Habitability is Potentially More Complicated Than Just the Goldilocks Zone

The search for habitable, alien worlds needs to make room for a second "Goldilocks," according to a Yale University researcher.

For decades, it has been thought that the key factor in determining whether a planet can support life was its distance from its sun. In our solar system, for instance, Venus is too close to the sun and Mars is too far, but Earth is just right. That distance is what scientists refer to as the "habitable zone," or the "Goldilocks zone."

It also was thought that planets were able to self-regulate their internal temperature via mantle convection -- the underground shifting of rocks caused by internal heating and cooling. A planet might start out too cold or too hot, but it would eventually settle into the right temperature.

A new study, appearing in the journal Science Advances on Aug. 19, suggests that simply being in the habitable zone isn't sufficient to support life. A planet also must start with an internal temperature that is just right.

"If you assemble all kinds of scientific data on how Earth has evolved in the past few billion years and try to make sense out of them, you eventually realize that mantle convection is rather indifferent to the internal temperature," said Jun Korenaga, author of the study and professor of geology and geophysics at Yale. Korenaga presents a general theoretical framework that explains the degree of self-regulation expected for mantle convection and suggests that self-regulation is unlikely for Earth-like planets.

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