MAY THE FORCE BE WITH YOU
Could ‘Star Wars’ Be Right About Habitable Moons?
The Star Wars movies got a lot of science wrong, but one thing might turn out to be right: Earth-like moons that can support life.
Much as I love them, the Star Wars films fail nearly every scientific test—even beyond the magical warriors in space motif. From faster-than-light travel to breathing in space with just a filtration mask to the sounds of explosions without air, they are unrealistic even by science fiction movie standards. But there are one or two places where they might be at least a little prophetic.
In both the first Star Wars and Return of the Jedi, the heroes visit worlds of a type potentially interesting to researchers. Both the unnamed moon of Yavin hiding the secret Rebel base and the Ewok-plagued Endor are very Earth-like moons orbiting giant planets. While the strong resemblance to Earth is unlikely, astronomers are taking the idea of habitable exomoons—moons orbiting giant planets in other star systems—very seriously.
The “habitable zone” is a region near a star that is warm enough for liquid water to exist on the surface, assuming the presence of a reasonable atmosphere. In our Solar System, only smaller, rocky planets orbit within the habitable zone. By contrast, most of the exoplanets in their host stars’ habitable zones are Neptune-size or bigger. That means they are composed of compressed gasses, with no solid surface.
While some researchers have speculated about life on gas giant planets, they aren’t promising. However, the giant planets in the Solar System are hosts to a flock of moons, many of which are planet-size. In particular, Saturn’s moon, Titan, is bigger than Mercury and has a nitrogen atmosphere thicker than Earth’s. Its surface is very cold: a lot of the “rocks” are actually boulders of water ice, and its lakes are composed of methane rather than water.
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?
The only potential exomoon astronomers have identified so far orbits a “rogue planet.” Rather than orbiting a star, rogue planets drift through interstellar space. Whether they formed in a star system and were ejected afterward by some chance, or if they formed in a stellar creche but never had enough mass to be a star themselves, these vagabonds and any satellites are likely as uninhabitable as can be. However, a rogue planet happened to drift between us and a more distant star, creating a brief gravitational magnification of the background star’s light known as “microlensing,” which depends on how massive the planet is.
In this case, the magnification had a second, smaller burst, which might possibly, maybe, perhaps be the sign of an exomoon orbiting the rogue planet. The principle is sound, but the effect is small enough that another source entirely could be responsible for the extra flare-up.
A more reliable method than microlensing, used to great effect by the Kepler mission, detects exoplanets when they transit—pass in front of their host star, blocking a small amount of light. In some cases, light shining through the outer layers of the exoplanet’s atmosphere has even allowed astronomers to determine some of its chemical composition.
With precision measurements, we might be able to find a secondary blip from an exomoon. It wouldn’t be a large effect, but by monitoring the same system over multiple orbits, we might have a chance. As with every exoplanet discovery, the chances are better for worlds orbiting very close to their host star, so habitable exomoons are still a lovely dream.
However, there’s another disturbing possibility. If an Earth-like planet has a Titan-like moon with an atmosphere, it might be hard for us to tell which atmosphere is which. That’s a problem if we’re looking for the chemical signatures of life. Early Earth had a very different atmosphere; microorganisms known as cyanobacteria produced oxygen as a waste product, until the balance of the atmosphere changed into something like we have today. Hunting for life elsewhere could involve looking for similar chemical imbalances, but an exomoon might spoil it for us: we’d see the chemistry of both exoplanet and moon, but be unable to tell which was which.
That’s a fairly distant worry for us. We have yet to find more than a few Earth-size exoplanets, and haven’t measured an atmosphere on any; the only exomoon possibility could be something else entirely. We don’t know how common Earth-like exomoons are either, but we can’t know without looking. If we’ve learned anything, science fiction—even fanciful examples like Star Wars—doesn’t always prepare us for the wonderful things we’ve discovered in our beautiful Universe.