A Comparative Evaluation of Earth Similarity Index (ESI) Methods for Exoplanet Habitability Assessment

Abstract

The Earth Similarity Index (ESI) is a quantitative metric designed to evaluate how closely an exoplanet resembles Earth based on key physical parameters. This study conducts a comparative assessment of four ESI calculation methods: the Radius–Flux method (ESI(R-F)), the Ratio and Exponent Method (ESI(REM)), the Weighted Difference Method (ESI(WDM)), and the Analytic Hierarchy Process (ESI(AHP)). These approaches incorporate combinations of planetary radius, density, escape velocity, surface temperature, and stellar flux, normalized to Earth standards. The manuscript systematically derives each method, applies them to hypothetical exoplanets, and extends calculations to a large sample of observed planets. Results show that the four methods vary in sensitivity to planetary parameters, with ESI(AHP) offering structured weighting and ESI(WDM) allowing more flexible parametrization. The comparative evaluation highlights the strengths and limitations of each method for identifying potentially habitable exoplanets. This work contributes to improving multi-criteria assessments of planetary Earth-likeness and provides a foundation for future refinement of habitability indices.