FEW discoveries could be bigger than detecting life on another planet. Whether it is a rocky ball or a giant cloud of gas, hot, cold, or somewhere in between, we arenât picky: so long as a world has life, we want to find it.
For as long as we have searched, we have had one image in mind: Earth. It might seem like vanity, but our focus makes a certain amount of sense. After all, Earth is the only planet in the universe that we know for a fact supports life. Even if faraway exo-Earths donât have oceans, continents, rainforests, deserts and polar caps, the long-standing assumption is that they will still be familiar in certain ways. There will be water on the surface, oxygen in the air, possibly even vegetation on the land.
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But Earth hasnât always looked the way it does now. In the 4.5 billion years our planet has existed, it has experienced dramatic transformations: ice ages and warming periods, times when the atmosphere was impossible to breathe, when large areas were desert, or when lush tropical forests hugged the poles. Throughout the vast majority of this turbulent history, life has somehow clung on.
If, armed with a spottersâ guide to the world we inhabit today, we found exoplanets resembling those early Earths, would we even recognise them for what they were? Maybe not. We know how to seek comparatively advanced signs of intelligent life, such as cacophonous radio transmissions and the bright lights of megacities. If a planet has less sophisticated inhabitants, however, we must rely on identifying other signatures of life. That is why scientists interested in filling in the blind spots of our search for intelligent life have started a lot closer to home. They want to imagine how early Earth would look if seen from far outside the solar system.
The first time we tried to study our planet from afar was when the Galileo mission launched in 1989. It was programmed to orbit twice around Earth, using the planetâs gravitational pull as a slingshot to power it on its way to its ultimate goal, Jupiter. The astronomer Carl Sagan suggested taking the opportunity to point our best alien-finding equipment at Earth itself. If it found nothing, he reasoned, that meant life elsewhere might also escape our attention.
Galileo was to be the first of several probes to use their instruments in this way, and they confirmed that our technological civilisation should be detectable by a distant, similarly advanced alien civilisation.
Where there are no hints of technology to home in on, by far the best life sign to latch on to is oxygen. Its abundance in our atmosphere â 21 per cent â would be difficult to sustain without plants pumping it out constantly. Methane, released by bacteria as well as flatulent livestock, is a more ambiguous biosignature, given that a number of non-biological mechanisms can produce the gas.
A bigger giveaway than either oxygen or methane on their own, however, is the presence of both. That is because the combination is combustible and releases energy when it reacts, forming carbon dioxide and water. âIf these two gases are left to their own devices, theyâll usually annihilate,â says Chris Reinhard at the Georgia Institute of Technology. âThe fact that theyâre both present at relatively high abundance suggests that theyâre being pumped into the atmosphere at very high rates, potentially by biology.â
Another reliable sign of life is the light reflected by plants. Although our planetâs vegetation absorbs most visible wavelengths, with the exception of green, it absorbs far less infrared light. The upshot is an abrupt jump â known as the red edge â in Earthâs reflectance spectrum, a phenomenon that would be hard to replicate without living organisms.
Searching for high levels of oxygen, methane coexisting with oxygen, or a red edge would be a good way of picking up planets that look like Earth now. But Earth has been inhabited â not to mention inhabitable â for far longer than it has displayed any of these features (see âdiagramâ).
The earliest known life forms emerged about 4 billion years ago, when the planetâs crust was cooling to form rocks and the beginnings of continents. At this time, known as the Archaean, Earthâs atmosphere would have been dominated by carbon dioxide produced by active volcanoes. In this hostile environment, primitive microbes emerged that made methane, and oxygen levels were at an all-time low. This was fortunate â if they had been any higher, life as we know it might not have emerged at all. âOxygen poisons some of the prebiotic chemistry that we think culminated in the origin of life on Earth,â says Stephanie Olson at the University of California, Riverside.
âFor about four-fifths of Earthâs history, we would not have been able to see evidence of life as we know it on the surfaceâ
A billion or so years later, the planet was entering a time called the Proterozoic. Photosynthesis was already well under way, but it was at this time that the ability to convert carbon dioxide into oxygen had lasting consequences for the planet. For the first time, oxygen, carbon dioxide and methane coexisted in the atmosphere, leading to the accelerated evolution of multicellular life.
During this time there were two âsnowball Earthâ events, when the entire planet was covered in ice. The trigger may have been something as trivial as a brief drop in global temperatures, allowing the polar ice caps to expand. This reflected more sunlight, reducing temperatures even more in a feedback loop that eventually froze the whole planet. The trouble is, we donât know how freezing over would have affected the chance of spotting life from afar. âThe composition of the atmosphere during the snowball Earth events is actually not terribly well known,â says Reinhard.
The Proterozoicâs defining feature, however, was a billion-year period of apparent stability from 1.8 billion years to 800 million years ago. After the frenzied emergence of multicellular life, and the chaos of the first snowball Earth event, the planet relished the opportunity to take a breather. The climate was constant, life appeared to be evolving very slowly, if at all, and oxygen remained at low levels. Little wonder that geologists have taken to calling this the âboring billionâ.
Oxygen masked
Some think that name is a tad unfair. After all, this was when sexual reproduction first evolved, and when the first eukaryotes â organisms with complex cells that ultimately gave rise to ourselves â appeared, âwhich is a big dealâ, says Nick Butterfield at the University of Cambridge. From the perspective of an alien Galileo space probe, however, little would appear to change.
By the time the boring billion ended, life was really stepping on the accelerator pedal. At this time, called the Phanerozoic, the diversity of life forms skyrocketed in what is known as the Cambrian explosion, oxygen finally reached levels that would be remotely detectable and plants began to flourish on the planetâs surface. This is when the red edge would have first been visible.
This complicated history offers a stark lesson for those hunting exoplanets based on Earthâs current appearance. âFor about four-fifths of Earthâs history we would not be able to see evidence of life on the surface,â says Reinhard. Whether searching for high oxygen levels, oxygen-methane coexistence or a red edge, you would mostly come up empty-handed. âThe obvious candidates for biosignatures arenât going to work as well as we thought,â says Olson.
Part of the problem is how little we know about early Earth. It is no exaggeration to say that, in some ways, we will soon know more about planets billions of kilometres away than we do about our own world billions of years in the past. With so little surviving evidence â and each piece so open to interpretation â reconstructing Earthâs history is a major challenge. Until the gaps are filled in, what life-hunting astronomers need is a broader-brush way of figuring out which planets to investigate further.
For Enric PallĂ© at the Institute of Astrophysics of the Canary Islands, one promising avenue is the ancient equivalent of red edge. Ever since the days of the Archaean, long before the continents became hotbeds for vegetation, they were probably swamped by purple mats of single-celled bacteria. If these tiny organisms were present in great enough numbers, says PallĂ©, their effect on the light reflected from Earth would be similar to that of vegetation, but shifted toward the far red end of the spectrum. âYou can envision a whole bunch of colours you can get,â says Abel MĂ©ndez, director of the Planetary Habitability Lab at the Arecibo Observatory in Puerto Rico, depending on which bacteria are most common.
Attractive though this sounds, its signal could be weak and tough to spot from afar. Olson and her colleagues think they have identified two more promising avenues. The first could be to observe planets over a long period, instead of just getting a snapshot of their atmosphereâs composition. Observing Earth in this way, for example, would reveal a seasonal change in atmospheric carbon dioxide levels. That is because plants use more carbon dioxide during their growing season, with the northern hemisphere dominating the effect because of its greater land mass. âYouâd see that itâs kind of wobbly up and down once a year,â says Reinhard.
On plant-free planets like Proterozoic Earth, it is not likely that photosynthesis by microbes would be enough to cause clear oscillations in carbon dioxide levels. Instead, respiring organisms might produce similar seasonal variations in oxygen. And although oxygen levels themselves might be too low to spot from afar, the effect of their fluctuations on levels of other chemicals such as ozone could conceivably be picked up.
One downside of using seasonality as a biosignature is that some worlds donât have seasons. MĂ©ndez points out that planets around the smaller, dimmer stars that are Earthâs closest neighbours must orbit close to their star to be potentially habitable, but doing so means they tend to end up keeping the same face pointing toward their star all the time. âThey are tidally locked,â he says, âso you wonât have any seasons.â
Delicate imbalance
The other idea Olson and her colleagues are working on could prove more fruitful. It involves rethinking how life might influence the make-up of an atmosphere. Much as methane and oxygen would not persist together on Earth if all present-day life disappeared, there are other combinations of gases that scientists regard as being in disequilibrium â that is, they would be hard to sustain without life. During the Archaean, for example, such was the imbalance of atmospheric methane with carbon dioxide, nitrogen and water that it would have been rapidly wiped out as soon as it stopped being produced.
But it isnât necessary to look for all of those gases at once. Olsonâs team argues that you need only see carbon dioxide and a sufficiently large amount of methane together in an atmosphere to realise something biological is probably afoot. And although the relative abundance of oxygen and methane would probably not have been measurable from afar at any point in Earthâs history, that of carbon dioxide and methane might be. âA detectable carbon dioxide and methane disequilibrium is more likely on a broader range of planets,â says Olson, âincluding those with undetectable levels of oxygen.â
Butterfield thinks looking for these atmospheric imbalances is an interesting idea, but cautions against falling for alien life as the explanation. âJust seeing disequilibrium is interesting,â he says. âBut it doesnât necessarily have to be biological activity.â
Although false positives are inevitable, Olson is hopeful that clues like disequilibrium and seasonality will help fill in some of those blind spots in our search for life. For one thing, says Olson, âtheyâre not tied to specific metabolismsâ. Alien life wouldnât necessarily need to be like us, or even be carbon-based, for these potential biosignatures to reveal its existence on a distant planet.
Getting a good enough look to find out, however, isnât going to be easy. Figuring out what is going on in the atmosphere of an exoplanet means gathering as much light as possible from it. But of course, planets donât produce light; they only reflect it, and that signal is dwarfed by the light of their star. To separate them, we will need enormous telescopes like NASAâs James Webb telescope, due to launch in 2021, or the next generation of extremely large, ground-based telescopes. Even with these devices, the task will be tricky. Seasonality on an exoplanet will require so much telescope time, says PallĂ©, that âwe will not be able to measure that, not as long as you or I are aliveâ.
No single measurement is ever going to be conclusive. By looking at the make-up of a planetâs atmosphere, how it changes over time and anything unusual that appears to be going on at the surface, researchers will instead build a slowly evolving picture of that worldâs chances of hosting life. âItâs not going to be like a discovery where you dig something and you say, âThatâs it! I found itâ,â says PallĂ©. âItâs a process where we are slowly choosing our best candidate.â
Our continuing ignorance of aspects of Earthâs primordial past could also hamper the hunt for distant life. What set off the snowball Earths of the Proterozoic, for example, is still a mystery. Even if we spot an exoplanet in an ice age, says Reinhard, we wonât know how to interpret what we see. One of the biggest discoveries in history could elude us once again.
This article appeared in print under the headline âLessons from early Earthâ
