IT’S (NOT) ROCKET SCIENCE: Distant Habitable Moons

Copyright Dan Durda, Fellow IAAA
Copyright Dan Durda, Fellow, IAAA

A recent report in Astrobiology raised the possibility of new places to look for extraterrestrial life: on the moons of distant planets.  Astronomers have now discovered evidence of well over 3000 extra-solar planets (that is, planets orbiting distant stars) but none of them seem to fit the blend of temperature, size and mass hospitable to life as we understand it.  However, more than 100 of these do appear to be gas giants (like our own Jupiter and Saturn) which, unlike our neighbors, orbit their stars in the so-called habitable zone where planetary liquid water can exist.  Though gas giants have no hard surfaces where water can pool, the researchers’ thinking is that some rocky moons in orbit around them might.

So what would prevent such an “exomoon” from harboring life?  First, it would be constantly exposed to a double whammy of radiation from both the star and the gas giant.  For example, Jupiter reflects about a third of the sun’s radiation striking it.  That means the moon would need to be about the same mass as earth in order to maintain an atmosphere and magnetic field strong enough to deflect much of that incoming destructive energy.   No moon in our solar system qualifies.  Second, it would have to survive extreme tidal forces.  A gas giant moon experiences planetary tides so powerful they can deform and heat it enough to sterilize any surface life.

Assuming that the exomoon was big enough and massive enough and at just the right distance from the gas giant to make it through all of that (a measure that the researchers call the “habitable edge”), then maybe, just maybe, life could thrive there.   No suitable candidates have been found yet, but we’ve only begun the search.

Living conditions on such an idealized moon would likely be very warm, but seriously cool from an SF perspective.  If the moon kept one side constantly facing the gas giant, like the moons in our solar system do, the ecosystem would be quite different from ours.

Copyright Dan Durda, Fellow IAAA.
Copyright Dan Durda, Fellow, IAAA.

On the planet-facing side, daylight would cycle from full darkness (when the gas giant eclipses the star), to twilight (when the exomoon eclipses the star and receives only reflected light from the gas giant).  Meanwhile, the side of the moon facing away from the planet would cycle from direct sunlight to utter darkness every cycle. In between, both sides would receive some mixture of direct and reflected light.  The planet-facing side would constantly be heated by radiation from the gas giant. If the exomoon also survived any early solar system jostling, like the impact that gave Earth its axial tilt, there’d be no seasonal variations in climate.

Whatever lifeforms arose there, we could be certain they’d be nothing we’d recognize as “life as we know it”.  It sounds like a playground for some world building writer.  I wonder if Larry Niven is busy.

If you’re interested in reading more, the paper reference is: René Heller and Rory Barnes. Astrobiology. January 2013, 13(1): 18-46. doi:10.1089/ast.2012.0859 

And to see more of Dan Durda’s incredible space art, visit his site at 3D IMPACT

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  1. According to an entry in the scientific journal "Wikipedia", the second moon of Endor has an additional twist: Endor itself orbits a binary star system. That's even more exotic, so maybe things cancel out and the place is just right for those cute little killer teddy bears. Ewoks live!

    1. Can you imagine if Earth were a moon of a gas giant, and that there were several other inhabitable worlds so close and easy to visit, in orbit around the same gas giant? Perhaps with their own life? You could go from the telescope to the rocket to colonization in the time we have and really get a quick go at space travel. It would be really exciting.

      1. That's true, Mike, some systems get all the luck. On the other hand, we were lucky to have a large moon close enough to Earth to allow us at least a baby step. The prevailing theory is that our Moon was a chunk of the Earth broken off when a planet-sized rock slammed into us about 4 1/2 billion years ago, while the solar system was forming. If it weren't for that, our nearest jump-to point would be Mars or an asteroid.

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