Geometry of the triple system of a star, planet and moon. Illuminations are indicated by the different shadings. Four orbital phases are shown.

Are Exomoons Habitable?

Title: Exomoon Habitability Constrained by Illumination and Tidal Heating
Authors: Rene Heller and Rory Barnes
First Author’s Institution: Leibniz Institute for Astrophysics, Germany

Some exoplanets seem to have walked directly out of the best science fiction movies. We’ve discovered a planet consisting purely of water (GJ1214b) and one with two suns (Kepler 16b). Do the movies “Waterworld” and “Star Wars” ring a bell? Others seem far more odd than even the most creative science fiction writer could have imagined. Some worlds scrape their host star every periastron passage. You had better make sure you’re on the opposite side of the planet at that time of the year! Other worlds exist in darkness without a star at all.

Considering the question of habitability for planets like these seems like a joke. But what if we stopped looking at these extreme worlds and turned our eyes to their moons instead? Perhaps their moons are less extreme. And given that our own Jupiter hosts 67 moons, surely they’re more abundant. Can such extreme planets host habitable moons? The 36-page paper written by Heller and Barnes, which appeared in the January issue of Astrobiology, attempts to address this question.

The photometric precision of Kepler now makes the detection of Earth-sized exomoons possible. If, during an exoplanet transit, an exomoon also passes in front of the host star, there will be an added dip in the observed light. See this video for a great demonstration. Another popular method for detecting exomoons is transit timing variations. The exact timing of a transit will vary if there are secondary bodies (either other planets or moons) in the system.

This paper does not address the observational difficulties of detecting exomoons (although it is a pretty cool puzzle to solve), but rather the theoretical question of what makes an exomoon habitable. The authors consider the physical and orbital parameters, which may affect the conditions for life. They argue that it will be possible to constrain an exomoon’s habitability based on the data available solely at the time they’re discovery is confirmed. Let’s look at a few of the factors that come into play.

Read more: Are Exomoons Habitable? | astrobites.

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