A Statistical Method for Constraining the Capability of the Habitable Worlds Observatory to Understand Ozone Onset Time in Earth Analogs

TL;DR

This paper develops a statistical method to assess how well future observations of Earth-like exoplanets can determine the timing of ozone appearance, a key indicator of life, considering observational uncertainties and sample size.

Contribution

It introduces a statistical framework to constrain ozone emergence timing on exoplanets, accounting for observational uncertainties and sample size effects.

Findings

Uncertainties are mainly influenced by sample size, not stellar age uncertainty.

A sample of 30 Earth analogs can constrain ozone emergence time with high confidence.

No ozone detections in a sample of 30 would still allow a 10σ limit on emergence time.

Abstract

The oxygenation of Earth's atmosphere 2.3 billion years ago, which on exoplanets is expected to be most detectable via the UV ozone feature at $\sim$0.25 $\mu$m, is often regarded as a sign of the emergence of photosynthetic life. On exoplanets, we may similarly expect life to oxygenate the atmosphere, but with a characteristic distribution of emergence times. In this paper, we test our ability to recover various "true" emergence time distributions as a function of 1) stellar age uncertainty and 2) number of Earth analogs in the sample. The absolute uncertainties that we recover, for diverse underlying distributions, are mostly independent of the true underlying distribution parameters, and are more dependent on sample size than stellar age uncertainty. For a sample size of 30 Earth analogs, and an HWO architecture sensitive to ozone at 1% of the current atmospheric level on Earth, we find that no ozone detections across the entire sample would place a 10$\sigma$ limit on the mean time of ozone emergence, regardless of stellar age uncertainty.