Gliese 581 d is one of a few potential habitable exoplanets confirmed so far

This is a list of potential habitable exoplanets candidates ranked by similarity with Earth using the Earth Similarity Index(ESI). The index was made by the Planetary Habitability Laboratory at UPR-Arecibo.

[edit]Methodology

[edit]Earth Similarity Index (ESI)

The ESI or the “easy scale,” measures how similar are planets to Earth in a scale from zero to one, with one being identical to Earth. ESI values between 0.8 and 1.0 correspond to Earth-like planets with a rocky composition that is able to hold a terrestrial atmosphere under temperate conditions. The ESI is a function of the planet’s radius, density, escape velocity, and surface temperature.

[edit]Standard Primary Habitability (SPH)

The SPH measures the thermal-water climate suitability of a planet for land primary producers (vegetation) in a scale from zero to one, with one being more habitable. It is correlated with the global distribution of vegetation and net primary productivity. The SPH is a function of surface temperature and relative humidity. Only the thermal component of the SPH is used for exoplanets, assuming that water is present.

[edit]Habitable Zones Distance (HZD)

The HZD is a measure of how far a planet is from the center of its parent star habitable zone (HZ) in habitable zone units (HZU). Planets inside the HZ have HZD values between -1 to +1 HZU, with zero being the exact center of the HZ. The negative and positive values correspond to locations closer and farther to the star, respectively. The HZD is a function of the star’s luminosity and temperature, and the planet’s distance.

[edit]Planetary Class (pClass)

The pClass classifies planetary bodies with a combination of three thermal zones and seven mass categories. The thermal zones are hot, warm, and cold. They are related to the orbital position of the body with respect to the HZ, warm being the HZ. The mass divisions are asteroidan, mercurian, subterran, terran, superterran, neptunian, and jovian. The classification can be used for any solar and extrasolar planets including moons.

[edit]Habitable Class (hClass)

The hClass is a classification only for habitable worlds (Earth-size planets inside the HZ) in five thermal categories. The categories are hypopsychroplanets (Class hP, very cold planets), psychroplanets (Class P, cold planets), mesoplanets (Class M, medium temperature planets; not to be confused with Isaac Asimov‘s coinage mesoplanet), thermoplanets(Class T, hot planets), and hyperthermoplanets (Class hT, very hot planets). These names where inspired by the names used in microbiology to describe the growth temperatures of microbial life. Class M planets also have the thermal surface requirements to support complex life (0-50°C), a name familiar from science fiction. The other categories represent conditions only habitable by extremophilic life. The generic Class NH is used for non habitable planets.

[edit]List

For comparison purposes, four solar terrestrial planets are included in the list. The only three confirmed exoplanets in the data, HD 85512 b, Gliese 581 d and Kepler-22b, still rank very low as compared to the NASA Kepler candidates.