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 analog. The index does not have any meaning for habitability, and even a grade 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 bulk density, and (2) associated with the surface which is associated with the escape velocity and surface temperature. An article on the preprint server 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.
Ratings[]
| Planet | ESI | Notes | Distance (ly) |
|---|---|---|---|
| Earth | 1 | Reference | 0 |
| KOI-4878.01 | 0.98 | Planet is unconfirmed. Orbits a Sun-like star. | 1075 |
| Teegarden b | 0.97 | 12 | |
| TRAPPIST-1e | 0.95 | Orbits a ultra-cool red dwarf star. Likely tidally locked. | 39.5 |
| Gliese 581 g | 0.94 | Planet is unconfirmed. Tidally locked. | 20 |
| Luyten b | 0.91 | Tidally locked. | 12.2 |
| Gliese 667 Cc | 0.9 | Tidally locked, temperature is about 277.4 K (4.3 °C, 39.7 °F) based on black body temperature calculation | 23.6 |
| TRAPPIST-1d | 0.89 | Tidally locked. | 39.5 |
| Kepler-438b | 0.88 | Likely tidally locked | 640 |
| Proxima Centauri b | 0.87 | Closest possibly habitable planet | 4 |
| LHS 1723 b | 0.87 | No density or atmospheric data available | 17.5 |
| Ross 128 b | 0.86 | Host star is inactive and quiet, should be habitable if it has an Earth-like atmosphere | 11 |
| Kepler-442b | 0.84 | In the center of the habitable zone | 1206 |
| Kepler-452b | 0.84 | 1402 | |
| Kepler-62e | 0.83 | 990 | |
| Gliese 832 c | 0.81 | Tidally locked and no plate tectonics. Likely very similar to Venus, but habitable if it possesses an Earth-like atmosphere | 16 |
| Kepler-283c | 0.79 | 1743 | |
| HD 85512 b | 0.77 | Habitable if it has an Earth-like atmosphere | 36 |
| Wolf 1061c | 0.76 | 13.8 | |
| Kepler-440b | 0.75 | 850 | |
| HD 40307 g | 0.74 | 42 | |
| Mars | 0.73 | Lacks global plate tectonics. Too small for atmospheric water vapor. | 0 |
| Kepler-61b | 0.73 | 1100 | |
| Gliese 581d | 0.72 | Unconfirmed, tidally locked. | 20 |
| Kepler-22b | 0.71 | Tidally locked. | 587 |
| Kepler-443b | 0.71 | While it has an 89.9% chance to be habitable, it only has a 4.9% chance of being terrestrial. | 2540 |
| Gliese 422 b | 0.71 | Unconfirmed. | 20 |
| Gliese 3293 c | 0.7 | Unconfirmed. | 20 |
| Kepler-62f | 0.69 | 990 | |
| Mercury | 0.6 | In 3:2 spin-orbital resonance | 0 |
| HD 69830 d | 0.6 | No density or atmosphere data available | 40 |
| 55 Cancri c | 0.56 | No density or atmosphere data available | 41 |
| Moon | 0.56 | No plate tectonics, too small for surface or atmospheric water. | 0 |
| Venus | 0.44 | 0 | |
| Gliese 1214 b | 0.42 | Likely to be an ocean planet, temperature is somewhere between 390 K (116 °C, 242 °F) - 552 K (278 °C, 533 °F) | 48 |
| Jupiter | 0.26 | Gas giant. | 0 |
| Gliese 581 c | 0.24 | Tidally locked. | 20 |
| Saturn | 0.2 | Gas giant. | 0 |
| Uranus | 0.17 | Gas giant. | 0 |
| Gliese 581 e | 0.16 | Tidally locked. | 20 |
| Neptune | 0.14 | Gas giant. | 0 |
| HD 209458 b | 0.08 | Also known as Osiris. | 159 |
| Kepler-70c | 0.03 | 4200 | |
| Sun | 0.01 | Yellow dwarf star. | 0 |