Earth Similarity Index


The Earth Similarity Index, ESI or “easy scale” is a measure of how physically similar a planetary-mass object is to Earth. It is a scale from zero to one, with Earth having a value of one. The ESI was designed to measure planets, but the formula can also be applied to large natural satellites and other objects. The ESI is a function of the planet’s radius, density, escape velocity, and surface temperature. These parameters are often estimated based on one or more known variables. Such variables depend greatly on the method of observation used. For example, surface temperature is influenced by a variety of factors including irradiance, tidal heating, albedo, insolation and greenhouse warming. Where these are not known, planetary equilibrium temperature is frequently used, or the variable is inferred from other known attributes.

A planet with a high ESI (values in the range from 0.8 and 1.0) is likely to be of terrestrial rocky composition.

ESI is not a measure of habitability, though given the point of reference being Earth, some of its functions match closely to those used by habitability measures. The ESI and habitable zone share in common the use of surface temperature as a primary function (and the terrestrial point of reference).

According to this measure there are no other Earth-like planets or moons in the Solar System (second-ranked Mars is 0.697), though a number of exoplanets have been found with values in this range. Kepler-62e has the highest Earth Similarity of confirmed exoplanets at 0.83, whereas Kepler candidate KOI-1686.01, if confirmed, would be likely to have an ESI of 0.89. Further, the candidate exomoon HD 222582 b m of a confirmed exoplanet, and several candidate exomoons (KOI 375.01 m, KOI-2933.01 m, KOI-422.01 m) of unconfirmed exoplanets, all have an ESI of 0.86.

On November 4, 2013, astronomers reported, based on data gathered by the Kepler spacecraft, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way. 11 billion of these estimated planets may be orbiting sun-like stars.The nearest such planet may be 12 light-years away, according to the scientists.

Related formulas


ESIEarth Similarity Index (dimensionless)
mplanetary property index (dimensionless)
itotal number of planetary properties (dimensionless)
xicorresponding terrestrial reference value (e.g. 288 K) for the property (K)
xi0one of the planetary properties (e.g. surface temperature) (K)
wnweight exponent for the property (dimensionless)