Exoplanet Habitability Score

This calculator computes the Earth Similarity Index (ESI), a metric introduced by Schulze-Makuch et al. (2011) in the journal Astrobiology that astrobiologists use to summarize how physically Earth-like a candidate exoplanet is, on a scale from 0 to 1. Rather than a simple average, ESI multiplies together four separately scored components — planet radius, bulk density, escape velocity, and surface temperature — each scored by how closely it matches Earth's own reference value (all set to 1, or 288 K for temperature) using the formula (1 − |x − x₀|/(x + x₀)) raised to a weighting exponent. The weights (5.58 for temperature, 1.07 for density, 0.70 for escape velocity, 0.57 for radius, normalized by their sum) come directly from that paper's published exponents, chosen so that temperature dominates the overall score, reflecting its outsized importance for surface liquid water and thus habitability as currently understood.

Because the four component scores are multiplied rather than added, one badly mismatched parameter — say, a scorching surface temperature — can drag the whole index down even if the planet is otherwise Earth-sized and Earth-dense; ESI rewards balanced similarity, not any single strong match. The calculator also reports an Interior ESI (radius and density combined) and Surface ESI (escape velocity and temperature combined) as sub-scores, plus a rough surface gravity estimate (density times radius) and a simple water-state guess based on whether surface temperature falls between 273 K and 373 K. Remember that ESI measures physical resemblance to Earth, not confirmed habitability or life — a planet can score high on ESI while having an atmosphere, radiation environment, or geological history that makes it entirely uninhabitable.