Follow the Water: Finding Water, Snow and Clouds on Terrestrial Exoplanets with Photometry and Machine Learning

TL;DR

This paper demonstrates that machine learning applied to broadband photometry can effectively detect water, snow, and clouds on Earth-like exoplanets, aiding in the prioritization of targets for detailed spectroscopic follow-up.

Contribution

It introduces a machine learning approach using XGBoost to identify water and surface features on exoplanets from photometric data, achieving high accuracy and feasibility for initial characterization.

Findings

XGBoost achieves over 90% accuracy in detecting snow and clouds.

70% accuracy in detecting liquid seawater at S/N ≥ 30.

Photometric detection can guide target prioritization for spectroscopic follow-up.

Abstract

All life on Earth needs water. NASA's quest to follow the water links water to the search for life in the cosmos. Telescopes like JWST and mission concepts like HabEx, LUVOIR and Origins are designed to characterise rocky exoplanets spectroscopically. However, spectroscopy remains time-intensive and therefore, initial characterisation is critical to prioritisation of targets. Here, we study machine learning as a tool to assess water's existence through broadband-filter reflected photometric flux on Earth-like exoplanets in three forms: seawater, water-clouds and snow; based on 53,130 spectra of cold, Earth-like planets with 6 major surfaces. XGBoost, a well-known machine learning algorithm, achieves over 90\% balanced accuracy in detecting the existence of snow or clouds for S/N$\gtrsim 20$, and 70\% for liquid seawater for S/N $\gtrsim 30$. Finally, we perform mock Bayesian analysis with Markov-chain Monte Carlo with five filters identified to derive exact surface compositions to test for retrieval feasibility. The results show that the use of machine learning to identify water on the surface of exoplanets from broadband-filter photometry provides a promising initial characterisation tool of water in different forms. Planned small and large telescope missions could use this to aid their prioritisation of targets for time-intense follow-up observations.