Sunday, February 6, 2011
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 from gas to liquid to solid. Many moons have signs of active and quiescent cryo-volcanism - from Enceladus to Triton and Europa. They also show good evidence for subsurface liquid water oceans that readily exceed the total volume of Earth's oceans. 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 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 circumplanetary disks of gas and dust much like a scaled-down version of planet formation itself, but there are many caveats. 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 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 cusp of the first radial velocity exoplanet discoveries. Lurking already in the bounty of Kepler data there could be evidence for exomoons as 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.
There are some new rules though. Stellar tides can be very bad for moons. The same forces that operate to eventually bring a planet into spin-synchronicity or tidal lock with a star 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 then there are in fact no stable moon orbits and any such objects will over time 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 then 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 this recent Kepler data release of planets within about 0.5 AU of their stars may not be the ideal place to look. Kepler release 3.0 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 perhaps the modern classic paper on the subject, by Williams, Kasting and Wade in 1997. By Jim Kasting's own admission the inspiration for this paper 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 was 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 then 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 lovely thing about it all was that the energy for this all geophysical warmth came from the spin and (ultimately) orbital energy of the giant planet. Life powered by angular momentum? Perhaps so.
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.
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.