The Earth Similarity Index (ESI) is a proposed 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; this is meant to simplify planet comparisons from large databases. The index can also be calculated for large natural satellites and other objects. It has no quantitative meaning for habitability.
Contents
Formulation
The ESI, as proposed in 2011 by Schulze-Makuch et al, incorporates the planet's radius, density, escape velocity, and surface temperature into the index.[2] Thus the authors describe the index as having two components: (1) associated with the interior which is associated with the mean radius and bulk density, and (2) associated with the surface which is associated with the escape velocity and surface temperature. For exoplanets, these parameters are not 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. A paper on the preprint server arxiv.org attempts to reproduce the ESI using only the temperature and mass of the planet.[3]
A webpage maintained by one of the authors of the 2011 Astrobiology paper, Abel Méndez at the University of Puerto Rico at Arecibo, lists his calculations of the index for various exoplanetary systems.[4] Méndez's ESI is calculated as
,
where and
are properties of the extraterrestrial body and of Earth, respectively,
is the weighted exponent of each property, and
is the total number of comparable properties.[4][undue weight? ] This formula has been used elsewhere.[5][6]
The ESI is designed so a planet with a high ESI value, in the range 0.8 to 1.0, corresponds to one that is of terrestrial rocky composition.
According to this measure, the closest planet to Earth in the Solar System is Mars with an ESI of 0.70.[7] A number of exoplanets have values in excess of this. Kepler-438b has the highest ESI[8] of confirmed exoplanets at 0.88, although others have indicated it is Kepler-62e[9] with an ESI of 0.83.[10]
Other ESI values include:[7]
- Earth - 1.00
- Gliese 581g - 0.89 (some astronomers do not believe Gliese 581g exists)
- Gliese 581d - 0.74
- Gliese 581c - 0.70
- Mars - 0.70
- Mercury - 0.60
- HD 69830 d - 0.60
- 55 Cnc c - 0.56
- Moon - 0.56
- Gliese 581e - 0.53
Relation to habitability
Although the ESI does not characterize habitability, given 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 paper 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 is one of the keys to surface conditions.[8]
Planets with an Earth-like size
![](https://web.archive.org/web/20160413170352im_/https://upload.wikimedia.org/wikipedia/commons/thumb/0/06/Relative_sizes_of_all_of_the_habitable-zone_planets_discovered_to_date_alongside_Earth.jpg/220px-Relative_sizes_of_all_of_the_habitable-zone_planets_discovered_to_date_alongside_Earth.jpg)
The classification of exoplanets is difficult in that many methods of exoplanet detection leave several 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.[11] Only Titan (a moon of Saturn) is known to hold on to a significant atmosphere despite an overall lower size and density. While Io (a moon of Jupiter) has a low average temperature, surface temperature on the moon varies wildly due to geologic activity.[12]
Use with other indices
The Habitable Exoplanets Catalogue ranks the habitability of planets and moons according to three criteria, i.e. surface temperature, similarity to Earth, and capacity to sustain organisms at the bottom of the food chain. For this, the catalogue uses the Habitable Zone Distance, the Earth Similarity Index, and the Global Primary Habitability scale.[13]
See also
References
- ^ "HEC: Data of Potential Habitable Worlds".
- ^ Schulze-Makuch, D.; Méndez, A.; Fairén, A. G.; von Paris, P.; Turse, C.; Boyer, G.; Davila, A. F.; Resendes de Sousa António, M.; Catling, D. & Irwin, L. N. (2011). "A Two-Tiered Approach to Assess the Habitability of Exoplanets.". Astrobiology 11 (10): 1041–1052. Bibcode:2011AsBio..11.1041S. doi:10.1089/ast.2010.0592.
- ^ Suresh Chandra; Subas Nepal; Mohit K. Sharma (July 2015). "Earth Similarity Index with two free parameters". arXiv:1507.06293v1 [astro-ph.EP].
- ^ a b "Earth Similarity Index (ESI)". Planetary Habitability Laboratory.
- ^ Gonzalez, A., Cardenas, R. and Hearnshaw, J. (2013). "Possibilities of life around Alpha Centauri B." (PDF). Revista Cubana de Física 30 (2): 81.
- ^ Planetary Habitability Laboratory. "Earth Similarity Index (ESI)". University of Puerto Rico at Arecibo.
- ^ a b "Most liveable alien worlds ranked". BBC. November 23, 2011. Retrieved April 10, 2016.
- ^ a b Armstrong, D. J.; Pugh, C. E.; Broomhall, A.-M.; Brown, D. J. A.; Lund, M. N.; Osborn, H. P.; Pollacco, D. L. (2016). "The host stars of Kepler's habitable exoplanets: superflares, rotation and activity". Monthly Notices of the Royal Astronomical Society 5 (3). arXiv:1511.05306. Bibcode:2016MNRAS.455.3110A. doi:10.1093/mnras/stv2419.
- ^ Rushby, A. (2013). "A multiplicity of worlds: Other habitable planets". Significance 10 (5): 11–15.
- ^ Mia, R. and Kushvah, B.S. (2016). "Orbital dynamics of exoplanetary systems Kepler-62, HD 200964 and Kepler-11". Monthly Notices of the Royal Astronomical Society 457: 1089–1100.
- ^ 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
- ^ 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.
- ^ Sample, I. (December 5, 2011). "Habitable exoplanets catalogue ranks alien worlds on suitability for life.". The Guardian. Retrieved April 9, 2016.
External links
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