Solar Change and Planetary Habitibility

Watery worlds like this may not last long.

Earth has been habitable for a very long time, virtually its entire 4.5 billion year life.  Water has existed on the surface for nearly all of that time, and so has life.  That life was only single-celled for most of that time, but there was life nonetheless.  The Earth is a fantastic place for life, and will be for billions of years to come, right?  Well, not quite.  The sun is slowly but surely getting brighter, and in a billion or so years from now, it will eventually render Earth inhospitable to life.

The slow evolution of the sun presents a problem for planetary habitability everywhere.  Stars everywhere exhibit this same behavior, it's the natural life cycle of a star.  The Sun, for example, was about 30% less bright when it first formed as compared to today.  The Earth of 4 billion years ago was in the habitable zone at that time, and liquid water formed on the surface.  According to the models, considering how the Sun has brightened, the Earth should be about 100°C warmer today than it was when it formed.  But it isn't, if anything, it's cooler now than before.  So, what's going on?

The answer, it seems, is that the slow evolution of our planet and the life on it has fortuitously balanced out the heating effects of an expanding Sun, moderating the temperature and keeping the planet cool.  This is not good for the search for complex and intelligent life in the universe.  While it is likely that life evolved on many planets, most of those planets will not have experienced the same good fortune the Earth has had.  Cancelling out solar heating through "geobiological" cooling is not a guarantee, and so there will be many worlds out there that have heated up beyond where life can restore planetary balance.

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