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-438b has the highest Earth Similarity of confirmed exoplanets at 0.88. 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
Extrasolar planets dominate the list of known Earth-like objects. However, the classification is made difficult in that many methods of extrasolar planet detection leave ESI parameters unquantified. For example, with the transit method, one of the more successful, measurement of radius can be highly accurate, but mass and density are often estimated; likewise with radial velocity methods, which provide accurate measurements of mass but are less successful measuring radius. Planets observed via a number of different methods therefore have the most accurate measures of ESI, though this is not possible in many situations.
The following exoplanets (go to the link below and then scan to the chart containing the data) have been determined to have higher ESI than 0.80 (Earth included for comparison):
Gliese 667Cc 0.84
KOI-3010.01 0.84
Kepler 442b 0.83
Kepler 62e 0.83
Kepler 452b 0.83
Gliece 832 c 0.81
The ESI can be applied to objects other than planets, including natural satellites, dwarf planets and asteroids, though comparisons typically draw lower global ESI due to the lower average density and temperature of these objects, at least for those known in the Solar System,.
The following non-planetary objects have relatively high global ESIs:
Moon (of Earth) 0.559
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-438b has the highest Earth Similarity of confirmed exoplanets at 0.88. 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
Planets with Relatively High
ESI
Extrasolar planets dominate the list of known Earth-like objects. However, the classification is made difficult in that many methods of extrasolar planet detection leave ESI parameters unquantified. For example, with the transit method, one of the more successful, measurement of radius can be highly accurate, but mass and density are often estimated; likewise with radial velocity methods, which provide accurate measurements of mass but are less successful measuring radius. Planets observed via a number of different methods therefore have the most accurate measures of ESI, though this is not possible in many situations.
The following exoplanets (go to the link below and then scan to the chart containing the data) have been determined to have higher ESI than 0.80 (Earth included for comparison):
PlanetaryBody ESI
Earth 1.00
Kepler 438b 0.88Gliese 667Cc 0.84
KOI-3010.01 0.84
Kepler 442b 0.83
Kepler 62e 0.83
Kepler 452b 0.83
Gliece 832 c 0.81
ESIs of Non-Planets
The ESI can be applied to objects other than planets, including natural satellites, dwarf planets and asteroids, though comparisons typically draw lower global ESI due to the lower average density and temperature of these objects, at least for those known in the Solar System,.
The following non-planetary objects have relatively high global ESIs:
Body ESI
KOI 433.02m 0.93
HD 222582b 0.86Moon (of Earth) 0.559
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