Planetary Science Institute, a joint initiative of Research School of Earth Sciences and Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 0200, Australia
As we get closer to finding planets beyond our solar system that are similar to Earth, we wonder if these extra-solar planets are habitable. Astrobiologists Dr Charles Lineweaver and PhD student Aditya Chopra at the Planetary Science Institute have examined what features of our own planet enabled the origin and evolution of life, and if these features are common elsewhere in the universe.
With the advent of instruments such as the space-based Kepler Telescope, we are on the verge of finding planets that are about the same size and mass as the Earth and in the 'habitable zone' of a star where it is possible for liquid water to be present on the surface. Would such a planet be sufficient to host life? Lineweaver says, "While liquid water is an important requirement for life, it is not the only constraint. On Earth, there are places where there is liquid water but without life." Lineweaver points out that if we are to seriously study habitability we need to consider the "astrophysical, geochemical, geophysical and biological limits on planet habitability."
In order to quantify the habitability of Earth, the authors describe the profile of the 'bioshell' - the small part of our planet, which is inhabited by life. "People are familiar with the term biosphere. Here we have introduced the term bioshell to highlight the relative thinness of the sphere within which we live.", says co-author Chopra. "The thinness is even more apparent when one considers that until a billion years ago almost all of the biomass was within marine sediments and only since plants spread on land has half of the biomass been distributed above the sea-floor."
The notion of habitability is not restricted to the present day. "The habitability of planets can change over time.", Lineweaver says. Life and its environment co-evolve. Our planet transitioned from an uninhabited planet to an inhabited planet with life forms that continually modify the environment, for example by oxygenation of the atmosphere. The authors propose the dynamic concept of an 'Abiogenesis Habitable Zone' (AHZ, where life can get started) which then could transition into a 'Habitable Zone' and suspect there might be planets which might be habitable but because they were not within the AHZ at any time, they remained uninhabited.
Reviewing discoveries of exoplanets, Lineweaver and Chopra find that nearly all of stars have planets around them and that a large fraction of them will be habitable. The expectation that the universe is filled with habitable planets is based on the finding that the presence of liquid water and the temperature range (between -20°C and 120°C) are the two most important parameters controlling habitability - features that should not be unique to Earth.
In this invited paper for the 2012 Annual Reviews of Earth and Planetary Sciences, the authors have considered habitability from the view point of what energy sources power the simplest life forms to identifying the habitable and inhabited regions on Earth. Not limited by scale, they also discuss habitable zones around stars and within galaxies.
Describing the research, Lineweaver says that over the years he has seen a many factoids dispersed in the literature about life on Earth and exoplanets. In writing this review, "we have tried to take a comprehensive view of habitability.