While we’ve looked several times in these pages at David Kipping’s work on exomoons, the investigation of moons much closer to home reminds us that finding a habitable satellite of another planet may not be out of our reach. After all, we’re gaining insights into possible habitats for at least microbial life on (or in) places like Europa and Enceladus, and speculations about similar biospheres within some Kuiper Belt objects also keep them in contention.

So what about a habitable moon around a distant gas giant? Kipping (University College London) has now gone to work on the question in relation to the Kepler space telescope. His findings are striking: A Saturn-sized planet in the habitable zone of an M-dwarf star would allow the detection of an exomoon down to 0.2 Earth masses.


Image: A habitable exomoon would offer an exotic vista, a view that may be more common in the galaxy than we have previously imagined. Credit: Dan Durda.

Now that sounds unusual, given that Kepler can’t find planets of such small size. How, then, does Kipping hope to find exomoons at this scale? The answer is that an exomoon detection depends on two measurements, neither of which demands observing the dip in starlight caused by the moon itself. What Kipping’s team is looking for is the effect the moon has on the planet, and the transit of that Saturn-class world around an M-dwarf is something Kepler can work with.

The method relies on two sets of observations, the first being transit timing — variations in the amount of time it takes a transiting planet to complete its orbit can be the signal of a moon. Add transit timing variation to the second measurement — transit duration — and you can nail down the presence of that moon. Transit duration measures the speed at which the planet actually passes in front of the star. Detecting Life-Friendly Moons, on the Astrobiology Magazine site, explains that the two signals occur separately when a moon is involved, screening out other possible causes.

The Kepler researchers have their hands full looking for exoplanets, but an exomoon survey using Kepler data is certainly in the running for future investigation. If a moon as small as a third of Earth mass can hold onto a magnetic field, life could develop despite the presence of nearby planetary radiation belts. Such a moon, Kipping believes, would be detectable in the right transit situation as much as 500 light years from the Sun.

“There may be just as many habitable moons as habitable planets in our galaxy,” Kipping tells Astrobiology Magazine, an audacious thought with all kinds of ramifications for the Fermi paradox. Remarkably, combining the two measurements his team works with would allow Kipping to estimate both the moon’s orbital period and its mass. Run that to its limit and you get this: Knowing the orbital period would allow prediction of a lunar eclipse whose spectroscopic study could then reveal the signature of atmospheric gases on the moon’s surface. That’s pushing current technology to the max, but it’s increasingly feasible as we tune up our resources.

The paper is Kipping et al., “On the detectability of habitable exomoons with Kepler-class photometry,” Monthly Notices of the Royal Astronomical Society, published online 24 September, 2009 (abstract).