Current and future observations by ground and orbital missions will be able to identify habitable exoplanets. As a first assessment, the surface temperature of Earth-like exoplanets is used as a proxy for habitability. Surface temperature is indirectly determined by the exoplanet's distance to the parent star together with the star's luminosity within the stellar habitable zone. So far, this is the easiest method to assess the potential for life of exoplanets and future observations will be needed for better assessments that include directly planetary temperatures measures together with atmospheric composition. However, not all habitable exoplanets are expected to be equally habitable. Even Earth's habitability changed through time and any habitable exoplanet could be in any stage.
The number of habitable exoplanets that will be detected by current (i.e. Kepler) and future missions (i.e. TPF) will require some way of classification. There are many ways to do this (i.e. Earth Similarity Index) but here we are focusing in constructing the simplest formulation, one based on temperature alone but restricted to terrestrial exoplanets with abundant water in their surface. Their terrestrial nature can be inferred by their size and density. Unfortunately, we need to assume that water, as a very abundant substance in the universe, is also present in necessary quantities for life (i.e. lakes or oceans) as we don't still have the capability for such observations, at least for terrestrial exoplanets.
It is interesting to note that there are no planetary habitability classifications schemes, at least in the scientific community. Only in the Star Trek sci-fi universe there is a planetary classification where Earth-like planets are classified as Class M Planets. Although appealing, this classification mix habitability, geological evolution,and atmospheric properties in a complex and unorganized way not suitable for exoplanets studies. There is also the less known Sudarsky's thermal classification of giant exoplanets but not one for habitable exoplanets. The field of exoplanetology is still growing and there was no need for a habitability classification before as Earth is the only habitable "item," so far.
To develop a planetary habitability classification based on temperature we need to consider our understanding of the thermal requirements of life in general. Microbial life such as bacteria and archaea has a wide thermal tolerance and growth has been measured at temperatures from -15°C to 121°C. Macrobial life like animals (metazoa) show a more restrictive tolerance usually between 0°C to 50°C. This is also true for most microbial (i.e. cyanobacteria) and macrobial (i.e. plants) primary producers, which generate most of the energy for consumers in the trophic scale. Most plants are particularly efficient at temperatures close to 25°C. Water is liquid between 0°C to 100°C at standard atmospheric pressures but is also known to stay liquid down to near -50°C in supercooled states or in combination with antifreeze agents (i.e. salts). Therefore, and for simplicity of the classification, it is practical to divide the thermal tolerance of life in bins of 50°C around the freezing point of water.
There is a well-known and simple thermal classification for microbial life in the microbiology field. Mesophiles are those microorganisms that growth best at moderate temperatures between 10°C to 45°C. Psychrophiles between -15°C to 10°C, thermophiles between 45°C to 80°C, and hyperthermophiles above 80°C. These limits were arbitrarily decided for historical reasons and there is no strict standard between the exact values. Still, it has been a very practical thermal classification in microbiology. It is also extensible to our planetary habitability classification using the same greek prefixes for exoplanets as mesoplanets, psychroplanets, and thermoplanets. However, we revised the limits to 50°C increments as they are easier to remember and are also relevant to both simple and complex life (Figure 1).
The proposed names of our classification can be further abbreviated as M-planets (mesoplanets), P-planets (psychroplanets), and T-planets (thermoplanets). All three classes represent potential habitable exoplanets based on their mean global surface temperature. Only mesoplanets (temperate) correspond to Earth-like planets, which may be potentially habitable by complex life as we know it, such as plants and animals. Psychroplanets (cold) and thermoplanets (hot) may only be habitable for microbial life. Even an extension of the classification as hypopsychroplanets or hyperthermoplanets might still be habitable, but these conditions are in the limits of our understanding of carbon-based life in aqueous environments.
Figure 1. Proposed Thermal Planetary Habitability Classification (T-PHC) for exoplanets. The classification suggests the use of the terms mesoplanets or M-planets to refer to "Earth-like exoplanets," or those terrestrial planets with mean global surface temperatures between 0°C and 50°C, conditions known to be necessary for complex terrestrial life.
As an added bonus, our proposed classification names matched the letters and general description of Class P and Class M planets of the Star Trek planetary classification. This was totally out of luck. Unfortunately, Class T are gas giants planets in the Star Trek classification and here used for hot planets. The name mesoplanet was also used before by Isaac Asimov to refer to planets between the size of Ceres and Mercury in various sci-fi stories. The proposed classification is easy to understand by the scientific community, familiar to microbiologists, and also easy to understand by the general public, specially trekkers. There was some fortuity pop-culture in its development.
Now that we have a classification we can start to put in context the recent discoveries of potential habitable planets. In particular, the exoplanets Gliese 581g (unconfirmed) and 581d, and the Kepler's candidates KOI-701.03 and 326.01. We can also include Earth and Mars for comparison (Figure 2). The other objects of the Solar System will be out of the scale and therefore non habitable by the proposed classification, at least at the surface. Planetary bodies like Mars, Europa, Titan, and Enceladus are more interesting at the subsurface levels. The evolution of Earth's habitability since the origin of life is interesting as it probably changed from a Class hT to a Class M planet in 4 billion years. Mars probably started as a Class M planet that rapidly changed to Class hP in less than one billion years.
Figure 2. Thermal classification of potential habitable exoplanets with Earth and Mars for comparison. Only Gliese 581g and KOI-701.03 fall in the category of mesoplanets or truly Earth-like exoplanets, if confirmed. Exoplanets like Gliese 581d and KOI-326.01 are in categories that only supports microbial life, if any. Mars is in one of the extremes on the categories where any surface microbial life is much less likely. Only future observations will be able to confirm these habitability assesments. There is still much uncertainty in current observations and large shifts in the exoplanets classifications are probable.
The proposed thermal planetary habitability classification provides a simple classification scheme based on temperature for terrestrial exoplanets. It is compatible with our understanding of the thermal limits for terrestrial life and the observations that are currently possible for exoplanets. The classification can be expanded with other planetary parameters preserving its functional meaning as more detailed information of terrestrial exoplanets are obtained. It also provides and easy method to communicate results to scientists and the media about future discoveries. Mesoplanets (Class M) are the main goal of current efforts in exoplanets searches, but psychroplanets (Class P) and thermoplanets (Class T) are still interesting. A future article will discuss the categories in detail.