Earth Similarity Index (ESI) is a measure of how similar a planet – solar or extrasolar – is to Earth, expressed as a number between zero (no similarity at all) and one (identical to Earth). The Earth Similarity Index (ESI) involves a number of different parameters and is akin to similarity indices used in other fields. The ESI can be used to prioritize observations of exoplanets, especially those of astrobiological interest, perform statistical assessments, and develop classification systems for planets. The basic expression for the ESI is

 

 

where x_i is a planetary property (e.g., surface temperature), x_io is the corresponding terrestrial reference value (e.g., 288 K), w_i is a weight exponent, and n is the number of planetary properties. The weighting exponents are used to adjust the sensitivity of the scale and equalize its meaning between different properties.

 

Earth-like planets can be defined as any planetary body with a similar terrestrial composition and a temperate atmosphere. As a general rule, any planetary body with an ESI value over 0.8 can be considered an Earth-like planet. This means that the planet is rocky in composition (silicates) and has an atmosphere suitable for most terrestrial vegetation including complex life. Planets with ESI values in the 0.6 to 0.8 range (e.g., Mars) might still be habitable too, but only by simple extremophilic life, as they are either too cold or too hot, assuming life as we know it.