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

Abstract

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 $CO_2$ 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.