Of Rebel bases and habitable exomoons

T-shirt by R. Stevens
T-shirt by R. Stevens

I’m an unabashed Star Wars fan. Sure, I know all the problems with the series: the dearth of female[*] and non-white human characters, some very troubling stereotypes, and of course the willingness of Lucas to screw up the story in the name of making money. That doesn’t even get into the science, but to me that’s not really a big deal. Despite the spaceships and alien planets, Star Wars isn’t really science fiction: it’s a sword-and-sorcery drama set in space, with elements of Westerns and samurai stories thrown in for good measure. In fact, one of the actual science-fiction plot devices — the “midi-chlorians” in the blood that grant people the ability to use the Force — was so rightfully reviled that it wasn’t brought up again.

However one plot device from Star Wars that appeared in at least two movies could contain some elements of real science: the habitable moons in the first film (“A New Hope”) and Return of the Jedi. (I don’t honestly remember if there are habitable moons in Episodes I through III, because who cares?) My latest column for The Daily Beast explains the science of exomoons: moons orbiting planets in other star systems.

Imagine, though, a Saturn-mass exoplanet with a Titan-sized moon orbiting its star within the habitable zone. That exomoon could have a sheltering atmosphere, sufficient to warm the surface to allow liquid water to form. Call the exoplanet “Yavin,” give the moon a few billion years for life to emerge and pump oxygen into the air, and we have a nice hiding place for a future Rebel Alliance.

So let’s assume we have a planet-like exomoon stably orbiting its planet, maybe in a system with a Sun-like star. How in the name of Yoda are we ever going to detect it? We have enough trouble finding small exoplanets; is there any hope of detecting a moon that would be as small or smaller? And how could we distinguish the small exomoon signal from that of its host planet? [Read more…]

As always, there are some looming questions. The most common stars are small and red, meaning the habitable zone is closer in than Mercury’s orbit around the Sun. As a result, many known exoplanets in habitable zones are “tidally locked”, always presenting the same face to their host star, as the Moon does with Earth and Titan does with Saturn. That complicates the orbits of potential habitable exomoons, which run the risk of getting gravitationally kicked out of their star system entirely, assuming they could form in the first place.

In other words, we need to start detecting exomoons and figuring out the important science of their formation, then worry about habitability. But it’s still an exciting possibility, one every person — Star Wars fan or not — could dream big dreams about.


  • The drinking game for teetotalers: drink every time a woman working for the Empire appears on screen. (I can’t find the source for this joke, but it’s been around for at least two decades.)

2 responses to “Of Rebel bases and habitable exomoons”

  1. Marshall Eubanks Avatar
    Marshall Eubanks

    Here is a really cool way to detect such exomoons – from ~30 MHz radio waves produced through their interaction with their primary and the local stellar wind. In the solar system, one of the two brightest radio sources is the cyclotron radiation (~ 10 – 40 MHz) coming from the Io flux tube (the so-called Jupiter S bursts). Any giant planet with a moon or two well inside their magnetosphere is likely to do this, and these would be visible across (relatively close) interstellar distances*. That the best way to detect such radiation would be on the lunar far side (or in a polar lunar crater) is just icing on the cake (for me, although if we are lucky LOFAR will do this first).

    *see, e.g., http://iopscience.iop.org/0004-637X/612/1/511/pdf/59362.web.pdf and http://arxiv.org/abs/0806.0327

  2. I suppose if the moon didn’t get kicked out of its solar system, orbiting around a gas giant might help it have some day-night changes whereas a lone habitable exo-earth would just get tidally locked around. If it had a Ganymede-sized orbit, it would get something resembling a “night” when it goes into the planet’s shadow. Isaac Asimov had that in his novel Nemesis (a prequel to his Foundation series).

    Some things do worry me about habitable exo-moons. The exomoons we know of are pretty tiny – all of the Galilean Moons put together have less than 7% of the mass of Earth. Maybe that wouldn’t be a problem, though, if the gas giant migrated into the habitable zone (it could pull together/capture more material).

    They might also be more prone to impacts, since the mother planet is much more massive and is going to pull in all kinds of comets and asteroids. Most of those will miss or hit the planet, but still . . .

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