Big Planets’ Moons and Life Outside “Habitability”

Astrobiology has long relied on the concept of a “habitable zone”, that is a zone around a star that is the right distance from said star to hold liquid water, and therefore life. This concept is absolutely valuable, especially insofar as it allows us to classify new exoplanets and identify potential exoplanets that may host life. However, we don’t have to leave the solar system to realize the limits of this concept, and how it potentially forecloses the search for life in many other environments.

The first key limitation of the habitable zone is that it assumes heat originates solely from the central star. By looking at the Galilean moons, we can immediately see that that assumption is flawed. Europa and Ganymede both are far outside the habitable zone, and both have liquid water beneath their surfaces. How? Tidal heating. As the moons pass by each other, and orbit Jupiter, differences in tidal forces heat the moons to the extent that liquid water becomes possible. This is critical since these moons are among the best possible candidates for life within our solar system, with the conditions of the subsurface oceans being reminiscent of what we think Earth looked like around the start of life. The existence of liquid water on these moons indicates that a focus on habitable zones may preclude an examination of all possible bodies on which life may exist

The second key limitation of the concept of the habitable zone is the assumption that liquid water is a necessary prerequisite to life. While this certainly maps to our understanding of life on Earth, it is theoretically possible that life could exist using methane or some other compound as the key ingredient. This is important, since methane can exist in liquid form far outside the bounds of the “habitable zone”. One example of a body where this life could exist is Saturn’s moon Titan, where while it is quite cold, there is a significant amount of liquid methane. Whether or not life could exist in an environment like this is an open question, but these solar bodies provide key challenges to the concept of the habitable zone.

While life likely won’t be found on any of these bodies (nor Enceladus, a moon of Saturn with liquid water and organic compounds), they provide important theoretical challenges to the concept of where we think life can exist. As we seek life elsewhere, we should consider the habitable zone as important, but remember that astronomy can be very diverse, and a larger variety of environments may support life than we initially expect.