In October 1995 Didier Queloz and Michel Mayor of Geneva university discovered a planet 300 trillion miles away, orbiting a star known as 51 Pegasi. With the poetry of the modern astronomer, they named it 51 Pegasi b. It was the first planet orbiting a sun-like star ever detected outside our solar system. The science of exoplanets was born.

Once the skies had been thronged with gods. Priests and soothsayers—the first astronomers—discerned the biddings of the divine in the stars and planets. Even as man’s vision of himself changed from the creation of a supernatural being to a chance byproduct of cosmological theory, from standing at the centre of the celestial spheres to being an infinitesimal speck clinging to a rocky ball spinning round a near-infinite universe, the primordial urge to populate the heavens remained. It is no longer just a spiritual quest. Astronomers now reckon there are billions of planets in the galaxy that are broadly similar to our own. Some of the most sophisticated scientific techniques of our age have been brought to bear in finding and studying them. Yet the question remains the same as it has for millennia: is there anybody out there?

It had long been suggested that planets existed beyond our solar system. Around 400BC, the philosopher Democritus spoke with uncanny prescience of a universe filled with worlds of all varieties. More recently sci-fi writers have filled the vast empty spaces between the stars with a multitude of alien spheres. Astronomers were unable to partake in this vacuum dreaming because they lacked any empirical proof, but, as Louis Pasteur said, chance favours the prepared mind, and when Mayor and Queloz found the first exoplanet they did so by accident. At the time, the kind of exoplanets that astronomers were trying to detect were thought to make long looping orbits around their stars, taking ten years or more, and as several orbits would be needed to confirm an exoplanet’s existence, the work was expected to take decades. Queloz, then a 29-year-old PhD student, thought he was simply supposed to “start the program, write the software, and debug the instrument”. So, when he and Mayor found an exoplanet with an orbit of just four days, it was a considerable coup. For centuries scientists had been wallflowers at the Alien World Ball; now they could finally down their drinks and hit the dance floor.

Or so it seemed. The sheer improbability of 51 Pegasi b led to bellows of disbelief from some astronomers. The fact was this discovery confounded—and threatened—contemporary scientific thinking. The planet 51 Pegasi b is the size of the gas giant Jupiter (our solar system’s largest planet), but it orbits much closer to its sun than Mercury. Astronomers had assumed that such a planet could only survive far from its star. After 30 years of space missions and billions of dollars spent peering into the void, how could they have been so wrong? With hindsight, it seems obvious. These new worlds had evaded detection precisely because they were so very different from the eight planets in our solar system. Even so, many astronomers couldn’t, or didn’t want to, believe in them.

The backlash was not entirely unwarranted. Earlier attempts to discover exoplanets had lent all planet-hunting activities a degree of doubt. In the 1960s, a Dutch astronomer, Peter van de Kamp, announced that, after three decades of painstaking study hunched over his refractor telescope, he had discovered two exoplanets revolving around Barnard’s Star, the second-closest star to our own solar system. For a time he was the toast of the astronomical community. But then it emerged that the days on which he had found the salient data exactly correlated to the days on which his telescope’s lens was cleaned. Instead of viewing foreign worlds light years away, he had been observing flaws within his telescope centimetres from his eye. To many astronomers, exoplanets became the fool’s gold of the galaxy.

Exoplanets are fiendishly hard to spot. On a dark night you can see the star around which 51 Pegasi b revolves with the naked eye, but trying to see the planet itself would be like standing in London and looking for a firefly fluttering a foot away from a searchlight in Baghdad. With a powerful telescope, you might glimpse the glow of the firefly by itself, but next to the glare of the searchlight you wouldn’t stand a chance. As a result, most exoplanets have been found by indirect means, such as the effect their gravitational pull has on the movement of their stars, or the fraction of light the planets block out when they pass in front of their stars. Exoplanet-hunters have been left, literally, chasing shadows.

Astronomers have identified a bizarre cornucopia of spherical objects twirling through the galaxy with more interesting properties than their names suggest. There’s TrES-4, a “puffy planet”, nearly twice the size of Jupiter, with the density of balsa wood. There’s 55 Cancri e, a third of which is believed to be made up of solid, and liquid, diamond. Or how about GJ 1214 b, thought to hold far more water than Earth, in many different varieties, including vast regions of hot ice.

Astronomers can even tell what the atmosphere of an exoplanet may be like by studying the starlight that trickles through its upper reaches. Crucially, this filtered light carries with it the fingerprint of that planet’s atmospheric composition, which means we can look for biosignatures, evidence of gases such as oxygen or methane that accompany living organisms. The search for exoplanets has become a detective story in reverse: instead of trying to find the cause of death, we are searching for signs of life.

Nearly 1,800 exoplanets have been discovered since 1995, largely thanks to NASA’s Kepler space telescope, which simultaneously measured the variations in brightness of more than 165,000 stars every 30 minutes for three and a half years. The great hope is to find an Earth-like planet—one that is habitable, or better yet, inhabited. In April, scientists trawling the oceans of Kepler data announced they had found the most similar-looking planet to ours yet. Known as Kepler-186f, this Earth-cousin (as distinct from an Earth-twin) is only 10% larger than Earth and lies in the same habitable zone around its star that the Earth lies in around the sun. Unfortunately Kepler-186f is one of those distant cousins you’re unlikely to visit, being 2.9 quadrillion miles removed. Not to worry. There are plenty more out there: by the latest estimate, the Milky Way has at least 8.8 billion Earth-like planets. Can life be far behind?

Calling occupants... An artist’s impression of 51 Pegasi b, the first exoplanet ever detected, and its sun

One person in particular has made it her mission to answer this question. A professor of physics and of planetary science at the Massachusetts Institute of Technology, Sara Seager talks as if her breath can’t keep up with her words and her words can’t keep up with her thoughts. A long scarf flutters around her neck, lending her the air of someone in perpetual motion, which doesn’t lie: she travels relentlessly in her role as the public face of exoplanet science.

The path to finding new worlds is rarely smooth. “All who found out about my project denounced it with laughter and ridiculed me,” wrote Christopher Columbus in 1501, after returning from his third voyage to the New World. Seager needed a similarly thick skin when she began to look for worlds trillions of miles from home. Brought up in Toronto, she was drawn to exoplanets while studying astronomy at Harvard in the 1990s. “It was very high-risk back then—almost nobody worked on it,” she tells me, sitting in her office in Cambridge, Massachusetts. “It was like ‘why are you wasting your time?’” That only spurred her on: “In the early days it was people like me who were, like, ‘screw the world, I’ll just do what I want’.”

Following the discovery of 51 Pegasi b, the steady accretion of newly discovered planets ensured that the science would endure. Rival groups of astronomers raced to discover the latest weird orb, cannily parlaying their scientific obsession into a media spectacle, with each discovery prompting a press conference, a gaudy artistic representation of the planet, and often a front-page story. The column inches brought academic stardom and plentiful funding. The planet-hunters went from being untouchables to feathers in the cap of their institutions. When Seager developed the techniques for finding the biosignatures of other worlds—and thus their chances of holding life—she greatly broadened the pull of the science. These days it attracts students and specialists from fields as diverse as geology and microbiology, all keen to lend their expertise. New planets are being discovered almost every day. The study of exoplanets has become the little science that could.

The range of projects is vast. Seager speaks out for an astonishing array, from reawakening interest in the Terrestrial Planet Finder, a giant space telescope that was set to scan nearby solar systems before NASA mothballed it in 2011, to promoting a tiny fleet of shoebox-sized “ExoplanetSats” that will focus on individual stars. She is even advising an asteroid-mining consortium in the hope of becoming wealthy enough to fund her own exoplanet endeavours. “We’ll see which one works, but I have to pursue all options.”

Behind her is a blackboard that covers a wall, festooned with the chalk filigree of advanced mathematics. Her celestial enthusiasms are backed up by an instant recall for scientific detail and an apparently infinite self-confidence. She seems by turns entrepreneurial, evangelical and monomaniacal, always looking to lure you down the rabbit hole into her world of other worlds. She is not unconvincing. Take her Starshade project, a beautiful, flower-like spacecraft that looks like the buttonhole of a giant debonair robot. It is designed to hover thousands of miles in front of space-based telescopes and block out the blinding light from suns so that the exoplanets orbiting them may be seen directly, rather than being inferred. She recently showed some of NASA’s grandest poobahs one of the sumptuous petals that make up the Starshade and detected a “childlike enthusiasm” rare in the field. “It doesn’t mean they’re going to build it,” she cautions, “but the level of joyfulness it creates is incredible.”

This excitement reflects Seager’s success—she has not only recruited experts to the field, but also generated a devoted popular following. Once an outlier, she is now a figurehead: in 2013 she received a MacArthur genius grant of $625,000, given to people who “show exceptional merit and promise for continued and enhanced creative work”. She is deluged by letters and e-mails from high-school students, business leaders and even incarcerated felons. There is a growing appetite for planet-hunting and the mammoth questions it seeks to answer. Seager has her detractors, especially now the study of exoplanets has become just another science in the pecking order for NASA funding. She has been accused of being too ambitious in her projects and too optimistic in her pronouncements. But Geoff Marcy, professor of astronomy at Berkeley, who has discovered more exoplanets than anyone else, thinks that both Seager’s mission and her manner are essential. “Sara is on fire. The world loves her. She’s obviously brilliant but there’s more than that to her. She’s extremely honest, she’s insightful, she calls it like it is and people love that. They love the fact that she can think out of the box and she doesn’t mind saying what’s on her mind.”

In December Seager testified in front of a congressional committee on the possibility of finding life on other planets, drawing bipartisan support from the politicians. “It’s really motivating the public,” she says. “Finding planets and finding life gets people in a way few other things do. People need hope and exoplanets could be that. A symbol of hope.”

The search for exoplanets is partly an existential quest. Seager describes the burgeoning interest in planet hunting as “the awakening”. She dates this to 2009, when she and 30 other eminent scientists attended a workshop on astrobiology—the study of life in the universe—at no less a venue than the Vatican. “We did not talk about religion, but it was an eye-opening thing to me that people started really caring about exoplanets for reasons that are more serious than just science-fiction and science.”

Joy, hope, awakening: it’s a strangely spiritual vocab­ulary for a scientist to use, especially one with Seager’s upbringing. In her childhood in multi-cultural Toronto, a confrontation with her Hebrew teacher was formative. “I said, ‘I don’t understand why all my friends at school have a different religion. They don’t seem consistent.’ I pointed out a few examples, and he could not answer. So I became an atheist.” Yet exoplanets now seem to offer not just scientific verifiability to a question once left in the hands of priests, but also a glimmer of the same transcendence.

As much as exoplanets are a source of hope to many, they act as a solace for Seager. Her public zeal stems partly from a personal tragedy: in 2011, just as she had climbed to the top of her profession, her husband died of cancer, leaving her as a single parent of two young children. “I was so young, I had to ask myself, ‘What am I going to do now?’” Her answer was to embrace exoplanets ever more passionately. Now 42, she has become a tireless champion for her science, harrying, challenging and demanding more from colleagues than is sometimes politic. Her drive seems emotional as much as professional. She sees engaging people’s feelings as essential, not just because excitement among the public should translate into funding from government, but because exoplanets could offer a greater reward than science for science’s sake. “You could call it arrogance, but I strongly believe that this is something that will be hugely impactful.”

What would happen if we found an Earth 2.0, as Seager calls it? You can almost feel the planet-wide crick in the neck that would follow the discovery of an alternative Earth—for that is what a habitable planet even hundreds of light years away would represent: an alternative. Imagine the race to reach it, the new technologies that would be developed to try to get there. Imagine the optimism. Then picture the existential somersaults we would all have to go through if life were found there too, the thrilling, humbling discovery that we were no longer unique—or alone. Even if the life we find is no more than photosynthesising pond scum, even if it is found in just one place, its very existence would crack open our minds.

To conceive of such far-off possibilities, we must first look a little closer to home. As much as the science of exoplanets stares out into the galaxy, it also forces us to look inward: to understand the puzzle of existence, we must first look at ourselves. What is life? Would we recognise it if we saw it? We still don’t know how it began, and the only examples we have to go by are those found on Earth. Even here we don’t know the half of it. In caves and deep-sea trenches, over the past 40 years, scientists have uncovered a multitude of bizarre creatures called extremophiles: microbes that not only survive but thrive amid massive amounts of radiation, extremely acidic environments, or temperatures above 120˚C. Such exploration is broadening our sense of what life forms might exist out in space. Although, as the discovery of the first exoplanet showed, our preconceptions about what is out there are almost certain to be wrong.

The problem is that the Earth is not just a living space but a head space too. It’s near-impossible to dissociate ourselves from earthbound thinking, to get enough distance from our small blue dot for our ideas and imaginings to escape its gravity. Seager recognises that this terracentrism—the belief that life in the galaxy will be like life on Earth—raises problems. “We’re assuming life uses chemistry,” she says. “If there was life that used mechanical energy—think of a windmill—we wouldn’t see that. If it took air currents and ocean currents, and directly transferred energy, we’d have no way of finding that.” Even her optimism is tempered by the possibility of encountering life but not being able to recognise it. “We’re just doing what we can do with astronomy. If we don’t search, there’s no chance [of finding life at all]. But you’re right. It’s a problem we stress about.”

There are some scientists who wonder why Seager is looking for life at all. The proponents of the Rare Earth hypothesis claim that our planet is unique in the galaxy and the search for life elsewhere is a grand folly. It is compelling to read of the multitude of coincidences that made Earth the way it is, and it seems far-fetched that they could have been repeated elsewhere, but the Rare Earth idea rests on an assumption of the infallibility of our scientific knowledge at the moment, something the study of exoplanets has consistently challenged. As it stands, there is still no overriding theoretical reason to think that human beings are unique. Rare Earth scientists represent a fundamental pessimism, battling the innate optimism of Seager and her gang.

The significance of the search is powerful enough to keep Seager moving forward, looking for answers, solutions, new ways of thinking. She has absolute faith in her calling, in the promise of her revelation, and no doubt as to its historical importance. “I believe that we will be collectively remembered as the society who first found the Earth-like worlds.”

For all its talk of heaven above, Christianity is a fundamentally terracentric religion. Barring the odd passage advising that Moon-worshippers be stoned to death, there is little in either testament about other worlds or their denizens. Why, then, is the Catholic church at the forefront of the search for exoplanets? “I like to use the ‘Star Trek’ phrase,” says Father José Funes, an owlish Jesuit who is director of the Vatican Observatory. “The final frontier!” He is speaking in an austere strip-lit office at the University of Arizona, where the Vatican Observatory has a research outpost. “But I use it in two senses. We are going far in space, and we are on the front line between science, philosophy and theology.”

In March the Vatican hosted a conference in Tucson entitled “Search for Life Beyond the Solar System”. This was not a strained metaphor for the promise of eternal life in the firmament, but a gathering of scientists to discuss exoplanets and their possible inhabitants. The Vatican is not renowned for its enlightened attitude to science. In the 17th century it branded Galileo a heretic for declaring that the sun was at the centre of the solar system. In one of history’s most notorious clashes of faith and science, Galileo was forced to recant his discoveries and eke out his days under house arrest. Only in 1992 did the church express regret at its treatment of him.

Yet Funes sees the Vatican’s present interest in exoplanets as consistent, even complementary, with the church’s mission, particularly that of the Jesuits. “The science of exoplanets fits very well with our spirituality to go to the frontiers.” He propounds a different history of the church’s involvement with science, dating its interest in astronomy to the adoption in 1582 of the Gregorian calendar, based on the observations of a Jesuit astronomer. The Vatican founded its first observatory in 1774. Its current headquarters are in the papal summer residence at Castel Gandolfo in the hills above Rome, with the research group and state-of-the-art 1.8-metre telescope in Arizona. The Vatican funds the two stations and the 15 Jesuits who staff them, to the tune of €1m a year.

The Vatican Observatory studies what any other obser­vatory might, from near-Earth objects such as asteroids to the formation of galaxies. But the science of exoplanets most directly challenges the notion of human exceptionalism that lies at the heart of Christianity. Funes smiles when he hears this, as if he is asked it a lot. He replies with the words of a 19th-century Jesuit astronomer, Angelo Secchi: “Those immense regions must be inhabited by intelligent beings endowed with reason, able to know, love and honour the Creator; and perhaps these inhabitants of the stars are more faithful than us...”

Funes argues that even though a belief that the Earth travelled around the sun was deemed heretical for centuries, the belief in extra-terrestrial life was not. A 15th-century German philosopher, Nicholas of Cusa, declared that beings existed on the sun, the Moon and all the planets known at the time – and was made a cardinal. The search for life on other planets is “not in contradiction with our faith”, Funes insists, “because we cannot establish limits to God’s creative freedoms.” He agrees that the challenge is to show that science and religion are not enemies. “That they can live in harmony, even more [that] they can help each other. Just by our work, we are showing it is perfectly possible to be a scientist and a believer.”

As he talks, I see how closely the religious calling mimics the scientific one, and vice versa. Funes recalls how as a child, in Córdoba, Argentina, he was transfixed by images of the first men on the Moon. “I knew everything a kid of six can know about the Moon and the astronauts.” You wonder if, at times, the scientific urge might outstrip the religious one. The Vatican Observatory has traditionally spent most of its time studying “orphan science”, projects that don’t tend to get support because they are not attractive enough to funding agencies. Yet its decision to study exoplanets, a hugely popular science, seems well beyond this remit. Is it a sign that the possibility of finding life on other planets is more real than ever before? “We want to be involved in the big questions that other astronomers have,” Funes says. “Are we alone, are there other earths?”

He is aware that the answers may be more complex than the questions, but he is optimistic. “Would the possibility of life in the universe, or intelligent life, help us better understand the creation, the incarnation, redemption? This would be a good challenge for theology, no?”

“Now we hav e the tools, we’d be crazy not to look.” So says Jill Tarter, the public face of the search for extraterrestrial intelligence (SETI). For nearly 40 years she has been at the forefront of the quest to satisfy our belief that there is something out there. “It could turn out that we are unique. We just don’t know the answer.”

SETI is not so much a single institution as an umbrella for a range of attempts to search for intelligent life in the galaxy. It was born in 1959 when a scientific paper suggested that interstellar communication from superior extraterrestrial civilisations might be heard on Earth using radio telescopes pointed into space. The assumption was not only that life existed somewhere out there, but that the beings concerned had evolved to a state of technical sophistication more advanced than that on Earth. If an alien society was only as technologically sophisticated as we are now, we wouldn’t be able to hear them with our current equipment.

Such assumptions can leave SETI looking as if it operates on unempirical, quasi-religious, lines. Tarter refutes this: “After millennia of asking the priests and the philosophers what we should believe, what we’re trying to do now is use some tools to explore and see what is.” But if SETI shuns the idea of blind faith, its mission can seem partially sighted. While exoplanet scientists are now taking slow but steady steps towards finding the most basic building blocks of life, SETI is still going for the extraterrestrial jackpot: intelligent life or bust!

It is rocket science Preparing the Kepler spacecraft for launch

Tarter became something of a celebrity in 1985 when she was revealed as the inspiration for the main character in “Contact”, Carl Sagan’s bestselling novel about discovering alien civilisations. Unlike her alter ego, she has yet to meet an alien. Instead she has become the guiding spirit—and fundraising hub—of a scientific endeavour that often draws scorn. In the 1990s Congress withdrew SETI’s NASA funding, accusing it of “Martian-hunting” at the taxpayer’s expense.

One of Tarter’s favourite ways of describing SETI’s mission is to compare the known universe to the size of all the Earth’s seas. Of all those seas, she says, SETI has so far listened to the equivalent of a glassful. Such a Sisyphean task makes SETI’s telescopes seem about as practical a technology for finding aliens as church spires are for discovering God.

Despite the size of the task and the paucity of data, SETI follows a strict methodology in scanning the heavens for signals from other planets and has fashioned a rich theoretical framework to bolster it. It has created a formula to calculate the number of active communicative extraterrestrial civilisations in the galaxy (the Drake Equation), found a rough way to describe how advanced the aliens are (the Kardashev Scale), and hypothesised about the possible nature of alien technology (sun-hugging structures known as Dyson Spheres). But assumptions still abound. Where Sara Seager assumes life on other planets will be the result of chemistry, Tarter supposes intelligent life will be innately technological. It is an equally terracentric vision—and a hubristic one. Tarter keeps champagne on ice at the SETI Institute in Mountain View, California, ready to be popped should contact be made; a discovery protocol has already been worked out. Only the signal is missing. The scientists of SETI can seem like evangelical Christians planning for the end of days, its members longingly waiting for a sign to be beamed down to them; a Rapture in reverse.

But preposterousness and breakthroughs often go hand in hand—look at the study of exoplanets itself. Geoff Marcy, one of the first exo-hunters, faced scorn for years. Now he is a legend in the field, often touted as a likely winner of a Nobel prize. “There’s no credibility,” he says of seti. “It’s not a science until you make one detection.” Yet in 2012 he was elected to the first-ever endowed chair in extraterrestrial studies, at Berkeley. He doesn’t shrink from the problems of plausibility: “I think, in an ironic sense, it’s a reasonable pursuit for someone like me. I’ve already had more success than I deserve and now I can fall flat on my face. I have the luxury of failing.” Seager too has worked on SETI projects in the past. It’s odd to find ambitious scientists holding a lottery mentality—you can’t win if you don’t play. But when it comes to searching for life, the importance of the grand prize makes even the extremely long odds of success palatable. SETI seems at once ridiculous, absurd, unbelievable and necessary. The search for life on other planets is not just a quest for knowledge; it is a sort of scientific redemption. Why do we search the universe for life? Because it’s there.

Four years before the first exoplanet was discovered, eight men and women wearing red jumpsuits were sealed up inside Biosphere 2, a glass and steel megastructure of geodesic domes, Mesoamerican pyramids and vaulted aircraft hangers that squat like an alien starship in the middle of the Arizona scrub. They were to spend two years living in three acres of burgeoning rainforests and blossoming savannas, fog deserts and saltwater oceans, to explore the possibility of creating a self-sustaining environment that could be replicated on other planets. As in all the best science fiction, things did not go quite to plan. Six months later they were starving, suffocating and had split into warring factions. The interplanetary dream was extinguished. While attending the Vatican conference in Tucson earlier this year, Sara Seager and other scientists went to visit this replica world. It seemed a cautionary tale, a temple to false hopes and dreams.

Seager was readying herself to leave. She was about to lecture on exoplanets in Italy before returning to MIT, her many galactic enthusiasms and countless media requests. She was due to appear with the actor Sir Patrick Stewart—star of “Star Trek: The Next Generation”—at The Future is Here festival, hosted by the Smithsonian Institution in Washington, DC. (“I will be calling him Captain Picard,” she admitted.) The hard science, the constant cajoling, the endless evangelising seem to stretch on with no end in sight.

Will the public interest in exoplanets continue to grow if life on other planets remains stubbornly hidden? Can the huge distances between these far-off worlds ever be bridged? What if Seager spends her life proselytising for a science that fails to find life on other planets? At first she laughs off the question, “I remember the first time a journalist asked me ‘What happens if you don’t find what you’re looking for?’ I said I’d never thought about that! You can’t live your life thinking about that.” But what about the nuclear physicist Enrico Fermi, who famously asked, “if they exist, where are they?” What if the great revelation never comes? At this she grows more steely. “You have to understand one thing about exoplanets: and that is that people are going to look. Nothing is going to stop us looking. Nothing.”