Planet Rotation Limits Habitability

Life can’t exist on a planet that rotates too fast or slow.  This is another Goldilocks problem for astrobiologists to consider.

NASA’s Astrobiology Magazine has added another factor to habitability: planetary rotation.  In “Rotation of Planets Influences Habitability,” Amanda Doyle reports on findings from a paper on the arXiv server scheduled for Astrophysical Journal Letters.  After giving the usual definition of the habitable zone as the inner and outer radius around a star where liquid water can exist, she complicates things:

However, this concept is rather simple. In reality, many other factors come into play that could affect a planet’s habitability. New research has revealed that the rate at which a planet spins is instrumental in its ability to support life. Not only does rotation control the length of day and night, it can also tug on the winds that blow through the atmosphere and ultimately influence cloud formation.

Doyle shows how a planet that spins too fast ends up with atmospheric Hadley cells that make the planet too hot:

This will decrease the temperature difference between the equator and the poles and ultimately weaken the Hadley cells. The result is fewer clouds at the tropical regions available to protect the planet from the intense heat, and the planet becomes uninhabitable.

At the other extreme, a slow rotator is more habitable, because Hadley cells will encompass the whole planet, allowing hot can flow to the cold side, and clouds form to protect the hottest hemisphere.  This should extend the inner edge of the habitable zone closer to the star.  But what about Venus, a slow rotator?  The authors’ model breaks down in that case; “This goes to show that just because a planet is rotating slowly does not automatically mean that it is habitable, rather it has the potential to be habitable if the right conditions exist.”  To rescue that observable case, the authors had to speculate that Venus used to rotate faster.

This adds a 12th factor to our growing list of habitable zone requirements.  It’s looking more like Earth hit the cosmic jackpot.  By chance or design?