Observational Tests of Terrestrial Planet Buffering Feedbacks and the Habitable Zone Concept

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Observational Tests of Terrestrial Planet Buffering Feedbacks and the Habitable Zone Concept

Carbonate-silicate weathering feedback behavior throughout the habitable zone (HZ). (a) Reproduction of O. R. Lehmer et al. (2020) results showing stable, Earth-like climates generated by their habitable zone weathering model. The x-axis shows stellar flux normalized to the solar constant (S/S), the y-axis shows atmospheric CO2 partial pressure (pCO2), and the z-axis shows the distribution of pCO2 values at each S/S. Black squares mark modern Earth and Mars (S, pCO2) values. Point color indicates relative density (yellow = low, purple = high). The model predicts increasing atmospheric CO2 with decreasing stellar flux within the HZ. (b-d) Kernel density estimates (KDE) of pCO2 for simulated planets at 1.0 (b), 0.75 (c), and 0.50 S/S (d). While panels b-d reflect model parameter variations in pCO2, similar distributions would be expected in observational data if a CO2-buffering feedback is operative, highlighting the ideal dataset for applying our method. — astro-ph.EP

The habitable zone is defined as the orbital region around a star where planetary feedback cycles buffer atmospheric greenhouse gases that, in combination with solar luminosity, maintain surface temperatures suitable for liquid water.

Evidence supports the existence of buffering feedbacks on Earth, but whether these same feedbacks are active on other Earth-like planets remains untested, as does the habitable zone hypothesis.

While feedbacks are central to the habitable zone concept, one does not guarantee the other-i.e., it is possible that a planet may maintain stable surface conditions at a given solar luminosity without following the predicted CO2 trend across the entire habitable zone.

Forthcoming exoplanet observations will provide an opportunity to test both ideas. In anticipation of this and to avoid premature conclusions based on insufficient data, we develop statistical tests to determine how many observations are needed to detect and quantify planetary-scale feedbacks.

Our model-agnostic approach assumes only the most generic prediction that holds for any buffering feedback, allowing the observations to constrain feedback behavior. That can then inform next-level questions about what specific physical, chemical, and/or biological feedback processes may be consistent with observational data. We find that [23, 74](95% CI) observations are required to detect feedback behavior within a given solar luminosity range, depending on the sampling order of planets.

These results are from tests using conservative error tolerance-a measure used to capture the risk of false positives. Reducing error tolerance lowers the chance of false positives but requires more observations; increasing it reduces the required sample size but raises uncertainty in estimating population characteristics. We discuss these trade-offs and their implications for testing the habitable zone hypothesis.

Morgan Underwood, Adrian Lenardic, Johnny Seales, Benjamin Kwait-Gonchar

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2509.02848 [astro-ph.EP] (or arXiv:2509.02848v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.02848
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Submission history
From: Morgan Underwood
[v1] Tue, 2 Sep 2025 21:32:22 UTC (8,881 KB)
https://arxiv.org/abs/2509.02848
Astrobiology, Exoplanet,

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