# Earth Similarity

The Earth Similarity property of an object represents the Earth Similarity Index (ESI), a measure of how similar the object's physical properties are to the Earth's. The Earth Similarity property is read-only and is calculated automatically by Universe Sandbox.

Universe Sandbox calculates an object's ESI based on its radius, density, escape velocity, and surface temperature. The ESI is given by

$ESI = (ESI_{radius} * ESI_{density} * ESI_{escvel} * ESI_{surftemp})^{1/4},$

where ESIradius, etc., are the ESI contributions for each of these four properties. For example, if the object's radius is Robj and the Earth's radius is Rearth, then the contribution of radius to the ESI is

$ESI_{radius} = \left(1 - \frac{R_{obj} - R_{earth}}{R_{obj} + R_{earth}}\right)^{w_{radius}}.$

The contribution of each property is weighted according to the following values: