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IT IS the biggest question in the universe: are we alone? Philosophers have debated the question for millennia. When 16th-century Italian astronomer and Dominican friar Giordano Bruno declared that the cosmos contained “an infinity of worlds of the same kind as our own”, he was directly contravening religious dogma. He was later burned at the stake during the Inquisition, in part for daring to question Earth’s unique status.

The debate continues, in more restrained fashion, to this day. For some, the sheer size of the universe makes it unlikely that life formed only once. For others, the remarkable complexity of life on Earth is testament to its uniqueness.

Until recently, vague philosophical answers of this kind were the best science could do. The signs of life were far too ambiguous to pin down for certain, and our nearest potentially habitable worlds were too small and distant to test.

But for the first time in human history we are reaching the technological sophistication needed to provide a genuine answer. Powerful telescopes are letting us study planets in other solar systems, giving us a glimpse into their atmospheres and a flavour of what type of life might be living on their surfaces. At the same time, improved analysis of our own planet is allowing us to redefine what life might look like from afar, and is helping us to distinguish the signs of a flourishing alien civilisation from the mere geological rumblings of a lifeless world. With these tools at our disposal, answers are finally within our grasp.

To understand my optimism, it is worth revisiting the work of astronomer Frank Drake. In 1961, Drake devised a formula to estimate how many advanced civilisations were capable of signalling their presence in the Milky Way. His eponymous equation depends on breaking down that big unknowable quantity into a number of more tractable ones that can be multiplied together, such as the number of stars in the galaxy and the fraction of those likely to have planets (see Quiet neighbourhood).

Even with pessimistic values, the existence of millions of technological civilisations seems likely. The main bottleneck on that apparent explosion of life, however, is in Drake’s final term: the average lifetime of a communicating civilisation. Humans have been broadcasting radio signals that escape into space for only about a century, and, in the current geopolitical climate, who is to say how many more years we have left. If you take the pessimistic assumption that intelligent life destroys itself rather quickly, the Drake equation suggests that statistically we are alone in the galaxy. If intelligent civilisations survive for millions, or even billions of years, however, then the Milky Way should be teeming with aliens.

This calls for optimism, but also caution. After all, if there are millions of alien civilisations out there, then why haven’t we seen signs of them already? This seeming contradiction is sometimes called the Fermi paradox, after Italian physicist Enrico Fermi, who gave it its most succinct expression. With a back-of-the-envelope calculation, he showed that a single space-faring civilisation could easily colonise a galaxy within a hundred million years. Because the universe is 13.8 billion years old, and no interstellar colonists have yet appeared on our horizon, Fermi asked: where is everybody?

Radio silence

There are many proposed answers to this question (see “Solutions to the Fermi paradox”). Perhaps, say some, the aliens are already here, just keeping their identities secret. Perhaps they are deliberately steering clear of Earth, treating it as a sort of cosmic heritage site that deserves their protection. Or alternatively, there are simply no aliens out there. As an astrobiologist, I prefer to believe that aliens are out there; we simply haven’t communicated with them yet.

It isn’t hard to imagine why this could be the case. Alien civilisations might well be millions of years ahead of us in their technological advancement. Trying to communicate with them using our primitive technology would be as absurd as teaching a ladybird to use a telephone. That hasn’t stopped us trying, of course, whether by including artefacts, such as plaques etched with celestial maps and images of humans, on our long-distance spacecraft or by broadcasting targeted radio messages into the depths of space. So far, no reply.

All hope is not lost. The Fermi paradox and Drake equation specifically deal with the question of intelligent life, with the ability to communicate, travel and colonise. But only a fraction of the life we know of would be capable of these feats. Today, the vast majority of Earth’s biosphere consists of microbes. Single-celled organisms dominated the planet’s surface for nearly 3 billion years before multicellular life began. What is more, microbial cells not only outnumber human cells on our planet, they even outnumber them on and in your body. If life exists elsewhere in the universe, chances are it is microbial.

This means that the first detection of alien life is unlikely to come from eavesdropping on an interplanetary conversation. Instead, we will need to scan the atmosphere of other planets for familiar molecules that primitive microbes are likely to emit: as close as we can get to a fingerprint of life.

An obvious place to start is with our own planet. If alien astronomers were observing Earth from a remote star system, would anything about it grab their attention? Compared with our rocky neighbours Mars, Venus and Mercury, the distinctive mix of oxygen and methane in Earth’s atmosphere would be sure to trigger interest. Oxygen makes up 21 per cent of the atmosphere now and is entirely due to life, entering the atmosphere from photosynthetic bacteria and plants that convert sunlight into energy. We aren’t sure when exactly oxygenic photosynthesis evolved, but there are clear signs that our atmosphere filled with oxygen 2.33 billion years ago. Methanogens, the microbes that produce methane, existed even earlier.

Despite the biological origins of both gases, neither on its own is a sure sign of life. Methane, for example, is also produced by volcanoes and hydrothermal vents, although methane with an organic origin has a higher carbon-12 to carbon-13 isotope ratio. Oxygen could be formed when radiation from an active star splits molecules of water into hydrogen and oxygen, with the lighter hydrogen escaping from the planet’s atmosphere. In combination, however, methane and oxygen tell a story of a planet swarming with life.

In the 1960s, astronomers realised that the existence of each gas was fatal to the other. Without large quantities of both oxygen and methane being continuously pumped into the atmosphere, these gases would quickly react and destroy each other. Individually you might expect a lifeless planet to contain either oxygen or methane. But geology alone doesn’t provide a way to maintain both.

This means that finding oxygen and methane coexisting in appreciable quantities on a distant planet is a pretty good indicator of life. What’s more, life on Earth produces thousands of other molecular gases that seem to be unique. Methyl chloride, dimethyl sulphide and nitrous oxide have all been proposed as promising biosignature targets.

“Finding no life elsewhere may lead us to take better care of our own world”

What if our search for all of these gases comes up empty? Does that mean a planet is an arid ball of rock? Not necessarily. Life on a distant world may be totally different to that on Earth. It could be hiding under the surface, within solid rock or in hidden seas, where it would be effectively invisible. More radical alternatives are also possible. It could be based on silicon, for example, rather than carbon, or run on unknown metabolisms that use a liquid other than water. For these types of weird life, synthetic biology and research into alternative biochemistries could help us understand what unique chemicals to look for.

Sara Seager at the Massachusetts Institute of Technology is trying to tackle this problem, working her way through all the molecules whose presence might indicate the existence of life. One of my favourite ideas comes from another MIT researcher, Clara Sousa-Silva, who says we should look for phosphine as a sign of life. Phosphine is a gaseous compound of phosphorus and hydrogen that is produced on Earth by anaerobic microbes, which don’t rely on oxygen to survive. Not only would it be relatively easy to detect in an exoplanet’s atmosphere, but it is the simplest gas that can’t be produced by any natural processes we know of. Detecting phosphine, in other words, could indicate an anaerobic biosphere.

If coming up with such hypotheses seems challenging, putting them to the test is something else entirely. The first step is to identify candidate exoplanets: those with the right temperatures to nurture the complex chemistry needed to sustain life. At present, finding worlds beyond our solar system is usually done by looking for the slight dimming that happens when a planet crosses in front of its star. It is a process hundreds of times more difficult than spotting a firefly crossing a searchlight on the other side of the Atlantic.

This detection method also opens the door to sensing different types of molecules in the atmosphere of a temperate and rocky planet. For example, when light from a star passes through the air cloaking such worlds it can reveal the composition of that air. Different molecules respond to different wavelengths of light, and by separating the light we collect in our telescope into different wavelengths, we could see the telltale spectra, or light signals, produced by substances such as oxygen, ozone, methane, water and carbon dioxide.

What makes it such an exciting time to work in this field is the number of missions being developed to perform this task. The first of these will be NASA’s James Webb Space Telescope, scheduled to launch in 2021. This will be our first hope at identifying molecules in the atmosphere of a habitable exoplanet. ARIEL, a European Space Agency mission due to launch in 2028, will continue this effort.

Another promising technique involves using large ground-based telescopes to do the same thing. These include the European Southern Observatory’s Extremely Large Telescope, currently being built in Chile and due to start working in 2025. Observing planet atmospheres from Earth’s surface is difficult because you must first remove our planet’s atmosphere from the signal. Next-generation ground observatories will be able to do just that by subtracting its effects from the light entering the telescope. This detailed technique can even allow us to distinguish isotopes on other worlds, subtly different versions of the same atoms that differ only by the presence of a single neutron in their nuclei. That is something I never dreamed would be possible in my lifetime.

For all the excitement surrounding far-flung planets, perhaps the first successful detection of extraterrestrial life will happen closer to home. Certainly, other places in our solar system have conditions suitable for life as we know it, such as in the liquid water ocean hidden beneath a thick ice layer on Jupiter’s moon Europa in the subsurface water on Mars. Alternatively, some have suggested that life could reside on Saturn’s moon Titan, swimming in its lakes of liquid methane.

Whatever we find on these nearby worlds, I am confident life exists elsewhere in the universe. But confidence isn’t enough. Over the next few years, our searches are going to become more accurate, more thorough and capable of looking further than before. The answers we find stand to fundamentally shift our understanding of the universe and our place in it. As the science fiction writer Arthur C. Clarke put it: “Two possibilities exist: either we are alone in the universe or we are not. Both are equally terrifying.”

To my mind, finding alien life would humble our apparently exalted status in the cosmos. We would be just one more example of life as a planetary process, crystallising out of the molecules that make up our universe. Searching widely and finding nothing would be equally sobering, however, indicating that even in environments we think of as habitable, the chasm between chemistry and simple life is vast. Hopefully, such an appreciation of life’s rarity would lead us to protect all forms of existence on our own world, reminding us that Earth is the only home we have.

The next two decades will witness a revolution in exoplanetary science. We have already found dozens of potentially habitable worlds and the next technological advancement in observations will be able to detect potential biosignatures in their atmospheres. Now we need to watch – and wait.