Habitable Zones

An important concept in the evolution of life on a planet around a star is that of a habitable zone or ecosphere. Generally, as for the porridge in Goldilocks and the Three Bears, planets too close to the star will be too hot for life, planets too far away will be too cold, and there should be an intermediate distance that is "just right". We do not know fully the range of conditions that could support life, but life on Earth requires liquid water and that sets strong constraints since water is liquid only over a 100 degree temperature range. If we define the expected "just right" temperature range around a star to be that where water is liquid, we find that the resulting habitable zone depends on the type of star that it is. This is illustrated in the following figure.

Notice that the volume of the habitable zone is generally larger around the hotter stars. For the G2 star, the orbits of Venus, Earth, and Mars are indicated. Only Earth lies clearly within the habitable zone within our own Solar System. Notice how small the habitable zone is around the M0 star. Comparing the G2 and M0 examples, we see that Earth would lie outside the habitable zone (it would be too cold for liquid water) if the Sun were an M0 star.