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The Earth Similarity Index (ESI or "easy scale") is a characterization of how similar a planetary-mass object is to Earth. It was designed to be a scale from zero to one, with Earth having a value of one. The index can also be calculated for large natural satellites and other objects.

The index incorporates the planet's radius, density, escape velocity, and surface temperature. These parameters are not always known with certainty and so must be estimated based on one or more observable features of a planet, but many uncertainties remain. 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 can stand-in.

The ESI is designed so a planet with a high ESI (values in the range from 0.8 and 1.0) corresponds to one that is of terrestrial rocky composition. The index does not characterize habitability, although given the point of reference being Earth, some of its functions match those used by habitability measures. As with the definition of the habitable zone, the index uses surface temperature as a primary function (and the terrestrial point of reference).

According to this measure the closest planet to Earth in the Solar System is Mars with an ESI of 0.797. A number of exoplanets have values in excess of this. Kepler-438b has the highest Earth Similarity Index of confirmed exoplanets at 0.88.

Formulation

The index is based on the planetary radius, Bulk density, Escape velocity, and Surface temperature. Thus the index has two components which characterized different aspects of physical similarity. The mean radius and bulk density are associated with the interior, while the escape velocity and surface temperature are associated with the surface.

Planets similar to Earth

Exoplanets dominate the list of known Earth-like objects. However, the classification is made difficult in that many methods of exoplanet detection leave certain planetary features unknown. 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 can provide accurate measurements of mass but are less successful measuring radius. Planets observed via a number of different methods therefore can be most accurately compared to Earth.

Similarity of non-planets to Earth

The index can be calculated for objects other than planets, including natural satellites, dwarf planets and asteroids. The lower average density and temperature of these objects give them lower index values. Only Titan is known to hold on to a significant atmosphere despite an overall lower size and density. While Io has a low average temperature, surface temperature on the moon varies wildly due to geologic activity.

See also

• Earth analog
• List of potentially habitable exoplanets
• Planetary Habitability Index

References

1. "HEC: Data of Potential Habitable Worlds".
2. "Earth Similarity Index (ESI)". Planetary Habitability Laboratory.
3. Schulze-Makuch, D., Méndez, A., Fairén, A. G., von Paris, P., Turse, C., Boyer, G., Davila, A. F., Resendes de Sousa António, M., Irwin, L. N., and Catling, D. (2011) A Two-Tiered Approach to Assess the Habitability of Exoplanets. Astrobiology 11(10): 1041–1052.
4. Suresh Chandra; Subas Nepal; Mohit K. Sharma (July 2015). "Earth Similarity Index with two free parameters". arXiv:1507.06293v1 .
5. pg 143. Multivariate and other worksheets for R (or S-Plus): a miscellany P.M.E.Altham, Statistical Laboratory, University of Cambridge. January 10, 2013
6. Keszthelyi, L.; et al. (2007). "New estimates for Io eruption temperatures: Implications for the interior". Icarus. 192 (2): 491–502. Bibcode:2007Icar..192..491K. doi:10.1016/j.icarus.2007.07.008.

External links

• HEC: Data of Potential Habitable Exoplanets and Exomoons
• Earth Similarity Index calculator

v t e Exoplanets
Planet Definition IAU Planetary science
Main topics	Exoplanet Exoplanet orbital and physical parameters Methods of detecting exoplanets Planetary system Planet-hosting stars
Sizes and types	Terrestrial Carbon planet Coreless planet Desert planet Dwarf planet Hycean planet Ice planet Iron planet (Super-Mercury) Lava planet Ocean world Mega-Earth Sub-Earth Super-Earth Gaseous Eccentric Jupiter Mini-Neptune (Gas dwarf) Helium planet Hot Jupiter Hot Neptune Gas giant Ice giant Super-Jupiter Super-Neptune Super-puff Ultra-hot Jupiter Ultra-hot Neptune Other types Blanet Brown dwarf Chthonian planet Circumbinary planet Circumtriple planet Disrupted planet Double planet Ecumenopolis Eyeball planet Giant planet Mesoplanet Planemo Planet/Brown dwarf boundary Planetesimal Protoplanet Pulsar planet Sub-brown dwarf Sub-Neptune Toroidal planet Ultra-cool dwarf Ultra-short period planet (USP)
Formation and evolution	Accretion Accretion disk Asteroid belt Circumplanetary disk Circumstellar disc Circumstellar envelope Cosmic dust Debris disk Detached object Disrupted planet Excretion disk Exozodiacal dust Extraterrestrial materials Extraterrestrial sample curation Giant-impact hypothesis Gravitational collapse Hills cloud Internal structure Interplanetary dust cloud Interplanetary medium Interplanetary space Interstellar cloud Interstellar dust Interstellar medium Interstellar space Kuiper belt List of interstellar and circumstellar molecules Merging stars Molecular cloud Nebular hypothesis Oort cloud Outer space Planetary migration Planetary system Planetesimal Planet formation Protoplanetary disk Ring system Rubble pile Sample-return mission Scattered disc Star formation
Systems	Exocomet Interstellar Exomoon Tidally detached Rogue planet Orbits Retrograde Trojan Mean-motion resonances Titius–Bode law
Host stars	A B Binary star Brown dwarfs F/Yellow-white dwarfs G/Yellow dwarfs Herbig Ae/Be K/Orange dwarfs M/Red dwarfs Pulsar Red giant Subdwarf B Subgiant T Tauri White dwarfs Yellow giants
Detection	Astrometry Direct imaging list Microlensing list Polarimetry Timing list Radial velocity list Transit method list Transit-timing variation
Habitability	Astrobiology Astrooceanography Circumstellar habitable zone Earth analog Extraterrestrial liquid water Galactic habitable zone Habitability of binary star systems Habitability of F-type main-sequence star systems Habitability of K-type main-sequence star systems Habitability of natural satellites Habitability of neutron star systems Habitability of red dwarf systems Habitability of yellow dwarf systems Habitable zone for complex life List of potentially habitable exoplanets Tholin Superhabitable planet
Catalogues	Nearby Habitable Systems Exoplanet Data Explorer Extrasolar Planets Encyclopaedia NASA Exoplanet Archive NASA Star and Exoplanet Database Open Exoplanet Catalogue
Lists	Exoplanetary systems Host stars Multiplanetary systems Stars with proto-planetary discs Exoplanets Discoveries Extremes Firsts Nearest Largest Heaviest Terrestrial candidates Kepler 1–500 501–1000 1001–1500 1501–2000 K2 Potentially habitable Proper names Discovered exoplanets by year before 2000 2000–2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025
Other	Carl Sagan Institute Exoplanet naming convention Exoplanet phase curves Exoplanetary Circumstellar Environments and Disk Explorer Extragalactic planet Extrasolar planets in fiction Geodynamics of terrestrial exoplanets Neptunian desert Nexus for Exoplanet System Science Planets in globular clusters Small planet radius gap Sudarsky's gas giant classification
Discoveries of exoplanets Search projects

• Planetary habitability
• Search for extraterrestrial intelligence