A harvest of new moons

WHERE science-fiction leads, science sometimes follows. In 2009, when James Cameron made “Avatar”, he set his colonial allegory not on an alien planet but on the moon of such a planet. Perhaps he felt that exoplanets themselves were passé as settings for fiction, since in fact so many had been discovered. If so, he got in just in time, for reality may soon imitate art in the matter of moons, as well as planets. That is because a few astronomers are indeed beginning the search for satellites around some of the thousands of known and suspected exoplanets. Besides the lure of being the first to find something new, there are Cameronian motives, too. For there is reason to suspect that if life does exist elsewhere than on Earth, then the moon of an alien planet might not be a bad place to search for it.
 
Finding something that is orbiting something that is orbiting a star hundreds of light-years away is hardly a trivial task, and not everyone is convinced it is possible. But that does not deter enthusiasts. They point out that similar objections were raised against the idea of looking for exoplanets in the first place. The best way to shut the naysayers up, they reckon, is to try it and succeed.

 
The most organised attempt at the moment is called the Hunt for Exomoons with Kepler, or HEK. It is led by David Kipping of the Harvard-Smithsonian Centre for Astrophysics, in Cambridge, Massachusetts, and it has the advantage that it makes use of data which have already been collected. Kepler, an orbiting telescope owned by NASA, America’s space agency, looks for exoplanets by identifying the tiny dip in brightness caused when one of them passes in front of its host star, as seen from Earth—a phenomenon known as “transiting”. So far, 105 planets have been discovered this way, and the existence of a further 2,321 has been suggested, though their reality remains to be confirmed by the observation of subsequent transits.
 
What the HEK, why not try it?
 
Dr Kipping’s technique relies on the fact that moons do not simply revolve around their host planets; planets also revolve around their moons—or, rather, the two bodies both revolve around their common centre of mass. If a planet is large and its moon small the distinction is trivial. But if the planet is small and the moon is large, it is not. In the case of Earth and its moon, for example, the common centre lies only around 1,700km (1,100 miles) beneath the Earth’s surface. Someone looking from afar at the movement of Earth would thus be able to deduce the moon’s existence without having to see it directly.
 
Dr Kipping hopes to see similar gyrations in the paths of transiting planets detected by Kepler. This sort of technique has already been used to identify the small perturbations induced by unseen, non-transiting planets which interfere with the orbits of their transiting brethren. For moons, though, the effects are likely to be not just small, but minute—right on the “hairy edge” of what Kepler can do, says Greg Laughlin, a planet-hunter at the University of California, Santa Cruz.
 
For a start, most of the exoplanets detected so far are much bigger than Earth, so for moons going around them to be detectable, they would have to be considerably bigger than any moon in the solar system. Specifically, Dr Kipping reckons, they would need to have at least a fifth of the mass of Earth. Ganymede, Jupiter’s most massive satellite (and also the biggest in the local solar system), has only a fortieth of the Earth’s mass.
 
Even supposing such bodies exist, the statistical alchemy needed to distinguish the signal they produce during a transit from any fluctuations in the light of the star being transited, or from gravitational effects caused by other planets that might be lurking nearby, is formidable. Dr Kipping is nevertheless hard at work. He and his team plan to present their first batch of data to a meeting of the American Astronomical Society in January.
 
If Dr Kipping’s approach does not come up with the goods, though, moon-hunters need not despair. In February NASA will decide whether to pay for a new planet-hunting spacecraft called the Transiting Exoplanet Survey Satellite (TESS). Unlike Kepler, which stares fixedly at a single patch of space, TESS would survey the whole sky. Crucially, the stars it is intended to examine are much closer—and which therefore appear brighter—than those Kepler is looking at. That makes both planet hunting and moon hunting easier.
 
If NASA does go ahead with TESS, the telescope could be in orbit by 2016. A couple of years after that the European Space Agency plans to launch a telescope of its own, the awkwardly acronymed PLATO (PLAnetary Transits and Oscillations of stars). This will also examine brighter stars than those studied by Kepler, offering another boon to exomoon-hunters.
 
A hot topic
 
There is also a sliver of hope that the biggest space telescope of them all—the enormous, exquisitely sensitive, massively over-budget and very late James Webb Space Telescope, now due for launch (so it is claimed) in 2018—could give astronomers the opportunity to take pictures of exomoons directly. In a paper posted on arXiv, an online repository of scientific papers, in September, Mary Anne Peters and Edwin Turner, of Princeton University, ponder whether some moons might shine brightly enough to be visible from Earth. Their idea relies on a phenomenon called tidal heating. In the same way that the gravitational pull of Earth’s moon causes tides in the oceans, a planet will tug more strongly on the near sides of its moons than on their far sides. These tidal forces knead the inside of the moon, and the resulting friction generates heat. The volcanoes of Io, Jupiter’s third-largest moon, are powered this way.
 
Dr Peters and Dr Turner propose to look for a sort of “super-Io”. They have calculated that if a large-enough moon was orbiting close enough to a planet sufficiently distant from its star for the signal not to be drowned out by the starlight, it would give off enough infra-red heat to be visible from Earth—if it were close enough.
 
That is a lot of “ifs” and “enoughs”. But you have to speculate to accumulate. And the prize in the accumulator may be a big one, for in the search for bodies other than Earth that might play host to life, exomoons are not a bad bet.
  
With only one example to go from, it is hard to be sure what conditions really favour the development of life. That one example does, however, suggest that something which is rocky, possesses an atmosphere and lies within the “habitable zone” of its star—ie, is warm enough to keep any water liquid—is a good place to start looking.
 
There is nothing that says such a body has to be a planet. Indeed, moons have some advantages. There are likely to be more of them, for one thing (the solar system of which Earth is part certainly has many more moons than planets). And though comparative solar-systemology is a pretty young science, there are suggestions that such systems are chaotic places in their youth, with gas-giant planets forming and then migrating in towards their stars, scattering any small, rocky planets that are in the way into more distant orbits or even out of the system altogether. The moons of gas giants, being tightly bound by their planets’ gravity, would be less susceptible to such early exits.
 
And moons might also be more habitable. Small planets close to their parental stars risk becoming tidally locked: that is, the lengths of their days come to match their orbital periods, and they thus have one permanently hot hemisphere and one that is permanently cold. This would probably not be good for the development of life. Though a moon may become tidally locked to its parent planet, it can never be locked to a star, so it will always have some sort of day-night cycle—even if it is an odd one from a terrestrial point of view.
 
The warmth provided by tidal heating might also help make some moons which are outside their stars’ habitable zones inhabitable nevertheless. Enceladus and Europa, for instance, are two icebound moons in the local solar system with surface temperatures hundreds of degrees below zero. But planetary scientists reckon both may have subsurface oceans, kept liquid by a combination of radioactive decay and tidal heating from Saturn and Jupiter, their respective parent planets.
 
For all these reasons, then, the search for exomoons seems a worthwhile enterprise. It is hard to believe they do not exist. And there is no better way to find something than to look for it.
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