characteriseable sub-Neptunes are likely to be magma-ocean or gas-dwarf worlds.

The methods that we present are generally applicable to constrain the planetary albedo of any exoplanet with measurements of its observable atmosphere, enabling the quantification of potential exoplanet habitability with current observational capabilities.

An observed transmission spectrum is linked to the planetary albedo through the optical depth, τ_∞, from the cloud top to the top of the atmosphere, and the scattering albedo, w₀ = τ_sca/(τ_abs + τ_sca), of the above-cloud atmosphere. High-altitude clouds/hazes that truncate observations and lead to flat transmission spectra can achieve high planetary albedos because there is little opportunity for stellar energy to be deposited above the clouds/haze (low τ_∞, panel A). If clouds/hazes exist deeper in planetary atmospheres, the planetary albedo depends on the scattering and absorption properties of the abovecloud atmosphere: if the atmosphere is very highly scattering then the albedo will be high and the transmission spectrum will be truncated due to the scattering continuum (high w₀ panel B); if the atmosphere is not highly scattering then some amount of the incident stellar energy must be deposited in the above-cloud atmosphere resulting in spectral absorption features, cloud truncation at more transparent wavelengths, and a relatively low planetary albedo (low w₀, panel C). If clouds or hazes are not visible in the transmission spectrum then the albedo will be low, due to the combination of surface reflection and Rayleigh scattering of the atmospheric gases (high τ_∞, panel D).– astro-ph.EP

Sean Jordan, Oliver Shorttle, Sascha P. Quanz

Comments: Submitted to AAS Journals
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2504.12030 [astro-ph.EP] (or arXiv:2504.12030v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2504.12030
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Submission history
From: Sean Jordan
[v1] Wed, 16 Apr 2025 12:39:54 UTC (517 KB)
https://arxiv.org/abs/2504.12030
Astrobiology,