The idea of habitable moons is, in my humble opinion, one of the most interesting science fiction tropes out there. And surprisingly, there’s nothing written on TV Tropes about them. I find this a little bizarre, because science fiction of all kinds is chock full of stories and settings where the action takes place on a moon in orbit around a giant planet!
Moons with habitable surfaces are everywhere in fiction. Most of Return of the Jedi takes place on “the forest moon of Endor”. In Avatar, Pandora orbits a gas giant. Several of the worlds in Firefly are moons. Arvuna in the Mass Effect universe is a water world with plentiful sea life. And of course, most recently there’s LV-223 from Prometheus. This list goes on… And, as with any truly good idea, the concept of habitable moons has escaped from fiction and actually made it into mainstream science.
The idea of looking for these “exomoons” has been seriously considered, and some enterprising astronomers even devised a way of looking for any particularly large moons with the Kepler telescope! Several high mass exoplanets, potentially carrying such moons in tow, are known to exist in the habitable zones of their parent stars, such as Gliese 876 b, 55 Cancri f, Upsilon Andromedae d, and 47 Ursae Majoris b. And that’s just a few of around 30 such planets which are currently known – some of which are listed here!
So let’s look at this rationally. We don’t know how prevalent moons are in the Universe, but if we look at our solar system we can find several moons large enough that if they were in orbit around the Sun, we’d unquestioningly consider them planets in their own right. Ganymede has its own magnetic field. Triton has a thin atmosphere with its own weather systems. Titan has a much denser atmosphere, and lakes on its surface. Europa contains more water than Earth does! That’s already as many interesting terrestrial objects as you can find in the inner solar system, and I haven’t even mentioned like Io or Callisto. And while it’s uncertain how many moons a gas giant might have closer to its parent star, if Earth can hold on to a satellite as massive as the Moon, then Jupiter could easily do the same!
In fiction, this opens up a huge number of interesting ideas which haven’t been explored outside the written word. A gas giant could have more than one habitable moon, for instance. Though more could be possible, suppose there might be two habitable moons around the same planet. Those two worlds could have very similar life, transferred by meteorites. Or they could be radically different. One moon might be some kind of planetary Australia full of things which are lethal to life from the other – and/or easily overrun by introduced life forms with no natural competition. And what of any alien cultures and societies which may have evolved in a system like this?
Extrasolar moons are a huge source of interest for both astronomers and fiction writers alike. However, this is once case where the scientists are putting the writers to shame with their creativity. While astronomers are busy using their imaginations to come up with all kinds of inventive ways to find these moons, fiction writers normally use a moon as little more than a backdrop for their story. We’re told that Endor is a moon, but in the movie we never actually see the planet it orbits. Pandora has some pretty scenery of a giant planet in the sky, but that’s about it. It would be lovely to see some fiction on the big screen where the fact that this place is a moon is at least part of a central plot point!
To have a look at some actual data on potentially habitable planets and moons discovered in the galaxy so far, have a look at the Habitable Exoplanets Catalog, run by Arecibo’s Planetary Habitability Laboratory!
EDIT– Also, there’s an interesting discussion on A Bit More Detail, in response to this article, about how massive a giant planet might need to be for an Earth-like moon to form.
Image manipulation by yours truly. Source images:
Gas giant – timbersavage90 @ deviantART
Planetary Rings – NASA/JPL/Space Science Institute
Forest Moon – digitalecho @ deviantART
Orange Moon – MistikFantasy @ deviantART
Moon 1 – Gregory H. Revera @ Wikimedia Commons
Moon 2 – NOAA @ WIkimedia Commons
And yes, technically Pitch Black features just such a plot point – but seeing as the word “moon” isn’t mentioned once in the film, I’d question whether this was not actually intended by the authors!
A trope is a recurring theme in any narrative which conveys information to the audience. These are snippets of information which have somehow ended up in our collective subconscious as ways in which storytellers have gotten their points across. Overused tropes end up as clichés.