Download: Our printable PDF poster of how to spot signs of life on other planets (also available as a high-res JPEG image)
JUST over 400 years ago, Galileo Galilei turned his newly constructed telescope towards Earth’s moon. What he saw there reminded him of home: a dappling of lighter and darker areas where sunlight played across the lunar surface. As he wrote in his 1610 treatise : “If the sphere of the Earth were seen from a distance, when flooded with the Sun’s rays, that part of the surface which is land would present itself to view as brighter, and that which is water as darker in comparison.” The moon, like Earth, Galileo concluded, was a world of lands and seas. And as his contemporary Johannes Kepler noted, where there are lands and seas, there could be life. Perhaps, indeed, life had built the moon’s large, near-perfectly circular craters.
They were by no means the last to be seduced by signs of life where no life exists. Wherever we look beyond Earth, we are eager to spot our ilk. William Herschel, the celebrated astronomer who discovered infrared radiation and the planet Uranus, noted roads, towns, pyramids and regions “tinged with green” on the moon. Early observers attributed fluctuating dark patches on the surface of Mars to the seasonal growth of vegetation. Venus’s cloud-covered face was long thought to obscure a humid twin of Earth with hot, lush jungles, or perhaps a warm global ocean teeming with life.
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Today we know the moon is lifeless and sterile, its blotches immense plains of solidified magma. Mars’s dark patches are dust storms; Venus’s friendly exterior obscures a choking greenhouse of sulphurous fumes. Time after time, life’s signatures have been written in disappearing ink.
That is something to chew over as we enter a new era of planetary discovery. In less than two decades, we have spied about 600 worlds orbiting other stars. This year alone, the Kepler space telescope, NASA’s dedicated planet hunter, has added over 1700 candidates to the list. A few finds from Kepler and elsewhere seem to be roughly the size of Earth and in orbits that just might allow liquid water to exist, providing a solvent bath in which carbon-based life could evolve and flourish. In short, it won’t be long before we spot another planet that might conceivably harbour life, if indeed we haven’t already. But would we know it if we saw it – and if so, how?
Fittingly, it was Galileo who first showed us what a life-bearing world looks like from afar. Not Galileo the astronomer, but . En route to Jupiter in 1990, it briefly turned its suite of remote-sensing instruments – cameras and spectrometers to record the emission and absorption of light – towards Earth from 960 kilometres away. Would it see an obviously living planet?
Astronomer and his colleagues reported that Galileo’s visible-light camera spied a contrast between darker areas and brighter areas. Earth’s absorption spectrum also contained the signatures of water vapour and carbon dioxide. Water vapour suggests abundant surface water, possibly in liquid form; carbon dioxide indicates a rocky planet, since in gas-giant worlds this gas would combine with readily available hydrogen to form simple hydrocarbons such as methane. Piecing all this evidence together, we might then interpret the dark areas as oceans and the bright areas as land ().
A beacon of photosynthesis
Some of those bright areas also had an odd spectral characteristic: they efficiently absorbed visible wavelengths corresponding to the peak of the available solar energy, but reflected less prevalent infrared. We might have recognised this as the signature of chlorophyll in surface vegetation, efficiently and selectively taking advantage of the light resources available to it. Earth’s plants shine a beacon of photosynthesis into space.
This was not the most compelling piece of evidence for life, however. That came in the form of the spectral signatures of abundant oxygen and a significant concentration of methane side by side in Earth’s atmosphere. This juxtaposition is chemically implausible. Oxygen is highly reactive; it readily oxidises methane into water and carbon dioxide. If even just a little methane persists in an oxygenated planet’s environment, some thermodynamically improbable process must be constantly replenishing it. In Earth’s case, that something is life. Oxygen is the metabolic by-product of photosynthesising organisms. A vanishingly small amount of methane is produced by slow geological processes in Earth’s crust, but most of it is the exhaust of a diverse and ancient group of microbes that reside in such oxygen-poor Earthly environments as swamps, rice paddies and the stomachs of ruminant animals.
All in all, the Galileo probe’s evidence added up to a reasonably convincing identification of life seen from about 1000 kilometres away. But would such signals be recognisable over the tens to hundreds of light years that separate us from most planets we have discovered in other solar systems?
In theory, the answer is yes. Indeed, NASA’s and space telescopes have already obtained crude atmospheric spectra for a handful of exoplanets. That, however, has largely been through happy chance: such observations are limited to hot, extremely large gas giants that are precisely aligned to move periodically across the faces of their stars as seen from Earth. This “transit” configuration allows starlight to shine through the planet’s puffy, extensive upper atmosphere, revealing spectral features.
The telescope uses the transit method to detect much smaller, cooler planets more akin to Earth. But it isn’t nearly powerful enough to collect the trickles of starlight that filter through these diminutive planets’ atmospheres. Typically, it can measure their sizes and orbital characteristics, but little else.
That situation is unlikely to change any time soon. “Kepler is finding worlds that look really intriguing, but there are no missions in the pipeline to follow up on these initial discoveries,” says Victoria Meadows, a planetary scientist at the University of Washington in Seattle. Capitalising on what we might see will require an alternative approach.
Meadows heads the (VPL), a network of scientists brought together by NASA, which aims to provide just that. Its goal is to use computing muscle to quantify the astrophysical, atmospheric and geological factors that influence whether a planet can harbour life, and model how these factors change a habitable planet’s appearance over time. We can then zoom out from these modelled virtual worlds and see how they would appear viewed from light years away, at all angles and at all stages in their history. The aim, says Meadows, is to create a “Swiss army knife” of tools that, when presented with data on a new planet from Kepler or elsewhere, will tell us something about its likelihood of bearing life.
“The aim is to create a ‘Swiss army knife’ of tools that will tell us about a planet’s likelihood of bearing life”
There’s one type of habitable planet we find easiest to model. A cornerstone of the VPL project is the “Earth in a box” – a sophisticated three-dimensional mock-up of our home world which can be viewed from all possible vantage points and under different solar illuminations. In a paper published in June this year, Meadows and her colleagues compared this model with actual space-borne observations and showed that it reproduces our planet’s fluctuating brightness and light spectrum to within 3 to 5 per cent accuracy ().
The advantage of this virtual home is that it is mobile: we can drag our twin to anywhere in the cosmos to see what we would make of it there. “It allows us to explore what Earth would look like if it were being observed from very far away as it orbited around its star,” says Meadows.
So how would it look? For a start, it would appear blue. That fits with our self-image as an ocean planet, but in fact it has less to do with the sea’s azure hue and more with why the sky looks blue: molecular gases such as oxygen and nitrogen in Earth’s atmosphere preferentially scatter the bluer portions of sunlight, colouring our view up and out – and also from outside looking in. The VPL’s modelling suggests that, by measuring the ratio of certain wavelengths of ultraviolet and visible light reflected from an alien planet, we might detect this scattering from very far away. That would be a tantalising hint that at least one other similarly lively world abides beneath a friendly blue sky ().
“A blue hue would be a tantalising hint that at least one other lively world abides beneath a friendly sky”
Leaps of faith
With luck we might still see oceans. If we just happen to observe a distant Earth-like planet from an angle so that the side facing its host star is almost entirely turned away from us, we will see a thin arc of illuminated surface, similar to a crescent moon. As an ocean-covered region rotates into this planetary sliver, light will bounce off the water’s flat, mirror-like surface, increasing the sliver’s reflectivity and glinting the signal of oceans across the light years (). An analogous shift in contrast between a planet’s thick atmosphere and an airless lunar surface might also reveal a large moon like ours as it orbited across a planet’s face. That would be a promising sign of habitability: our unusually large satellite is a crucial factor in correcting Earth’s otherwise chaotic axial tilt, and helps ensure stable climatic conditions in which life can thrive.
All this is very well, but just because life might plausibly be somewhere does not mean it is. Relying too much on finding characteristics similar to those of our own planet invites the same leaps of faith about the existence of life that we have been prone to in the past – with the same likelihood of a fall. That is why VPL’s modelling aims to aid the more direct approach of looking for the by-products of life in planetary atmospheres.
Even without the added problems of viewing over many light years, this is a fraught business. The most obvious signatures of life are spread out through the spectrum, with water vapour and oxygen at visible and near-infrared wavelengths and methane, carbon dioxide and oxygen’s molecular cousin, ozone, at longer infrared wavelengths. That makes it difficult to observe all of them with any one telescope.
Reliable models that can be applied to many environments could help sort out some of this confusion. A typical task for the VPL’s researchers might be to compute how light from a star different in size, temperature, and luminosity from our sun will affect a planet in a habitable orbit, by heating it more or less, or through creating and destroying compounds in the air or on the surface – for example turning molecular oxygen, O2, produced by life into ozone, O3.
“These sorts of modelling efforts will tell us things like how important it is to look for oxygen versus ozone, or where the best spectral region is to look for water, or which star types are really worth looking at,” says of the NASA Exoplanet Science Institute in Pasadena, California. “In each case, we want to look for whatever gives us the most leverage for learning exactly what kind of planet we’re studying.”
History being what it is, the need to identify false positives looms large. Oxygen is a case in point. Our near neighbour Venus possibly once had an ocean billions of years ago, until plentiful sunlight and its CO2-rich atmosphere contrived to spark a runaway greenhouse effect that boiled off its seas. Ultraviolet sunlight would then have gradually broken these airborne water molecules down into oxygen and hydrogen. The lighter hydrogen would have drifted off into space, but the oxygen would have persisted for many millions of years, gradually bonding with the surface rocks. Catch such a planet at the right time, and its hellish landscape would be veiled beneath an inviting facade of water vapour and breathable oxygen.
A similar deception could take place on a frozen, geologically inert world rather like modern Mars, but slightly more massive and with a more substantial atmosphere. There, for lack of volcanic gases and fresh rocks with which to react, the oxygen from any sublimating water ice might build up essentially indefinitely – a sign of life where none exists. “For an Earth-sized planet in the middle of the habitable zone of a sun-like star, perhaps most of the time atmospheric oxygen will be a biosignature,” says Meadows. “But outside of that narrow space we have to be very careful.”
Earth’s own history shows that even more caution is necessary before interpreting an absence of oxygen as an absence of life. Though life is thought to have sprung up relatively quickly on Earth, probably within Earth’s first billion years, it took a further billion years before oxygen reached appreciable atmospheric levels, about 2.4 billion years ago. Before that, methane from the ancient, anaerobic microbial biosphere would have been the most readily observable atmospheric biosignature. But methane alone is no cut-and-dried indicator of life: it can be made in small amounts through lifeless interactions of hot water and rock deep within a planet, as indeed it is on Earth. That methane could slowly seep out into an atmosphere and, in the absence of oxygen to mop it up, linger there. That leaves us with a conundrum. “We’ve only had an oxygenated atmosphere for half of our planet’s lifetime,” says Meadows. “So what the heck do we look for besides methane in an anoxic environment?”
“Earth has only had oxygen in its atmosphere for half its lifetime. So what the heck else do we look for?”
According to , an astrobiologist at NASA’s Ames Research Center in Moffett Field, California, the answer to that question might come from seeing life in terms of its function as a chemical catalyst. “Methane in Earth’s atmosphere represents the fact that life could gain chemical energy by putting carbon dioxide and hydrogen together,” he says. But that is not a one-step process – and the intermediate products vented from known anaerobic metabolisms might give us new clues as to what to look for on a distant planet.
Meadows and her team have taken this idea on board and focused on how “organosulphur” compounds such as methanethiol or dimethyl sulphide, which are produced in great quantities by some anaerobic microbes on Earth, might manifest themselves on a faraway planet. “We identified fluxes that could come from life at the surface, had them interact with the planet’s atmospheric chemistry and ultraviolet light from the parent star, estimated their atmospheric lifetimes, and generated the resulting spectra,” says Meadows.
What emerged was a surprise. Methanethiol and dimethyl sulphide were plentifully produced, but the action of ultraviolet light rapidly knocked off their methyl groups, which combined in pairs to form ethane gas. Ethane, while colourless and odourless, has a clear infrared spectroscopic signature and a significant atmospheric lifetime. A possible signature of an anoxic, but living, planet, the researchers concluded, is a high atmospheric ratio of ethane to methane ().
That is a significant result, as this ethane build-up should be particularly potent in planets around cool, diminutive stars called M-dwarfs. For a planet in such a situation to have liquid water, it would orbit practically cheek by jowl with its stellar host – well within Mercury’s distance from the sun – making it easier to discover through the gravitational wobble that its tight orbit would induce on the star. These could be the first living worlds we discover beyond our solar system – although the wrenching gravitational forces and intense ultraviolet radiation on such planets might make even familiar life forms difficult to recognise.
For such worlds and more familiar ones, the modelling initiative is slowly producing a long list of esoteric processes that could both help and hinder the identification of life. “We are revealing the complexity behind initially simplistic assumptions about habitable planets and how to find them,” says Meadows.
Ultimately, though, that can only take us so far without the right data. NASA’s , slated for launch no earlier than 2018, could probably obtain low-resolution atmospheric spectra for a few potentially habitable M-dwarf planets as they transit across the faces of their stars. Besides that, we are likely to be left guessing about what the light of life will look like. “The VPL and similar projects are going to help guide the design of future telescopes by telling us what astrobiologists really want to look at,” says Beichman.
Whether those telescopes are built is another matter. Just imagine, though, some day seeing with imperfect vision blurry pricks of blue scattered through the sky, some winking with oceans, a few blanketed with what could be the breath of life. Who wouldn’t want to take a closer look?
Not in my backyard?
As we begin to consider what signs of life might come from other solar systems (main story), intriguing possibilities still exist closer to home.
• Mars A few billion years ago, Mars was a lot more like Earth, with a clement climate and abundant liquid water. Today, liquid water might exist in subsurface reservoirs. Mars orbiters and remote-sensing instruments on Earth have also detected faint whiffs of methane seasonally wafting from the depths – signs, perhaps, of life that has long since retreated to a safe and hidden refuge.
• Venus This near neighbour was also once a much more amenable environment for life. Close examination of Venus’s stifling clouds today has revealed mutually reactive hydrogen sulphide and sulphur dioxide, as well as carbonyl sulphide, a substance produced on Earth chiefly by biotic processes. That nourishes a controversial theory that microbes could live on high in the planet’s atmosphere, shielded from harmful ultraviolet rays by protective sulphur rings.
• Europa If evidence from NASA’s Galileo mission is anything to go by, this moon of Jupiter boasts a deep salty ocean beneath its icy crust, which is itself filigreed with fissures and veined with organic compounds (picture, right). If the crust is not too thick, those energy-rich organic compounds might channel nutrients into an aquatic biosphere.
• Enceladus In 2005, the Cassini orbiter spied active geysers pluming from near this Saturnian moon’s south pole, providing evidence of warm pockets of liquid water just beneath the surface. It is unclear whether life could have arisen there – but any microbe vacationing from Earth would find conditions there quite hospitable.
• Titan The Cassini mission’s observations of Saturn’s largest moon reveal not one but two promising abodes for life. Titan’s crust appears to float on a vast ocean of liquid water and ammonia in which life not that dissimilar to Earth’s could perhaps exist. Meanwhile, something seems to be depleting the hydrocarbons ethane and acetylene on the moon’s frigid surface. That might indicate exotic carbon-based life with a hydrogen-fuelled metabolism that uses liquid methane rather than water as a solvent. Could this be a first glimpse of a “second genesis” of life on a completely different basis to Earth’s?
Exotic possibilities
Just as oxygen, methane and other gases can suggest the presence of a biosphere hungry for chemical energy (main story), the prevalence of easily metabolised, energy-rich materials in a warm, rocky planet’s atmosphere might constitute an “anti-biosignature” – evidence for a world bereft of life. According to planetary scientist of the California Institute of Technology in Pasadena, finding an abundance of something easily digestible like hydrogen in such a planet’s spectrum would be equivalent to seeing masses of windblown paper currency billowing through a city’s streets. In both cases, the chances are good that nobody lives there.
This, of course, at least partially assumes the primacy of carbon-based chemistry. The signatures of life built out of strange things like silicon or magnetised plasma or neutrinos, or organisms using energy sources such as radioactive decay or matter-antimatter annihilation, are extremely ill-defined. In large part that is because it is unclear how such creatures would perform many of the activities that underpin our definitions of life, such as metabolism, growth, reproduction and evolution.
An alternative possibility that does not depend on biology is to look for inadvertent “technosignatures” of life. Taking cues from Earth’s own recent history, when artificial chlorofluorocarbon (CFC) refrigerants proved remarkably adept at disrupting the photochemistry of our atmosphere’s ozone layer, perhaps we might seek on other worlds the spectral signatures of gases believed to be produced solely through technology and engineering. It might be a bleak identification: such artificial substances could endure as mute testaments to life, regardless of its biochemistry, long after a civilisation had burned itself out.
