Over the past several years, the search for planets around other stars show that most stars harbor planets and terrestrial planets are indeed very abundant. This leads to the idea of defining the types of planets where life can exist. Since our Earth is the only viable example of a planet that is habitable, planets with environments that can potentially support the kind of life on Earth are investigated. For this reason, the kind of life that is considered here uses carbon-based molecules with liquid water as a solvent and has one or more sources of energy to support it. Speculative forms of life based on other substrates such as liquid ammonia or even plasma ions are not considered due to the absence of any such examples. With regards to detecting the signatures of life on an exoplanet or exomoon, a biosphere which significantly modifies the planetary environment is much easier to detect and characteristic. On the other hand, a subsurface biosphere is unlikely to modify its planetary environment in an observable way, making the detection of such biospheres extremely difficult.
Class I habitable worlds consists of Earth-like planets where liquid water and sunlight are available on the planet's surface. Here, life derives its energy from sunlight either directly through photosynthesis or indirectly by consuming things that do. Due to the abundance of energy, complex multi-cellular life can evolve and thrive on the planet's surface. Sun-like stars which comprise of F, G and K-type stars are the most suitable stars around which class I habitable worlds can exist.
Low mass red dwarf stars can also harbor class I habitable planets even though a significant fraction of such planets are probably tidally-locked. This is because atmospheric and oceanic circulation can be sufficient to distribute heat between the permanent day and night hemispheres to prevent temperature extremes from building up. Class I habitable planets can also exist within the habitable zones around white dwarfs or brown dwarfs. However, as the white dwarf or brown dwarf cools, the habitable zone will shrink and eventually leave the class I habitable planet out in the cold, beyond the outer edge of the habitable zone. This can turn a class I habitable planet into a class III habitable planet.
Figure: Artist’s impression of a class I habitable world.
Class II habitable worlds consists of planets where life can exist but the planet evolves differently from the Earth. This class of habitable worlds includes planets that initially harbored Earth-like conditions but had evolved to be unable to sustain surface liquid water. In this case, life could have migrated to whatever limited pockets of habitable environments that still remain. Venus and Mars are potentials candidates for class II habitable worlds. For Venus, life could have established itself in the cool upper atmosphere, and for Mars, life could have migrated down into deep subsurface aquifers.
Figure: Mars, a potential class II habitable world.
Class III habitable worlds consists of planetary bodies with subsurface oceans of liquid water that directly interact with a silicate core. On such a world, the surface temperature is too low for liquid water to exist on the surface and the subsurface ocean lies beneath a surface layer of ice. An example of a potential class III habitable world is Jupiter's moon Europa which may be the only place in the Solar System with a global subsurface ocean of liquid water that is in contact with a silicate core. The ocean on Europa is kept warm through tidal heating and it is expected to contain a factor of a few times more liquid water than all oceans on Earth combined. The benefit of being in direct contact with a silicate core at the bottom of the ocean is that interactions with silicates and hydrothermal activity can provide the ocean with materials that are essential for life. A class III habitable world can transform into a class I habitable world if the temperature on its surface exceeds 273 degrees Kelvin, allowing surface liquid water to exist.
Class IV habitable worlds consists of water-rich worlds with liquid water oceans existing above a layer of solid ice. Here, the water layer is thick enough that pressures at the bottom are sufficiently large for water to exist as high pressure forms of solid ice (ice polymorphs). In the solar system, examples of potential class IV habitable worlds include Jupiter's moons Ganymede and Callisto, and Saturn's moon Titan. For each of these 3 moons, their liquid water oceans are sandwiched between a thick overlying layer of normal ice and a bottom layer of high pressure ice.
Class IV habitable worlds also include "ocean planets" where the surface temperature is high enough for liquid water to exist, resulting in a deep surface ocean overlying a layer of high pressure ice. Such "ocean planets" have no analogues in the Solar System. For some "ocean planets", volcanic and tectonic activity can create undersea mountains that may be sufficiently high enough to penetrate above the layer of high pressure ice. This allows the ocean to have some interaction with silicates, thereby blurring the distinction between a class I and a class IV habitable world.
Figure: Artist’s impression of a class III or class IV habitable world where a liquid water ocean exists beneath the surface.