Today we cannot be certain that any exoplanet is really habitable, less inhabited. Life depends on many planetary characteristics that are simply not known for exoplanets. Therefore, the list of worlds of the Habitable Exoplanet Catalog is only a selection of the best objects of interest based on current observations and knowledge of what makes a planet habitable.
Habitable means suitable for life and not necessarily that life is present. Life could be simple such as bacteria or complex like plants or animals, either terrestrial or extraterrestrial, if any. Millions of years ago Earth was only able to sustain microbial life, today it can also maintain plants and animals and support a more diverse biosphere. The potential for life of any planet is a function of time.
Any habitability assessment of a planet is limited to only the thing we can measure of them now. Generally, this includes their size (i.e. mass or radius), and orbital characteristics (i.e. stellar flux). Scientists usually focus on surface life using water as solvent, or life as we know it, since it should be easier to understand and detect. Subsurface life or life using other solvents are always an open possibility.
4. Extended Definition
Habitability metrics can be used to assess and compare the potential for life of exoplanets as a function of many stellar and planetary properties. They provide a system for exoplanets identification, ranking, observational prioritization, and comparisons. Most of these metrics requires stellar and planetary properties that are already available for many exoplanets but models are used otherwise.
Five habitability metrics and two classification systems are used as part of HEC. Exoplanets data are obtained from the Extrasolar Planets Encyclopaedia, the Exoplanet Data Explorer, the NASA Exoplanet Archive, and NASA Kepler Mission.
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.
The Habitable Zone Composition (HZC) measures how compatible for life is the bulk composition of an exoplanet . HZC values between -1 and +1 have a habitable composition with an iron-rock-water mix. Values below -1 correspond to unlikely high dense iron bodies (i.e. a core from a dead gas giant). Those above +1 correspond to gassy bodies like Uranus, Neptune, Jupiter, and Saturn. HZC values closer to zero are generally better candidates for a habitable bulk composition. The HZC is a function of the planet's mass and radius.
The Habitable Zone Atmosphere (HZA) is a measure of the potential of an exoplanet to hold a habitable atmosphere . Values between -1 and +1 have the capacity to thermally hold an atmosphere with basic ingredients for life, such as carbon dioxide, oxygen, nitrogen, water vapor, ammonia, and methane. Values below -1 correspond to bodies with thin or without atmospheres. Those above +1 correspond to probably dense atmospheres composed mainly of hydrogen and helium. HZA values closer to zero are not necessarily better. The HZA is a function of the planet's mass, radius, distance, and the star’s luminosity.
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.
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 (NPP). 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.
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.
The hClass is a classification only for habitable worlds (terrestrial planets inside the HZ) in five thermal categories. The categories are hypopsychroplanets (Class hP), psychroplanets (Class P), mesoplanets (Class M), thermoplanets (Class T), and hyperthermoplanets (Class hT). 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.