In a recent review article Lammer et al. (2009) proposed a classification of four habitat types. Class I habitats represent bodies on which stellar and geophysical conditions allow Earth-analog planets to evolve so that complex multi-cellular life forms may originate. Class II habitats includes bodies on which life may evolve but due to stellar and geophysical conditions that are different from the class I habitats, the planets rather evolve toward Venus- orMars-type worlds where complex life-forms may not develop. Class III habitats are planetary bodies where subsurface water oceans exist such as on Europa, which interact directly with a silicate-rich core, while class IV habitats have liquid water layers between two ice layers, or liquids above ice.
Class I habitable planets where complex multi-cellular surface life forms as we know on Earth can evolve also need to orbit around the right star. G-type stars and K and F-types with masses close to G stars should fall in this category. In such a case the activity of the host star decreases fast enough so that an evolving atmosphere and life may not be in danger of losing the atmosphere or the planet’s water inventory. Furthermore, the large distance of the corresponding HZ of such star systems lessen the efficiency of the non-thermal loss processes. The possibility that various atmosphere compositions and the water inventory can remain stable on such planets over geological time spans exists as long as the environment can keep plate tectonics with all its related consequences active over billions of years.
Class II habitat environments where life may originate but a planet evolves differently from Earth could be expect within HZs of low mass M and K-type stars which are located very close to these stars, so that their atmosphere-magnetosphere environments experience extreme stellar radiation and plasma exposures over very long time periods or even during most of their life-time. In such cases thermal and non-thermal atmospheric escape processes could modify the atmospheres and water inventories of the planets in such a way that they may end up after some hundreds of million years as geophysically inactive, dry Venus-like or cold Martian-like planets, although they originated and orbit within the classically defined HZ. It seems possible that should life have started early in the history of Class II habitable planets and if favorable conditions prevailed long enough to allow for an evolution that it might persist even after the loss of (almost) all water. The production of complex and diverse ecosystems, however, depend on the carrying capacity of the planet and the circumstances how fast life may develop. While in the cases of Class I and Class II habitable planets discussed above the main question is whether conditions favorable to life persist long enough to allow life to develop and evolve after it started, the question here is if life could start at all.
Class III habitats where subsurface oceans are in contact with silicates on the sea-floor open the question of where the building blocks for life could come from. By assuming that the organic material necessary to start life is supplied by impact of meteorites and comets and by precipitation of interplanetary dust, this material impacting on the surface has to find its way into the subsurface oceans. Also this material has to reach meaningful concentrations in some (small) compartment of the ocean, which is hard to imagine in a connected body of water as large as a planet-wide subsurface ocean. However, one should keep in mind that synthesis of organic material by either Fischer–Tropsch reactions or catalytic cycles are possible under the high pressure/high temperature conditions occurring at deep-sea vents. In such environments on Earth, reduced radicals such as H2 are contained in the hot fluid and can provide energy for a variety of organisms. However, the source of energy necessary to power an organism could be another problem.
Class IV habitats and exoplanets where a water ocean is in contact with a thick ice layer a much better situation for the influx of organic material from outside the planet compared to bodies like the Jovian satellites Ganymede or Callisto. The main problem encountered in Class IV habitats is, however, much more severe: that of the concentration of the necessary ingredients for life. A planet whose surface is completely covered in water several kilometers deep with nothing to act as a concentrating “sponge” for organic chemistry is probably too vast for any two or more interesting molecules to meet. While a sea/ice system could in theory provide such a means of concentrating life’s ingredients, most likely the starting concentrations needed for a system like that may be be reached in addition to the quite specific temperature conditions needed.
