Moonday: Searching for Endor

We may discover habitable moons orbiting around exoplanets, but Ewoks are unlikely to be present.

I know this will probably shock you all, but I’m a big Star Wars fan. In fact, I’ve been rewatching the original trilogy in the last few days: Star Wars on Saturday night, Empire Strikes Back last night, and probably Return of the Jedi either tonight or Wednesday. (Only the original theatrical releases for me, if you please. Han shot first!) While the movies are far from scientific, it’s interesting how some of the worlds depicted aren’t terribly far-fetched: for example, Luke’s home planet Tatooine orbits a binary star, which we know now may be a pretty common situation.

Two other worlds in the films are also types of worlds of interest to astronomers: the forest moon of Endor in Return of the Jedi and the unnamed moon orbiting the giant planet Yavin that houses the Rebel base in Star Wars. Both are habitable, very Earthlike moons in orbit around planets. Since many of the exoplanets we know about are very massive (even compared with Jupiter) and orbit their host stars closer than Earth does, it’s possible they may have habitable moons. Not only that, detecting at least some of those moons may be within reach!

That’s not to say the process will be easy. Despite the huge number of exoplanets that have been discovered (763 confirmed discoveries as of this afternoon), it’s still not an easy process. Planets don’t emit a lot of light, so direct detection isn’t generally possible. Instead, astronomers use either the wobble of their host star as the planets’ gravity tugs on them, the amount of light that gets blocked as planets transit (eclipse) their host star, or the slight brightening of a star as a planet crosses in front of it due to gravitational microlensing. While all of these methods are going to work best for the most massive planets orbiting closest to their host stars, the transit method does have the potential to reveal large exomoons: the satellites of exoplanets.

Here’s how it works in brief: just as a massive planet can noticeably swing its host star via gravity, a massive moon can have the same effect on its planet. That means when the exoplanet transits its host star, it might take slightly more or slightly less time, depending on whether the exomoon is trailing or leading the planet (respectively) during the passage. (Of course, several moons within our own Solar System are potentially habitable: Enceladus and Europa both have liquid oceans underneath the ice. However, they’re much smaller than the exomoons we’re discussing here.) If the exomoon is massive enough and we are able to observe many transits of the exoplanet, the fluctuations in transit time should contain enough data to determine not only how massive the moon is, but how large its orbit is. That’s a frankly mind-blowing idea to me, but it shows how much better astronomers have gotten at spotting exoplanets that this is even a possibility. Nobody has yet claimed an exomoon discovery, but with more data and better observations, I suspect it’s only a matter of time.

Probably no Ewoks, though.

(Caleb Scharf has a lot more about potentially habitable exomoons.)

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