In the otherworldly Chilean desert, this huge telescope could reveal alien life

In Chile's Atacama Desert, the biggest telescope ever built is taking shape – and it could radically change our view of the Universe.

Over the past 200 years, telescopes have got ever-bigger – which has posed a conundrum about what to call them. Some are named after eminent astronomers. Others after their location. But many get their titles from their sheer size.

The astronomer William Herschel set the trend in the 18th Century with his Great Forty-Foot Telescope. And today, there's the Southern African Large Telescope, Spain's Gran Telescopio Canarias, and the Large Binocular Telescope in Arizona, to name a few.

But what do you call a telescope that's bigger than large? For the scientists and engineers of the Paranal observatory in Chile's Atacama desert, the answer is simple: a process you might call "nominative magnification".

In the 1990s, they needed a title for a particularly whopping telescope, which combined several individual instruments into one virtual mega-mirror. They scratched their heads for a while, and decided to name it the Very Large Telescope (VLT).

Now the same astronomers are going even more superlative, building a facility on a second Chilean mountain that will become Earth's biggest eye on the stars. Its name? Naturally, they're calling it the Extremely Large Telescope (ELT).

And they are not kidding about the size: the ELT's dome is 80m tall (260ft), and the mirror inside is 39m wide (128ft). With an outer structure due to be completed in 2027, the organisation behind it – a consortium of 16 member states called the European Southern Observatory (Eso) – hope it will be peering into the cosmos by 2030. It's proven to be an enormous engineering challenge too, requiring tens of thousands of tonnes of concrete and steel, and 798 precisely engineered mirror segments, to be shipped to the top of an uninhabited mountain in the desert.

Why build such a massive telescope in such a harsh and remote location? To find out, the BBC visited the ELT site in Chile, and discovered an extremely big telescope poised to make extremely big discoveries. In fact, it might even make the biggest discovery of them all.

From a distance, it's difficult to get a handle on the size of the ELT. Approaching its mountaintop perch on Cerro Armazones, it first appears as a dot of silver glistening beneath an empty blue sky. Apart from a few road signs and the occasional industrial mine, there is almost no imprint of human habitation in the Atacama Desert, so nothing to offer a sense of scale. All the construction materials must be transported for hours from the nearest city, piece by piece, and the engineers here work in shifts of eight long days and nights before travelling back to their homes, sometimes hundreds of miles away.

Look east and there is only boundless wilderness – from some places, you can see as far as Argentina – and to the west, the Pacific Ocean, topped by a sea of inverted cloud.

The Atacama is not a desert of sand, but of rough rock, volcanic peaks, and salt flats. In some locations, rain comes only a few times a century, or not at all. With so little moisture, a visitor finds themselves continually licking their dry lips. Dehydration is a real danger, the altitude makes some nauseous, and the thin, cloudless air means that Earth's nearest star will burn your skin faster than almost anywhere else on the planet.

Yet within this unforgiving setting, one of the world's most advanced telescopes is gradually assembling. To appreciate how complicated a project the ELT is, you have to step inside.

Within the retractable, rotating dome lies a complex lattice of steel beams and moving parts that will hold the giant mirror in place. After a while, the back of your neck begins to ache because you're so frequently gazing upwards. "It's a cathedral in terms of the size and scale," says Michael Booth, the on-site mechanical engineering lead for the ELT, who is accompanying the BBC on a tour of the site.

However, "extremely large" does not mean lumbering, or imprecise. On the contrary, assembling the telescope has required extensive planning and minute accuracy, says Booth. "It's too big and too complicated for us to say, 'I hope it works.' It's not like, 'let's hit this button and see if it makes any big noises,'" he says.

To illustrate his point, Booth points at removable, millimetre-wide "shim packs" between the giant beams. These are necessary for tiny structural adjustments as the telescope's massive concrete base shrinks over time. That concrete, when it was laid, needed to be extraordinarily flat, he says, to an accuracy of 1/3,600th of a degree, and to account for the Earth's curvature too. (When they were finished, the engineers used the excess wet concrete to make a basketball court on the mountainside, because it was too far to the nearest city to return it before it hardened.)

To gather light that has travelled from distant stars and galaxies, and to account for the atmosphere's blurring effects, astronomers need the telescope to be incredibly stable; the slightest wobble will lead to a fuzzy image.

"The thing that makes construction of a telescope like this so hard is that usually the bigger you get, the bigger your tolerances – like how much wiggle room you have. But for us, obviously, the light waves don't get bigger," explains Booth. "And so we have to build to the scale of a large commercial building but with the tolerances you find on an optical bench."

It's not a static structure, either. During operation, the entire building will rotate to face in different directions, suspended on high-pressure oil. "We float the whole telescope, so it can move basically frictionlessly," says Booth. "It's like an air hockey table." This puck, however, weighs many thousands of tonnes.

The ELT's record-breaking mirror, in the centre of the structure, will be assembled from 798 individual, removable hexagonal segments, delicately polished. But unlike your bathroom mirror, there's no protective glass to shield against scrapes or dents. "That's the hard part," says Booth. "Most of the time if something weighs 10 tonnes you can hit it with a hammer. We can't even scratch the surface of it."

The mirror segments, shipped from Europe in containers, are currently stored in a warehouse nearby:

But when they are placed inside the structure, they will join to form one big mirror that is 39m (128ft) wide:

No operating optical-infrared ground telescope currently has a mirror this huge. By comparison, the ELT's little brother – the Very Large Telescope, around 20 kilometres (12 miles) away – is made up of four telescopes each with mirrors of only 8.2m (27ft) diameter (plus four small 1.8m (6ft) diameter auxiliary telescopes). Each has to work together to combine their light.

The ELT mirror's size – and the fact that it can change shape in real-time to account for atmospheric turbulence – will allow astronomers to scrutinise many objects with higher resolution than currently possible. In other words, the mirror is a "bigger bucket to collect light", says Eso astronomer Mariya Lyubenova.

There's a limit to the sharpness of the images that the current generation of telescopes can create, even space-based telescopes like the James Webb Space Telescope (JWST), currently orbiting the Sun (which incidentally, has a mirror that's only 6.5m (21ft) wide.)

Elyar Sedaghati, an Eso staff astronomer at the Paranal observatory, explains why the ELT offers a step-change for astronomers. "I​magine if you have a car that's coming from very far away at night," he says. "The car has two headlights but when it's really far away you cannot distinguish them. You only see a single light. The closer it gets, at some point, you start to separate the two lights." It's the same with stars, but a giant mirror essentially brings such overlapping starlight closer to Earth, allowing astronomers to distinguish what's in the field of view.

Therefore, the ELT promises advances right across astronomy and physics. "It's just so multifaceted. It will really push the boundary and push the needle at every possible astronomical subfield that you can think of," says Sedaghati.

So for example, the ELT will follow up on discombobulating discoveries recently made by JWST about the distant, early Universe, by imaging very far, very old galaxies that have never been scrutinised before. "It will be really amazing when the ELT starts, because it has a higher angular resolution compared to the James Webb, so we can combine observations," says Sedaghati. A space telescope alone can show us a lot, but a space telescope and a ground telescope together make for a great team.

Closer to home, astronomers hope the ELT will be sensitive enough to help them calculate the ages of our own galaxy's oldest stars. The hope is also to peer closer at the stars revolving around the galactic centre, says Lyubenova, which could reveal deviations from current gravitational theories. And in our own neighbourhood, the telescope will be able to track objects like the interstellar comet 3I/ATLAS that passed through our Solar System recently.

But the biggest prize of all? It could well be the discovery that would change everything: evidence of life beyond Earth.

In the 21st Century, we have entered a new era in the search for aliens. Some observers have called it the "age of alien agnosticism": a time when a growing number of serious scientists strongly believe a basic form of extraterrestrial life is close to showing itself, but cannot yet prove it – their evidence accumulating piece-by-piece.

It was once assumed that the discovery of aliens would involve flying saucers, radio signals or Martian megastructures. But it's more likely that we'll spot a more basic form of life first, revealing itself slowly to telescopes. We've already had signs: a compelling recent example was the apparently organic chemicals detected on the exoplanet K2-18b: a tantalising hint of marine organisms. It could be just the beginning.

If you talk to serious astronomers about aliens, most are cautious, burnt by hyperbolic media claims, and slightly ticked off that you're not asking them about more sober activities like cataloguing galaxies. But they don't totally discount the possibility. In a recent survey of astrobiologists, physicists and other scientists, almost 90% agreed that it's likely that basic extraterrestrial life exists in the Universe. The numbers in the survey were lower for complex or intelligent life (around 60%), but they were also far from zero. The cosmos, after all, is a big place.

"As an astronomer that works in this field, you have to be very careful," says Sedaghati. He is doubtful of finding intelligent life within the same broad expanse of space and time as us. But something like microbes or algae? That's different. "If you think about the number of stars that are only in our own galaxy and the number of planets around that are around these stars, and the proportion of those planets that are in a zone where you could possibly have life… if you just look at the sheer numbers, the probability that life exists somewhere else in the galaxy it's, for me, extremely high. There must be life out there."

The sticking point is: what will count as strong evidence? And that's where the ELT comes in. It's not possible to image an exoplanet and see an alien waving back; it's not even possible to image landmasses. But we can study their habitability in other ways.

For example, astronomers can infer whether a planet is rocky or gaseous. To do this, "we observe very tiny deviations in the wobble of a star", explains Lyubenova. This reveals the otherwise invisible nearby planet's gravitational effect, which in turn allows a calculation of its mass. Gas giants tend to have higher mass than rocky planets. Crucially, the ELT will be able to measure the masses of smaller, lower mass worlds than currently possible, she says, significantly expanding the candidates for hosting alien life.

Astronomers can also analyse the constituents of exoplanet atmospheres, via the light spectrum of the starlight that occasionally passes through them on its way to Earth. "Each chemical element has its own imprint in the spectrum. It's like a fingerprint," says Lyubenova. So if an alien bacteria or some form of strange algae is belching out organic gases, it could show up.

At the moment, it's difficult to study exoplanet atmospheres in the habitable zone of other solar systems, because such worlds are too close to the star. Like when you're dazzled by a car headlight, you can't see who is at the wheel.

Claims about biosignatures on K2-18b in 2025 were controversial, because as well as disagreements about whether the detected gases are organic, the planet is also right at the edge of possible habitability. The ELT, however, could explore many more candidate worlds.

In short, within as little as a decade, it is here – deep in the Chilean desert – that we may finally gain the evidence that aliens are out there, posing existential questions about our position in the cosmos. The age of alien uncertainty would end: we would finally realise that we are no longer alone.

More like this:

• The hotel where astronomers live like James Bond spies

• The light disturbing a dark zone full of telescopes

• The spacewalk that saved Hubble

What comes after that is unknown. But already, astronomers have their eye on the next telescopes – which could be even bigger.

"We are already thinking about other ways of pushing the boundary," says Sedaghati. For example, there are ideas like building lots of telescopes, kilometres apart, and connecting them to make a mega-telescope. In principle, if the existing Very Large Telescope was connected to a new telescope on another mountain-top via an "optical bridge" it would significantly boost its abilities, says Sedaghati.

In the more distant future, people have talked about sending a space telescope beyond our Solar System's most distant planets so that we can use the Sun – and the fact its mass bends distant starlight – as a "gravitational lens". Essentially, the telescope would be the size of the Solar System. "This way you would actually not just image a planet around another star – an Earth-sized planet – but you would even be able to map the surface of it," says Sedaghati. "You would be able to tell more or less, where is oceans, where is land? These ideas are out there but it's just our technology needs to catch up."

It begs the question: what do you call these behemoths if "extremely large" is already taken?

A couple of decades ago, Eso was already considering whether to build the "Overwhelmingly Large Telescope". But a clue might come from an XKCD cartoon that jokes where they could go next: "oppressively colossal" is one option, or perhaps "mind-numbingly vast".

For now though, astronomers wait. They know that by the 2030s, a cathedral to human curiosity will finally be peering into the cosmos. What they don't know yet is what it will find.

--

If you liked this story, sign up for The Essential List newsletter – a handpicked selection of features, videos and can't-miss news, delivered to your inbox twice a week. 

For more science, technology, environment and health stories from the BBC, follow us on Facebook and Instagram.