Hunt for life
Astronomers have long asked whether or not we are alone, and now we’ve entered an exciting new era
The new techniques and telescopes looking to uncover life elsewhere in the universe
For thousands of years humans have speculated that the cosmos is teeming with planets, many of which could support life. Our questioning has tapped into a long-held desire to know our place in the universe – a core human yearning which has preoccupied some of history’s greatest minds. But speculation is about as far as humans got until we invented telescopes and developed a proper understanding of the scientific method a few centuries ago. Now scientists are making considerable progress in the search for alien life, and the past decade has proven pivotal. Some big discoveries may be coming soon, but where has the hunt for life taken us, and where is it heading?
One of the first modern searches for life took place in August 1924, when astronomer David
Peck Todd and an inventor called Charles Jenkins wanted to listen for messages from Mars. They asked the US Army and Navy to turn off their stations so they could use their radio-photo message machine to carry out a search. Alas, they drew a blank. In 1960, however, the search for extraterrestrial intelligence (SETI) intensified when Cornell University astronomer Frank Drake used a radio telescope in West Virginia to listen for interstellar radio waves coming from the stars Tau Ceti and Epsilon Eridani. Called Project Ozma, this effort incorporated ideas from a seminal 1959 paper by Giuseppe Cocconi and Philip Morrison. But again it detected no recognisable signals.
Undeterred, scientists have been scanning the heavens for technosignatures ever since. Initially they focused almost exclusively on radio signals, but flashes of light are now being sought too.
These are the targets of increasingly common ‘optical SETI’ efforts. But then SETI scientists have to keep an open mind. After all, we don’t know what sorts of messages advanced alien civilisations might beam out. It means astronomers in the field are generally looking for signals that appear weird and artificial. They search for something coming from deep space that isn’t produced by any known natural astrophysical phenomenon.
An identified signal would ideally recur so it can be studied repeatedly and in detail. One-offs can remain forever mysterious, as 1977’s famous Wow! signal shows. In that case, a radio dish operated by Ohio State University picked up something so intriguing that astronomer Jerry Ehman wrote ‘Wow!’ on the data printout. Researchers scoured that same patch of sky again and again, hoping to get another ping, but they never did.
What’s perhaps more frustrating is that SETI has historically been a shoestring operation, and finding enough money to keep the telescopes running has been a consistent problem. US Congress axed a planned NASA-SETI project in 1993, with Senator Richard Bryan saying: “This hopefully will be the end of Martian hunting season at the taxpayer’s expense.”
ET hunters have mostly had to turn to the private sector for cash, and without steady funding, progress became slow for several years. Things changed recently when private money began to flow more freely into SETI research. Most of it comes from one man – tech billionaire Yuri Milner – but his passion for the search for alien life prompted him to establish an ambitious program called Breakthrough Initiatives in 2015.
Projects under the Breakthrough umbrella include the $100 million (£73 million) Breakthrough Listen SETI campaign and the $100 million (£73 million) Breakthrough Starshot, which aims to develop the technology required to send tiny robotic probes to nearby exoplanet systems at about 20 per cent the speed of light. There’s also Breakthrough Message, which aims to help humanity craft the best possible message to send out into the cosmos and encourages debate and conversation about SETI in general.
Even so, there is considerable debate within the scientific community about SETI. Some people, including the late physicist Stephen Hawking, have argued that it’s unwise to advertise our presence to aliens, whose nature and intent are complete mysteries to us. “We should be wary of answering back,” he said. “Meeting an advanced civilisation could be like Native Americans encountering Columbus. That didn’t turn out so well.”
Other researchers think that any creatures advanced enough to travel to Earth to enslave or eat us would already know we’re here. Just to be on the safe side, Breakthrough Message pledges not to actually broadcast any SETI signals until this
debate has played itself out – though humanity has already beamed out messages on multiple occasions, most famously in 1974 with the Arecibo message. Humanity is also leaking radio signals in all directions at all times, providing cosmic bread crumbs for anyone close enough to find them. Around the same time that SETI was getting off the ground, planetary scientists began getting their first good look at alien worlds.
In 1965, Mariner 4 flew by Mars, returning the first close-up images of the Red Planet. Those photos revealed a dry, heavily cratered and seemingly desolate world, forcing many scientists to recalibrate previously optimistic notions of
Mars’ habitability. Hopes of a life-supporting Mars had been famously stoked around the turn of the 19th century by astronomer Percival Lowell, who claimed that channels on the planet were actually canals built by intelligent creatures.
But the optimists got some good news in 1971 after Mariner 9 arrived in orbit around Mars, becoming the first spacecraft to circle another planet. This intrepid probe spotted river channels and other evidence of past liquid-water activity on the Martian surface. These discoveries helped spur NASA to develop two ambitious life-hunting Mars missions, Viking 1 and 2, which launched a few weeks apart in 1975.
The twin Viking landers each carried four biology experiments which hunted for signs of microbial life in the red dirt. One of those, called the Labeled Release (LR) experiment, returned data consistent with evidence of microbial life. Indeed, LR principal investigator Gil Levin argued that the Vikings found evidence of Mars life. However, most scientists who studied the data disagreed with Levin, determining that the data could be explained by abiotic chemical reactions.
The Viking results taught NASA some valuable lessons – chiefly that we didn’t know enough about Mars to mount a proper life hunt there. The space agency eventually embarked on a long-term ‘follow the water’ exploration strategy, seeking to learn more about ancient environmental conditions on the Red Planet and how they changed over time.
This strategy gave us many prominent Mars missions over the past few decades, including
Mars Odyssey, the Mars Reconnaissance Orbiter, Mars Atmosphere and Volatile Evolution (MAVEN), the rovers Spirit, Opportunity, Curiosity and Perseverance and the Phoenix lander. These robotic explorers did their jobs well, finding lots of evidence that ancient Mars was quite wet, and helping scientists better understand why, how and when the Red Planet transitioned to the frigid desert world it is today. Curiosity has taken this work the furthest, finding that its landing site, the 154-kilometre (96-mile) Gale crater, hosted a longlived lake-and-stream system billions of years ago that could have supported Earth-like life.
Meanwhile, other scientists have continued the hunt for Mars life, focusing on aliens that may have fallen fortuitously to Earth. Billions of Red Planet rocks have made their way here from Mars after being blasted into space by powerful asteroid or comet impacts. A lot of Earth material has ended up on Mars as well, but the ledger is decidedly unbalanced – the Sun’s powerful gravity pulls more stuff inward, towards Earth. This extensive rockswapping has led some scientists to postulate that life actually arose first on Mars, then made its way to Earth later.
In 1996 researchers announced they’d found potential signs of life in one such Mars meteorite, known as Allan Hills 84001 (ALH84001). It was a very big deal. The result was published in the prestigious journal Science, and President Bill Clinton held a press conference about the news on the White House lawn. But the ALH84001
story ended up going down a Viking path. Other scientists picked at the claim, and a consensus emerged that the meteorite evidence was ambiguous at best. But like Levin, the team held firm in its findings, and continues to do so today.
NASA and the broader exploration community weren’t focused solely on Mars for all these years, however. The Cassini-Huygens mission, which ended in September 2017, transformed scientists’ understanding of the Saturn system and our Solar System’s potential to host alien life. That mission found that Titan, Saturn’s largest moon, has a hydrocarbon-based weather system and that the frigid moon’s surface harbours lakes and seas of liquid ethane and methane. Life could swim around in these seas, though it would have to be very different from the life we know here on Earth.
Cassini also spotted geysers blasting from the south pole of another Saturn moon, the ice-covered
“Meeting an advanced civilisation could be like Native Americans encountering Columbus” Stephen Hawking
Enceladus. This discovery, among other Cassini observations, revealed that Enceladus harbours a big ocean of salty liquid water beneath its shell.
These geysers blast up huge plumes of water ice and other material in clouds so substantial that they create Saturn’s E ring. Cassini flew through one such plume on multiple occasions, gathering samples that scientists analysed for clues about the moon’s subsurface environment.
The researchers found carbon-containing organic compounds and free hydrogen, the latter of which suggests the existence of a hydrothermal system in Enceladus’ buried ocean. Undersea hydrothermal vents are one popularly invoked environment for the origin of life on Earth. But Cassini didn’t look for signs of life in plume material; the spacecraft wasn’t equipped to do so because nobody knew about the plumes before the mission launched. Now scientists have come to realise that buried oceans are relatively common in the outer Solar System. “There are more oceans in the universe than previously thought, making the existence of extraterrestrial life more plausible,” says Shunichi Kamata of Hokkaido University in Japan.
Multiple ice-covered Jupiter moons seem to have these oceans: Ganymede, Callisto and, most intriguingly, Europa. Europa’s huge subsurface sea seems to be in contact with the moon’s rocky core, like the ocean of Enceladus is, making possible a range of complex chemical reactions that could theoretically have led to life. Scientists think the oceans of Ganymede and Callisto are more boring, sandwiched between layers of ice. Titan seems to have a buried ocean of salty water as well, meaning the moon likely has two very different potentially habitable environments. Observations by NASA’s New Horizons spacecraft indicate that liquid water may slosh beneath Pluto’s surface, too.
And the list goes on. The abundance of water worlds in the outer Solar System suggests that looking for ‘Earth 2.0’ may not be the best lifehunting strategy after all – most of the habitable real estate in the cosmos may be buried under ice. And these revelations about our celestial backyard have come in parallel with big news about the cosmos at large.
Over the past decade or so, we’ve learned that our Milky Way galaxy is teeming with potentially life-supporting worlds. Much of this knowledge comes courtesy of NASA’s pioneering Kepler space telescope, which operated from 2009 through to November 2018. Kepler is responsible for nearly two-thirds of the 4,400 confirmed exoplanet discoveries to date, and mission data reveals that planets outnumber stars in our galaxy. Many of those planets might bear more than a passing resemblance to Earth. Kepler found that at least 20 per cent of Milky Way stars probably host rocky planets in their habitable zones, the just-right range of orbital distances where liquid water can persist on a world’s surface.
Some of these potentially habitable worlds are just a stone’s throw away in the cosmic scheme of things. For example, the nearest star to the Sun – Proxima Centauri, which is about 4.2 light years away from us – hosts a roughly Earth-sized planet in the habitable zone. This world, called Proxima b, is a prime Breakthrough Starshot target. And the TRAPPIST-1 system, which lies 39 light years from us, boasts seven rocky worlds, three of which may be capable of supporting life as we know it. But both Proxima Centauri and TRAPPIST-1 are red dwarfs, like 70 per cent of the Milky Way’s stellar population. Red dwarfs are small but very active stars, and their intense flaring may severely dampen planets’ habitability.
Kepler’s legacy is being carried on by other exoplanet missions, such as NASA’s Transiting Exoplanet Survey Satellite (TESS), which is expected to find thousands of alien worlds circling nearby stars, and the European Space Agency’s (ESA) CHEOPS (CHaracterising ExOPlanet Satellite), which aims to characterise some of these neighbouring worlds. The avalanche of exoplanet discoveries, as well as finds much closer to home, have brought astrobiology from the scientific fringe firmly into the mainstream. NASA is openly prioritising the search for alien life these days, as some current and coming missions show.
In July 2020 the space agency launched the Perseverance rover, which landed in February 2021 to hunt for signs of ancient Mars life and collect samples for eventual return to Earth. Finding evidence of long-dead microbes is expected to be a very tricky task, one ideally carried out by teams of scientists in well-equipped labs studying pristine pieces of Mars specifically selected for their lifepreserving potential. The ESA planned to launch its own life-hunting Mars rover, called Rosalind Franklin, in July 2020 as well, but technical issues pushed the launch back to the next window – it’s now scheduled for autumn 2022.
In 2024 NASA’s Europa Clipper is scheduled to launch towards the Jupiter system. Clipper will orbit the gas giant but make dozens of flybys of Europa, characterising the moon’s subsurface ocean and scouting out good touchdown sites for a future life-hunting lander, among other tasks. And in 2027 NASA plans to launch Dragonfly, a probe that will fly through Titan’s thick, smoggy
skies. Dragonfly’s main goals involve investigating the complex chemistry that could set the stage for life’s emergence and assessing Titan’s habitability, but the rotorcraft will also search for biosignatures.
The agency will soon start hunting for aliens much farther afield, too. NASA’s $9.7 billion (£7.1 billion) James Webb Space Telescope, the oftdelayed successor to the iconic Hubble Space Telescope, is scheduled to launch in November 2021. One of the many things the powerful new telescope will do once aloft is probe the atmospheres of nearby exoplanets for potential biosignatures – gases such as oxygen and methane, whose simultaneous presence in a world’s air would provide a strong case for life.
Three highly anticipated megascopes will begin doing similar work from the ground in the next decade if all goes according to plan. The Giant Magellan Telescope (GMT) and the Extremely Large Telescope (ELT) will do their observing from the mountains of Chile, whereas the Thirty Meter Telescope (TMT) will sit atop Hawaii’s Mauna
Kea volcano – if the telescope team and the local community can come to an agreement.
SETI activities may ramp up considerably soon, too, and not just because of Breakthrough Listen. The biggest radio telescope ever built, China’s Five-hundred-meter Aperture Spherical Telescope (FAST), nicknamed Tianyan, came fully online in early 2020, and searching for technosignatures is one of its many charges.
This is just a partial list of the coming lifehunting activities and technologies. The full list may eventually become gloriously ungainly thanks to the continuing drop in the cost of building and launching spacecraft. This trend could eventually make astrobiology missions feasible for a variety of interested parties, from university groups to private citizens. Indeed, Milner has already mused about launching a lifehunting mission to Enceladus or Europa.
Some of this alien searching will continue to occur in Earth-based studies, and it won’t just involve inspection of Mars meteorites. There’s an ongoing search for a ‘shadow biosphere’ on our planet – an entire tree of life separate from the one that includes bacteria, bats, birds and everything else we currently recognise as alive.
This peculiar pursuit isn’t so crazy if you think about it. After all, life appeared on Earth about 4 billion years ago – very quickly considering that our planet formed just 4.5 billion years ago and
remained hot and inhospitable for a long time thereafter. Life’s emergence really doesn’t seem that miraculous, which in turn implies that it could have happened here more than once. Given the incredible abundance of potentially habitable real estate – and that’s just for Earth-like life, saying nothing of the environments that could support ‘strange life’ of various types – why haven’t we found extraterrestrials yet?
Nobel Prize-winning physicist Enrico Fermi famously posed this question in 1950, specifically referring to intelligent aliens. Seven decades later, the answer to the so-called Fermi paradox remains elusive. ‘Answers’ is probably a better formulation, however, because multiple factors are most likely working together to keep us from finding intelligent aliens. Among the foremost is the vastness of space, which makes it difficult for two civilisations to touch base. Consider that Proxima b is just 4.2 light years away in a galaxy 100,000 light years wide. But 4.2 light years is an expanse that would take humanity’s current spacecraft tens of thousands of years to cross.
Contact with intelligent aliens would require temporal and temperamental alignments as well: their civilisation would have to rise in sync with ours, no mean feat in a universe that’s 13.8 billion years old. And ET would have to want to reach out – that’s no given, either. There are many reasons why some aliens may want to keep quiet, as the pessimists have pointed out. Or maybe intelligence is rare throughout the cosmos, even if life isn’t. Planet Earth has been inhabited for about 4 billion years, after all, but we’ve only been sending out radio waves for just a century or so and launching spacecraft since 1957. And it’s very tough to find faraway microbes, which presumably have not yet invented the radio.
Our technological youth may be the biggest factor of all: we’ve only just begun the search for our cosmic neighbours. And that search has been halting and haphazard, conducted by small teams of dedicated researchers who have had to scrounge money to keep the lights on. But that’s changing, as the exciting new missions and instruments currently in development show. We may start getting some answers very soon.