Solution 42 in my book Where is Everybody? is entitled “The Moon is Unique”. What has the Moon got to do with the Fermi paradox? Well, it seems quite likely that our Moon has played an important role in the development of life on Earth (for example, it stabilises Earth’s axial tilt and thus prevents extreme climatic variations) and it’s not entirely implausible that it played a role in the creation of life in the first place. However, the Moon was created in a giant collision between Earth and a Mars-like object. Had the parameters of that collision been slightly different, our Moon would not have formed with the size it has – and its effect on life would have been different. So, the argument goes, an Earth-Moon system such as our own might be rare – and so therefore might life.
I don’t believe that a scarcity of moons resolves the Fermi paradox – but for all sorts of reasons it would be good to understand more about moons in other planetary systems. And large moons – satellites such as Saturn’s Titan, for example – could themselves be hosts for life. The difficulty, of course, is in finding exomoons.
A recent paper by Kipping, Bakos, Buchhave, Nesvorny and Schmitt (The hunt for exomoons with Kepler (HEK): I. Description of a new observational project) explains how we might be able to search for exomoons with current technology.
The Kepler mission, as we know, searches for exoplanets by looking for the periodic dip in a star’s brightness that occurs when a planet transits the star. The technique has resulted in the discovery of hundreds of planets. Well, suppose the planet has a moon that orbits in more or less in the same plane as the planet orbits its star: when planet and moon were side-by-side they would block more light than when one object was in front of another. By looking in detail at the periodic variation in brightness of the star it should be possible, in principle, to determine the moon’s mass and diameter (and hence its density).
In the paper, Kipping and his co-authors calculate that Kepler should in principle be able to discover exomoons with a mass as small as 0.1 Earth masses. Such an object would be much bigger than Ganymede or Titan. The discovery of such an exomoon would be important for science, but would not in itself shed much light on the question of extraterrestrial life (other than increasing, perhaps, the potential number of abodes for life). But if the hunt for exoplanets has taught us one thing, it’s that observations that once appeared technically impossible eventually become routine. Right now it might be impossible to search for an exomoon that’s similar to our own Moon. In a few years time it won’t be.