THE diminutive sun set hours ago, but a giant orb suspended in the sky still suffuses the scene with a low, orange-red reflected glow. The air is thick with volcanic soot, yet surprising activity is discernible through the fog. Large-eyed predators roam the landscape, padding through black, wide-leaved foliage.
A weak sunrise inches over the horizon, as yet another seismic rumble ripples through the surface. A faint new day will soon dawn on this moon of a gas-giant planet, orbiting a red dwarf star thousands of light years from Earth.
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Lunar life is a staple of science fiction. Lush imagined moons such as Endor, forested home of the Ewoks in Star Wars, or the magical Pandora of Avatar readily capture our imaginations. Perhaps that’s because they represent something familiar in many ways, but altogether different.
An Endor or Pandora is possibly too much to ask for, but something like the gloomy, volcanic moon orbiting a gas giant might not be too wide of the mark. And as we learn more about planetary systems – in our solar system and elsewhere – there’s a growing feeling that, in the search for alien life, we are better off looking not at planets, but at moons. And although no “exomoon” in another solar system has been discovered quite yet, we could be just the blink of a telescope away.
The possibility of discovering alien life is one big reason why the discovery of exoplanets orbiting other stars has generated so much excitement over the past two decades. Instruments such as NASA’s Kepler space telescope, the most prolific of planet hunters, have been providing us with an almost constant stream of other worlds, with the confirmed total now pushing 2000.
The search continues for Earth’s twin – or at least, any form of habitable planet. There have been some close calls, but in many ways exomoons are more promising. Partly that’s a pure numbers game. Our solar system has only eight major planets (and only one habitable one), but between them they have 168 known moons. If our backyard is anything to go by, there are likely to be far more moons than planets out there.
The crucial hunting ground for life has conventionally been a star’s so-called habitable zone – the thin girdle around it where water, life’s solvent, can exist in liquid form. Kepler has shown us that the most abundant planets orbiting here aren’t small, life-friendly rocks like Earth, but gas giants the size of Jupiter or bigger. It’s hard to conceive of such planets harbouring life, but easy to think they might have rocky moons that can. “Moons might even outnumber terrestrial planets here and therefore be the most abundant type of inhabited world in the universe,” says exomoon hunter of McMaster University in Hamilton, Ontario, Canada. So the search is on – with the added inducement of having your name associated with that first discovery. “You really want to find an exomoon rather than planet number 4350,” says Heller.
But while there has been the odd unconfirmed candidate in recent years, there has been no definite exomoon sighting yet. “It’s mostly because they are probably very small,” says of the Harvard-Smithsonian Center for Astrophysics.
Kepler, for example, finds exoplanets by looking for the dip in a star’s brightness caused by an orbiting body transiting across its face. The size of this dip diminishes with the circumference of the transiting body, so something half the size will be four times harder to see. Kepler was designed to find Earth-sized planets; the largest moons in our solar system, Jupiter’s Ganymede and Saturn’s Titan, each have a radius about 40 per cent of Earth’s. If that size is typical, exomoons are right at the limits of existing telescopes.
But it’s not just size. Planets sail across their star at nicely regular intervals, but a moon can be behind its planet, in front of it, or at some point to the side, making any additional small dimming effect appear irregularly (see diagram). “It could happen a few hours before or after the transit, and sometimes early and sometimes late,” says Kipping.
Nevertheless, exomoon hunters are growing in confidence. “I think David Kipping and I would agree that maybe we are at the stage that the exoplanet hunts were at in the late 80s and early 90s,” says Heller. “The techniques are there, so we now have to search for these moons.”
辱Բ’s project is perhaps the furthest advanced. His team starts with Kepler information on known planets and painstakingly develops predictions for a range of effects such as when a possible moon would transit and how its gravity might affect the planet’s speed at certain points, changing the duration of its transits. They then search for hints of such things in the data. “Each of these effects may only just be detectable by themselves,” says Kipping. But see several of them with the same planet, and you might be on to something (see diagram).
Kipping has enlisted NASA’s to model 57 possible planetary systems, and hopes to have 300 completed by the end of the year. Their technique to discover the smallest exomoons capable of sustaining an Earth-like atmosphere in at least 1 in 4 of the systems the team surveys, if they exist. “In terms of moons that could have life on them, and by that I mean Earth-like life, we are already definitively sensitive,” Kipping says. “When we’ve finished this survey, I will be able to tell you how often Pandora actually exists.”
Heller’s own modelling, meanwhile, suggests a planet several times the size of Jupiter of Mars – and that should be big enough for Kepler to spot. His team has developed a technique that involves comparing multiple transits of the same planet and looking for any variations in light that might indicate the presence of a third body. He is currently applying for funding to use the approach on Kepler data.
“Exomoons can remain habitable far further out from their stars”
In June, meanwhile, Joaquin Noyola of the University of Texas at Arlington started listening for exomoons. That might sound unfeasible, but Jupiter’s moon Io is known to trigger radio emissions , and Novola hopes exomoons will do the same. A moon-induced radio signal would be very weak by the time it reaches Earth, so Noyola is testing the idea by listening in to Epsilon Eridani b, which at 10 light years away is one of the closest known exoplanets. Just on the off chance, he is also eavesdropping on two other nearby star systems not yet known to contain exoplanets.
One way or another, the moon hunters are closing in. “My gut feeling is that we’ll find an exomoon in the next few years,” says Heller.
That’s when the hard work will begin. To answer that all-important question about life, we need to know what the exomoons are like – do they have liquid water, or an atmosphere containing suggestive gases such as oxygen?
With exoplanets, that has proved tricky. The best way to sniff a distant planet’s atmosphere is to look at the spectrum of starlight reflected from its surface, which will look different depending on what atmospheric gases are present to absorb various wavelengths. But for an exoplanet to be toasty enough to retain liquid water, it must be relatively close to its star, and that generally means the delicate signal of light reflected from the planet gets drowned out by the star itself.
Not so with exomoons, says at Princeton University – for the simple reason that they can remain habitable further out. There has been a lot of talk in recent years about moons surrounding the gas giants in the outer reaches of our own solar system, such as or Enceladus, being potential homes to life – albeit only primitive microbial life (see “Endomoons“). These bodies lie way outside our star’s traditionally defined habitable zone, but in the neighbourhood of a giant planet additional sources of energy become available – light reflected from the planet, for example, as well as heat radiated as a young planet takes in gas and shrinks.
“A large, heated moon far enough from its star might even be visible directly”
Then there is the effect known as tidal heating. In a system with more than one moon, varying gravitational pulls as the bodies orbit the central planet can stretch and squeeze a moon’s interior, causing friction that can generate enormous internal heat. (Our moon’s smaller pull doesn’t generate much heat, but does create ocean tides.) Such effects could extend the region around a star in which liquid water can exist well beyond the habitable zone towards a “habitable edge” much further out.
It’s a fine balance – and any life on such a moon would have had to evolve in conditions of low light and high seismic activity (see “Plants of Pandora“). Then there are the strange phases of day and night that a body illuminated both directly by a star and indirectly by a planet would experience (see “By the light of an evening planet”).
Nevertheless, recent work by Turner and Vera Dobos, now at Konkoly Observatory in Hungary, shows how natural feedbacks , increasing the likelihood that they will have surface water. For example, if an ice-covered moon is close to its neighbouring planet, strong tidal forces might melt its ice, but the resulting water and slush would deform more easily and so generate less heat. That would prevent the world steadily getting hotter and burning off the water. The opposite effect would tend to keep ice – or indeed any surface material – near its melting temperature on moons further out from their planet. “This is an indication of why there might be a lot of liquid water in tidally heated moon systems,” says Turner.
A large, heated moon far enough away from its star’s glare might even be visible directly without any sophisticated detection algorithm. “You would point your telescope at it and take a picture of it at infrared wavelengths,” says Turner. “It’s plain old straightforward astronomy.”
Even so, that would be right at the limit of existing infrared telescopes. The would only see a nearby exomoon that’s brutally hot – around 700 °C, in fact. But NASA’s , due for launch in 2018, should be able to detect one at a relatively cool 27 °C and much further away from Earth.
There’s even a chance the right infrared telescope might see signatures of atmospheric gases such as carbon dioxide and methane directly imprinted in the glow of a tidally heated planet, without being overwhelmed by light from the star. The fact the infrared light would be radiated from the surface of the moon, rather than being reflected starlight, changes the interpretation of what we see somewhat, says Turner. “But in general, the same principles would apply.”
All this means we could be characterising habitable moons far sooner than habitable planets, he says. “If you are talking about anything remotely Earth-like, then I think 10 years is very optimistic, and 20 years maybe. But moons we could be working on within a few years.”
Don’t get your hopes of alien life up just yet, though, say both Heller and Turner. For a start, all ideas about exomoon habitability remain speculation until we have actually found some examples to test them on. And “until we find them we don’t even know they are there. The solar system has lots of moons, but there’s no guarantee that other systems do,” says Turner.
But assume our solar system is representative, it seems increasingly likely exomoons could be the first locations where we detect tantalising indications of life from afar. And that is a prospect far more exciting than any moon in the movies.
Endomoons
While excitement grows about the chances of finding life on “exomoons” in other solar systems (see main story), we haven’t yet exhausted the possibilities offered by the icy moons of our own outer solar system: the endomoons.
ENCELADUS: Saturn
Radius: 252 km
Average distance from sun: 1.4 billion km
Environment for life: A hidden ocean of salty water perhaps the size of Lake Superior, most obviously apparent through geysers shooting through the moon’s icy surface near its south pole. Earlier this year NASA’s Cassini probe found evidence the ocean might be warmed and supplied with minerals through undersea hydrothermal vents.
Possible life: Anything near these vents may resemble microbial life that lurks near Earth’s deep-sea vents – a possible cradle for life here.
EUROPA: Jupiter
Radius: 1561 km
Average distance from sun: 780 million km
Environment for life: Ridges criss-crossing Europa’s surface might be generated by tidal forces warming a voluminous subsurface ocean that has probably been around for 4 billion years.
Possible life: One of the closest analogues for Europa’s ocean is the subglacial Lake Whillans in western Antarctica, home to nearly 4000 species of microbe. But Europa’s waters are likely to be alkaline and salty, chemically more akin to soda lakes in eastern Africa – so any microbes there are likely to be unique.
GANYMEDE: Jupiter
Radius: 2634 km
Average distance from sun: 780 million km
Environment for life: In March, the Hubble Space Telescope provided evidence that a vast ocean of salty water – amounting to perhaps more than all of Earth’s surface water – exists beneath the icy surface of the solar system’s largest moon.
Possible life: Microbial ocean-dwelling lifeforms.
TITAN: Saturn
Radius: 2576 km
Average distance from sun: 1.4 billion km
Environment for life: Titan’s frigid surface has seas of liquid methane. Its atmosphere is rich in organic compounds formed when sunlight breaks down methane – but could some form of life also be generating fresh methane here?
Possible life: A recent study suggests cells on Titan , rather than the lipid bilayers of our cells.
Plants of Pandora
Exomoons orbiting planets in other solar systems present us with many possible habitats – but not all are as lush as fictional creations.
Red dwarf moon
Red dwarfs are the most abundant stars in our galaxy. Chances are these small, dim stars wouldn’t illuminate lush green worlds. Photosynthetic life would have to draw light from as broad a range of the spectrum as possible, so is most likely to be black – as would life on a moon orbiting some distance from a sun-like star.
Earth-like moon
A moon positioned further out than the stellar “habitable zone” – where temperatures are right for liquid water to exist – could still have Earth-like conditions. It would need to be large and get a tidal heating effect from its planet, and the mix of land and sea would be important. On a large watery moon, the oceans could be hundreds of kilometres deep and the pressures at depth too severe for life to get started. Overall light levels would be lower, too.
Hot rocky exomoon
Far away from its parent star, this large moon gets most of its energy from tidal forces that drive plate tectonics. Depending on how much water it started out with, there might not be much there. Hyperthermophilic bacteria and archaea that thrive at temperatures above 80 °C on Earth could provide the best indication of what any life might look like.
The extreme tidal moon
If a moon were orbiting particularly close to a large planet, the tidal forces could be enormous. Here, as with Jupiter’s Io, the rocky surface would , and the intense volcanic activity and searing heat would probably rule out life.
With thanks to , research fellow in astrobiology at the University of Leicester, UK
This article appeared in print under the headline “Home, sweet exomoon”
