ONE/6 Life Zone
STEP ONE: Use the formulae below to determine the inner, optimum and outer limit of the life zone. Calculate the life zone for all stars in a multiple system, but for Very Close binaries you can add the luminosities together before calculating the life zone.
WHAT IS THE LIFE ZONE?
It is the approximate distance from a star where a planetary body similar to Earth could be warm enough to have liquid water, but not so warm as to boil said water away. Earth-like life require liquid water, thus the name "life zone". We generate the life zone to determine which worlds-if any-are candidates for life based upon liquid water.
DO ONLY PLANETS WITHIN THE LIFE ZONE HAVE LIFE?
Not necessarily. First of all, the life zone is an approximation. The exact temperature of a world is calculated in much more detail in the section on Atmospheric Data. A planet might have a very high greenhouse effect, or a moon be heated by tidal deformation, and thus be warm enough despite being outside the life zone. For the same reason, a planet within the life zone might be inhospitable to life. Life not based upon water may have very different life zones - a life zone based upon liquid ammonia would be more distant, for instance. Life of low complexity have wide life zones, while complex life has a less wide one.
LIFE ZONES IN MULTIPLE STAR SYSTEMS:
Though it may be rather rare, a world might lie outside the life zones of two stars but inside their combined life zone. Consider a planet orbiting just outside the life zone of a small red star, but that star in turn orbit a much brighter star which provide enough additional radiation to heat the world. This may work well for a planet orbiting two close red dwarves too.
LIFE ZONE AND SYSTEM AGE:
We have already seen that stars change luminosity with age. Typically, the life zone moves outward, and this means that worlds which once were habitable will become too hot with time. It also means cold planets may be heated in the later life of the system. The optimum for a world is to be within the life zone (near the outermost limits) during the early years of a system, as this grants the longest possible time to evolve life. If a world which was too cold much later gets into the life zone, it is likely that life wouldn't start as easily. Much of the primordial gasses would have escaped, volcanism might have died down etc. To approximate where the life zone was in the early system age, check the luminosity modifications from the System Age section and use that luminosity to calculate life zones. In very young systems planets will still be warm from their formative stages and have fairly extensive atmospheres which have had little chance of escaping, but at 1 Gy this will have settled down.
SOME WORLDS BECOME COOLER ANYWAY:
On the other hand, there are possible ways a once habitable world can get too cold to sustain life. One way is if the star loses luminosity - most typical among brown dwarves who while they may have life-bearing worlds, usually tidally locked, lose luminosity due to their cooling. A large brown dwarf may drop 200°K/Gy for the first billion years.
Another way to cool a world is if the atmosphere changes, perhaps by becoming thinner due to escaping gas (typical on low-gravity worlds) or by sunlight breaking down greenhouse gasses. A third one is a change of axial tilt or eccentricity of the orbit. Also, all worlds cool off after their creation, and if volcanism dies down gasses like carbon dioxide and water vapor might not be released in the same degree as they are removed. Other options include growing glaciations which lead to a high albedo and thus lower temperature. Such glaciations could be triggered by continental drift and axial/orbital changes, but also by huge amounts of dust in the atmosphere which would block sunlight - an asteroid impact or especially explosive volcanic event are possible triggers.