On a moon, far, far away...

One of the biggest thrills of exoplanetary science is seeing how it combines the new and the old, with every discovery bringing startling perspective on the nature of our own very familiar solar system. I thought I'd dig out a post from the Life, Unbounded archives that helps illustrate this. A freshly edited version of the post is directly below, originally written in February 2011, and after that is a small followup.


Our solar system plays host to an extraordinary array of natural satellites, or moons. Many of these are entirely comparable in size, composition, and even chemical and geophysical activity to bona-fide planets. The only real difference is that these worlds reside deeper in the orbital hierarchy. Nine regular satellites in our system have diameters greater than 1500 km, the largest (Ganymede and Titan) are over 5000 km in diameter - larger than the planet Mercury. Io around Jupiter has extensive and active silicate and sulfur-rich volcanism. Titan has a frigid atmosphere that is somewhat denser than the Earth's, and a diverse and global hydrocarbon cycle taking compounds from gas to liquid to solid phases. Many moons have signs of active and quiescent cryo-volcanism - from Enceladus to Triton (check out the movie made in 1990 by NASA from Voyager 2 images) and Europa. They also show quite compelling evidence for subsurface liquid water oceans that readily exceed the total volume of Earth's oceans - although probably rich in compounds like ammonia. It is little wonder than many of the current concepts for future solar system exploration missions focus on these objects - they are tremendously interesting.

It is however also true that our models of how moon systems form are even less well developed than our models of planet formation. It seems that moons around giant planets probably form out of circum-planetary disks of gas and dust much like a scaled-down version of planet formation itself, but there are many caveats. And there's another sneaky truth; simulations of forming planetary systems are not typically set up in ways that allow us to track satellite formation, capture, or loss (embarrassed cough). In this sense we're even further behind in our theoretical framework than the equivalent situation for planets two decades ago.

Intriguingly though the prospects for detecting moons around exoplanets may not be too bad. It may even be on a par with the situation in 1994, on the eve of the first radial velocity exoplanet discoveries. There could already be evidence of exomoons lurking in the bounty of Kepler data, causing transit duration and transit timing variations. Moons make their planets wobble just as planets make their stars wobble by offsetting the system center-of-mass. The opaque bulk of moons can also implant tiny variations in the amount of light seen during a planetary transit, again with a signature that may raise a flag for their presence.

However if we want to look for moons capable of bearing life there are some new rules. Stellar gravitational tides can be very bad for moons. The same forces that operate to eventually bring a planet into spin-orbit-synchronicity, or tidal lock, also perturb satellite orbits and can pump their orbital ellipticity to a point where the moon just sails off. Additionally, once a planet becomes tidally-locked to its star - with a permanent day and night side - then there are in fact no stable moon orbits, and over time any satellites will spiral inwards due to moon-planet tides. The upshot of all this is that within about 0.6 astronomical units of a solar-mass star in all but the youngest systems you might not expect to find any moons - assuming of course that they formed in the first place. So the recent Kepler data release of planets within about 0.5 AU of their stars may not be the ideal place to look. Kepler releases 3.0 and up may be another story when we begin to confirm planets on longer orbits.

My own interest in exomoons was in part stimulated by what is arguably the modern classic paper on the subject, by Williams, Kasting and Wade in 1997. By Jim Kasting's own admission, the inspiration for this article titled 'Habitable moons around extrasolar giant planets' came from a viewing of a certain episode of a certain sci-fi franchise depicting a place called Endor. It's a lovely paper. A key point in it is that gravitational tides in moon systems due to moon-moon interactions could be pivotal in dissipating enough energy to make up for a moon being well outside the classical habitable zone of a star. Instead of stellar heating you'd have more geophysical heating. In 2005 I attempted a bit of a followup of my own and with some funding from NASA made a small study of the potential for 'habitable' moons around the then known exoplanets. The idea was simple, we knew the stellar input for these planets and any moons they might have, so what kind of tidal forces would be needed to push them to temperatures that could sustain liquid surface water? I was surprised to find that it could all work out pretty well. Although there are several caveats, tidal heating in a plausible range could effectively double the size of the habitable zone in these systems if we were willing to consider moons as well as planets. The nice thing about it all was that the energy for this geophysical warmth came from the spin and (ultimately) orbital energy of the giant planet. Life powered by angular momentum? Perhaps so.

How tidal heating can extend the classical habitable zone around a star for exomoons

Five years later and I was sitting on a tediously long flight watching a movie about blue-skinned aliens romping around on a lush tropical moon orbiting a gas giant planet in the Alpha Centauri system. It occurred to me how funny it was that two epic Hollywood productions framed the interim works on exomoons, obviously we should listen to scriptwriters more often. It also occurred to me that exomoons might just be ready to fully emerge from the astrophysical subconscious. A few recent publications seem to have confirmed that.

We may talk about finding the first 'Earth-like' planet (once we figure out what that actually means). What if we're more likely to find an 'Earth-like' moon around an ice or gas giant? The odds quite conceivably favor such a situation. There may be a few million rocky planets in habitable zones in the galaxy, but there could be as many or more rocky, watery moons in the extended habitable zones around giant worlds. This doesn't even include the enormous potential reservoir of sub-surface chemically rich liquid water that may lurk out of sight beneath the icy crusts of moons well outside any classical habitable zone.

I'm not for a moment suggesting that we divert attention from hunting exoplanets. I also hope that some of the pioneers who devoted themselves prior to 1995 to what was seen as a fringe pursuit are the ones to find that Earth-twin, they deserve to. However, if we find barren world after barren world it will be time to turn our gaze on those strange and fantastic places that are held in thrall of giant planets.

Ever closer

And here we are barely eight months later. No one has yet laid claim to the detection of an exomoon, but I'm increasingly convinced that someone (it might even be me if I could ever find the time - he says with his Fermat cap on) is going to do this within the next year. I think it will be a claim, it will remain to be seen if it holds up, this is going to be a tough measurement to make. People are hard at work on it though; David Kipping at Harvard has recently added several neat papers to the literature describing the effects to look for. He's also pointed out that if you can measure both the variation in when a planet transit happens due to the tugging of a moon, and the variation in the length of these transits due to the moon's presence then lo and behold, the mass of the moon and the size of its orbit can be estimated. In some quarters the detection of a gas giant planet in a 1-year orbit around a Solar-equivalent G-dwarf star has prompted others to consider the possibility of directly habitable moons. Astronomers are actively examining data from high-precision ground-based telescopic studies of planet transits to sniff for exomoons, and the analysis of Kepler data increasingly comes along with calculated limits on just how big exomoons (as of yet undetected) could be.

If exomoons exist somewhere, and there is no reason to think they don't, another chapter of exoplanetary science will open up. Perhaps we'll finally be able to find the proper context for those little blobs of light that Galileo saw appearing and disappearing around Jupiter just a little over 400 years ago, and the old will become new - again.