Earth Similarity Index

The Earth Similarity Index boils a planet down to a single number between 0 and 1 that says how Earth-like it is, with Earth set to exactly 1.0. gravityfinder computes it from the two things we can actually pin down for thousands of worlds, the planet's radius and its equilibrium temperature, refined by density and escape velocity where those are measured (the Schulze-Makuch 2011 weighting).

A high ESI is a filter, not a verdict. It tells you a world is roughly Earth's size and gets roughly Earth's warmth, which is a good place to start a search, but it is silent on air, water, and everything else that makes Earth livable. A hot, airless rock the size of Earth can still score well. Read it as "worth a closer look," not "a second Earth."