Various habitability metrics can be used to assess and compare the potential for life, as we know it, of exoplanets. The main idea behind these metrics is to simplify the complex interaction of life and many environemntal factors into fewer manageable numbers. They also provide a system to rank exoplanets, establish observational priorities, and even recognize those more habitable than Earth itself (i.e. less deserts).Here is a description and test of some of the habitability metrics and classifications that will be used as part of the Habitable Exoplanets Catalog (HEC). Most of them require a few stellar and planetary properties that are already available for many exoplanets but models can be used otherwise. A paper describing one of these metrics was recently accepted for publication.
Of the four presented habitability metrics (Table 1), the GPH and the SPH are better indicators of habitability than the HZD and the ESI, but require more information about the exoplanets. All metrics focus on surface habitability and does no consider subsurface environments. They are limited to what we can measure today for exoplanets and assume that all the other requirements of life are presence (i.e. oxygen). There are also two classifications systems for any planet or just habitable planets (Table 2). These metrics and classifications are being tested with Solar System and exoplanets data and some future changes to their formulations are possible (Figure 1 and 2).
Figure 1. Example of a view of the Habitable Exoplanets Catalog (HEC) for Solar System planets. Three complementary habitability metrics are shown. The HZD is a measure of how far is the planet from the center of the habitable zone, zero being exactly in the center (using a Venus maximum greenhouse habitable zone definition). The ESI compares the similarity of various planetary properties with Earth, a value of one being identical to Earth. The SPH is a measure of how good is the surface to support life, with a value of one being optimum. The SPH of Earth is 0.33 because all its land areas are not equally good for life due to deserts (maximum SPH is 0.51 without water limitation on land). Habitable planets are those with ESI values over 0.8 or SPH over zero (CREDIT: PHL and NASA Solar System Imagery).
Habitable Zones Distance (HZD)
The Habitable Zones Distance (HZD) measures 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 corresponds 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 Earth Similarity Index (ESI), or the "easy scale," measures how similar is a planet to Earth in a scale from zero to one, with one being identical. Planets with ESI values between 0.8 and 1.0 can be considered Earth-like as this means that they have a similar rocky composition and are able to hold a comparable atmosphere under temperate conditions. The ESI is a function of the planet's radius, density, escape velocity, and surface temperature.
Global Primary Habitability (GPH)
The Global Primary Habitability (GPH), or "gp-hab," measures the surface suitability of a planetary body for a global biosphere of primary producers in a scale from zero to one, with one being more habitable. The GPH is a function of the star's luminosity, temperature, age, and metallicity, and the planet's albedo, mass and radius. It is the most complete evaluation of planetary habitability given the information we know of exoplanets. The GPH is still under development.
The Standard Primary Habitability (SPH), or 'sp-hab,' measures the thermal-water climate suitability of a planet for land primary producers in a scale from zero to one, with one being more habitable. It is correlated with the distribution of vegetation and net primary productivity (NPP). The SPH is a function of the planet's surface temperature and relative humidity. The SPH was originally develop for local scales but it can be extended to global scales for exoplanets studies.
Planetary Mass Classification (PMC)
The Planetary Mass Classification (PMC) categorizes planets in seven mass divisions using Solar System analogs for comparison. The categories are asteroidan, mercurian, subterran, terran, superterran, neptunian, and jovian. The classification can be used for any solar and extrasolar planets including moons. The PMC is not itself a habitability classification but gives context for such.
The Planetary Thermal Classification (PTC) categorizes terrestrial planets in five temperature divisions using the known thermal requirements of life for comparison. The categories are hypopsychroplanets (Class hP), psychroplanets (Class P), mesoplanets (Class M), thermoplanets (Class T), and hyperthermoplanets (Class hT). Mesoplanets are considered Earth-like planets in the PTC as they have the basic requirements to support complex life. The generic Class NH is used for non habitable planets.
Figure 2. Comparison of terrestrial similarity (ESI) of Solar System planets with respect to the habitable zone (HZD). The plot clearly shows that most Earth-like planets are closer to the habitable zone in our Solar System. A fact that is not necessarily true for other stellar systems. A similar plot for known exoplanets looks very interesting and will be shown as part of the Habitable Exoplanets Catalog.