Earth Similarity Index

The Earth Similar Index (ESI) is a proposed characterization to detail how similar a planet or natural satellite is to the planet Earth. The index has a scale of "0.00" to "1.00", with "0.00" meaning that the planet is completely different to Earth, while "1.00" is either Earth or an Earth-like. The index does not have any meaning for habitability, and even a index of 1.00 would not guarantee a planet is habitable.
Currently, the highest rated planet on the ESI is KOI-4878.01, an unconfirmed exoplanet located about 1075 light years from Earth, which has a rating of 0.98. The highest rated planet which is confirmed is TRAPPIST-1e, which has a 0.95 rating located around 39 light years from Earth.
== Formulation and History ==
The ESI was first proposed in 2011 by Schulze-Makuch et al. in the journal "Astrobiology". The formula incorporates a planet's radius, density, escape velocity, and surface temperature into the index. The authors describe the index as having two components: (1) associated with the interior which is associated with the mean radius and mean density, and associated with the surface which is associated with the escape velocity and surface temperature. An article on arxiv.org attempts to reproduce the ESI using only the temperature and mass of the planet.
== No relation to habitability ==
Despite the implication of the rating, the ESI does not characterize habitability, so a large rating does not necessarily mean a planet is habitable, even if it has a 1.00 rating. However, given that the point of reference is the Earth, some of its functions match those used by habitability measures. As with the definition of the habitable zone, the ESI uses surface temperature as a primary function (and the terrestrial point of reference). A 2016 article uses ESI as a target selection scheme and obtains results showing that the ESI has little relation to the habitability of an exoplanet, as it takes no account of the activity of the star, planetary tidal locking, nor the planet's magnetic field (i.e. ability to protect itself) which are among the keys to habitable surface conditions.
== ESI table ==
Planet 	ESI	Notes 	Distance (ly)
Earth 	 	Reference 	0
KOI-4878.01 	 	Planet is unconfirmed. Orbits a Sun-like star. 	1075
Teegarden b 	 	 	12
TRAPPIST-1e 	 	Orbits a ultra-cool red dwarf star. Likely tidally locked. 	39.5
Gliese 581 g 	 	Planet is unconfirmed. Tidally locked. 	20
Luyten b 	 	Tidally locked. 	12.2
TRAPPIST-1d 	 	Tidally locked. 	39.5
Kepler-438b 	 	Likely tidally locked 	640
Proxima Centauri b	 	Closest possibly habitable planet 	4
LHS 1723 b 	 	No density or atmospheric data available 	17.5
Ross 128 b 	 	Host star is inactive and quiet, should be habitable if it has an Earth-like atmosphere 	11
Gliese 667 Cc 	 	Tidally locked, temperature is about 277.4 K (4.3 °C, 39.7 °F) based on black body temperature calculation 	23.6
Kepler-442b 	 	In the center of the habitable zone 	1206
Kepler-452b 	 	 	1402
Kepler- 	 	 	990
Gliese 832 c 	 	Tidally locked and no plate tectonics. Likely very similar to Venus, but habitable if it possesses an Earth-like atmosphere	16
Kepler-283c 	 	 	1743
HD 85512 b 	 	Habitable if it has an Earth-like atmosphere 	36
Wolf 1061c 	 	 	13.8
Kepler-440b 	 	 	850
HD 40307 g 	 	 	42
Mars 	 	Lacks global plate tectonics. Too small for atmospheric water vapor. 	0
Mercury 	 	In 3:2 spin-orbital resonance 	0
Kepler-61b 	 	 	1100
Gliese 581d 	 	Unconfirmed, tidally locked. 	20
Kepler-22b 	 	Tidally locked. 	587
Kepler-443b 	 	While it has an 89.9% chance to be habitable, it only has a 4.9% chance of being terrestrial. 	2540
Gliese 422 b 	 	Unconfirmed. 	20
Gliese 3293 c 	 	Unconfirmed. 	20
Kepler-62f 	 	 	990
HD 69830 d 	 	No density or atmosphere data available 	40
55 Cancri c 	 	No density or atmosphere data available 	41
Moon 	 	No plate tectonics, too small for surface or atmospheric water. 	0
Venus 	 	 	0
Gliese 1214 b 	 	Likely to be an ocean planet, temperature is somewhere between 390 K (116 °C, 242 °F) - 552 K (278 °C, 533 °F) 	48
Gliese 581 c 	 	Tidally locked. 	20
Neptune 	 	Gas giant. 	0
Gliese 581 e 	 	Tidally locked. 	20
Jupiter 	 	Gas giant. 	0
Kepler-70c 	 	 	4200