The "habitable zone" is shorthand for that region of a solar system which is sutable for life. The big question facing astronomers and planetary scientists is whether that zone is wide, in which case there should be many other independent examples of life that evolved on other water-rich worlds elsewhere in our galaxy, and in the wider Universe; or whether the zone is narrow, in which case it may be a long time before we finds of life anywhere else than on Earth.

In recent years, considerable interest has been raised in this topic and the example of Mars has been used to suggest that liquid water could be stable much further out in our solar sytem than the Earth's orbit. Values in excess of 2 or 3 AU have been quoted, based on very optimistic models of atmospheric behaviour. Unfortunately, my new model for Mars refutes the existence of liquid water on Mars' surface, so the habitable zone is rather more restricted - at least in terms of warm blue planets with abundant life on their surface.

My work shows that the maximum distance that a planet could orbit our Sun and be spontaneously habitable (i.e. without terraforming) is about 1.25 AU. At this distance a Terrestrial planet with the standard inventory of CO2 and H2O would have started off some 4.5 billion years ago with a surface temperature and pressure very similar to modern Mars. As the sun warmed, the planet would slowly warm too. After about 1.5 billion years, the planet would thaw enough for liquid water to be stable at the equator, and the process of conversion of CO2 to carbonate rock would begin and the extensive CO2 pole caps would begin to disappear. Once consumed, their depressing effect on temperature (caused by the reflection of sunlight back into space) would be removed and the planet could warm up appreciably. At the present day, such a planet would have Earth-like temperatures but would need a large proportion (15%) of CO2 in the atmosphere to acheive this. 15% CO2 is poisonous to higher life forms, but extremophiles can survive and if this was the planet's atmosphere, then life could well evoilve to tolerate it.

There's no reason why life couldn't evolve on a colder planet, further from the Sun. But it would be forever restricted to underground habitats where volcanic activity warmed the ground. In nature, this life would resemble extremophile bacteria and algae on Earth, which appears to have been the first form of Terrestrial life from which everything else evolved. In my new model for Mars, any life would be of this nature. The search for fossils on the surface and in the sediments of Mars is likely to be unprofitable (but success would mean an enormous revisoin of our understanding of the nature of Mars, the solar system, and life itself).

The minimum distance is more difficult to constrain. One hard constraint is to avoid a Venusian hothouse (where the temperature exceeds the critical point of water and the entire global oceans evaporate) This gives a value around 0.8 AU for an orbit where the hothouse doesn't set in immediately (Venus is at 0.723 AU).

Another possibility is offered by the speculation that the origin of life on Earth may have required liquid CO2 and liquid water to coexist. This may narrow the range even further, but it will take much more work to make this a serious scientific proposal, rather than just idle speculation.
 

      Created: May 2002
      Last modified: May 2002
      Authorised by:  Head, Earth Sciences

      Maintained by: Nick Hoffman
      Email: nhoffman@unimelb.edu.au