LIFE IN THE UNIVERSE

From Mars to distant exoplanets, the search for life is entering a new era of discovery. Armed with powerful telescopes and fresh ideas, astronomers are closing in on one of science’s greatest questions. In this special report, Colin Stuart explores how we might finally detect signs of life beyond Earth.

Colin Stuart

COVER FEATURE

SPECIAL REPORT

In the darkness of space, 78 million kilometres from Earth, the European Space Agency’s Venus Express pivoted to look back towards home. The blue–green light of our planet slipped into the probe’s spectrometer instrument and was instantly split into its constituent colours. Buried within this spectrum were the fingerprints of Earth’s composition – including the invisible gases and molecules, such as life-giving oxygen and water, that hide within our atmosphere.

It has been almost 20 years since Venus Express made these landmark observations. At first, the endeavour may seem strange. Why scan our atmosphere from a neighbouring planet when we can measure it right here on Earth? Simple: from the lofty distance of Venus Express, our home planet appeared as no more than a single pixel. No surface details were visible; it was just a whisper of light. This replicates the way astronomers observe planets beyond our Solar System. They are too small, faint and far away to see in detail. Instead, the whispers are all we have.

Analysing the Venus Express observations proved that we could decode a living planet’s chemistry from afar. It also underscored the difficult job faced by those searching for life elsewhere in the Universe; Venus also shows water and oxygen in its atmosphere, yet it is a baking, acidic, high-pressure hellscape.

When it comes to searching for life, simply finding the correct atmospheric ingredients is no guarantee that the conditions are right for life itself.

“The biggest challenge is not just detecting a molecule, it is proving that it really comes from life,” Ji Wang, an astronomer at Ohio State University, told Astronomy Now . “Many molecules can also be produced by volcanoes or other geological processes,” he adds.

For all our great studies of the wider cosmos, Earth remains the only place we know for sure is home to living things. Despite the old scientific saying that you cannot draw conclusions from a statistic of one, when it comes to searching for life, Earth is all we have to go on. It is therefore the single touchstone to which astrobiologists must anchor their searches.

What we know is that an abundance of life is not a subtle presence. It reshapes and moulds a planet, screaming its existence light years into the void. So where else can we search for life? And how could we find evidence of alien biology?

For the first time in human history, we have the tools to look.

The early Earth

Our planet was born 4.56 billion years ago, when lumps of rock and metal coalesced around the newborn Sun. Initially, the Earth was a super-heated, molten world, still bubbling with the fires of its birth. It is no accident that geologists refer to this time as the Hadean era, after Hades, the god of the underworld. It was literally hell on Earth.

By the time our planet was around 600 million years old, this intense era was over. Life then appeared exceedingly quickly. There is undisputed evidence of microorganisms in the fossil record 3.5 billion years ago, when the planet was around 1 billion years old. Other researchers claim to see signs of life even earlier, perhaps even eking out an existence before the curtain fell on the Hadean. “The problem with studying the origin of life is that we cannot travel back in time,” Zita Martins, an astrobiologist at the Instituto Superior Técnico in Portugal, told Astronomy Now . “We haveto reconstruct what happened using geology, meteorites and laboratory experiments.”

Early life on Earth appears to have been simple, dominated by single-celled organisms, but billions of years brought monumental change. Microorganisms called cyanobacteria began to photosynthesise, the same chemical process that sees modern plants turn sunlight into sugar. This produced copious amounts of oxygen, filling the skies in what is now known as the Great Oxidation Event (GOE, see box).

“When microorganisms begin producing oxygen, it accumulates first in the oceans and then in the atmosphere,” says Martins. “At that point the chemistry of the entire planet starts to change.”

Any alien astronomers monitoring Earth’s atmosphere from a distance could have seen this abrupt shift. Earth was announcing itself to the wider Universe as a living planet. Microbes, plankton and, later, forests altered the chemistry of the air and oceans, leaving signatures that could be seen from afar. The GOE shows just how profoundly biology can lead to planetary- scale changes. It is on this basis that astronomers and astrobiologists are hopeful of being able to detect life across the yawning chasms of space – even if that life is just pondweed.

Transmission Spectrum of an Earth-like Atmosphere

Light blocked

More

Less

Ozone

Carbon dioxide

Water

Methane

Ozone

Carbon dioxide

Water

5,000

10,000 Wavelenth (nanometers)

15,000

20,000

Detecting Exoplanets: Techniques

Although some direct images exist, most alien planets are too small and faint to be photographed. Instead, astronomers have to tease out their presence from subtle clues.

A star may dim regularly when one of its planets passes between us (the transit method). The amount of dimming reveals the size of the planet. The gap between transits betrays the length of the planet’s year and so indirectly gives us its orbital distance and likely temperature. Planets also pull on their stars via gravity, causing the star to wobble slightly. This can show up as changes to the star’s position in the sky (astrometry) or as alterations to the spectral features hidden within a star’s light (radial velocity). The latter technique is so sensitive that it can detect changes in a star’s speed of just a few tens of centimetres per second.

Then there is gravitational microlensing. Sometimes a foreground star passes in front of a more distant background star, briefly magnifying its light. If the foreground star has a planet, astronomers see a second, smaller magnification.

Life on Mars?

Although Earth may be the only example we have so far, there are other places in the Solar System that can be used to stress-test our understanding of exobiology. Take our other planetary neighbour, Mars.

When the twin Viking landers touched down on Mars in the 1970s, they carried experiments capable of detecting life in the Martian soil. Some results were positive. The American engineer Gilbert Levin was the principal investigator on the Viking Labeled Release experiment. He went to his grave in 2021 convinced that his experiment’s positive result was at least worthy of direct follow-up investigation.

Most other scientists, however, decided that the results were probably the product of non- biological chemistry and changed tack to look for evidence of former habitable conditions rather than directly search for life on Mars.

There was another brief period of excitement in the 1990s when US President Bill Clinton announced that a meteorite from Mars found in Antarctica contained structures that looked like signs of ancient microbial life. However, later work showed that they could have been formed by non-biological processes, so the interpretation remains inconclusive.

In a nutshell, this underlines the challenge faced by astrobiologists: just because something looks as though it may have a biological origin, it might not. In the courtroom of science, the existence of life is hard to prove beyond reasonable doubt.

From today’s perspective, we can see that Viking 1 and 2 were pathfinders to a cavalcade of orbiters, landers and rovers sent to Mars. Together, they have opened a window into the planet’s past, which could be completely different from the Red Planet today – a dry, dusty and deserted wasteland.

“On the Martian surface you are dealing with daily temperature swings from roughly minus sixty degrees to plus twenty or thirty,” Stewart Gault, an astrobiologist at the University of Edinburgh, told Astronomy Now . “On top of that there is intense radiation, an extremely thin atmosphere and very unstable water.”

Combine all of those factors and modern Mars is clearly an incredibly hostile place for life. Billions of years ago, however, the planet was considerably warmer and wetter.

The Curiosity rover found rounded pebbles and thick layers of lakebed mudstone in Gale Crater, showing that persistent rivers and lakes may have lasted for millions of years. The Perseverance rover landed in what used to be a river delta feeding a crater lake. Earlier this year, Perseverance also found evidence of wave- shaped shoreline features, implying that the lake had open water with wave activity and not just transient flooding.

The conclusion is inescapable: at the time life was beginning on Earth, conditions on Mars were likely similar. So, could biology have gained a foothold there too? The ingredients certainly seem to have been present. In 2025, scientists detected thelargest organic molecules ever found on Mars. They are long-chain hydrocarbons, the type of molecules that make up the fatty acids found in cell membranes.

Although not evidence of life itself, this discovery shows that Mars had both the chemical building blocks for life and the conditions in which complex organic molecules could survive. However, the Martian soil also contains salts known as perchlorates, which may prevent these organic molecules from becoming living things. “These chemicals can destabilise proteins and other biological molecules, which makes it even harder for terrestrial-style life to survive on the surface,” says Gault.

If life did indeed get started on Mars billions of years ago, could it still be clinging on today?

Methane could provide a big clue. The Curiosity rover has seen seasonal spikes in methane levels around Gale Crater. Methane is short-lived. On Earth it survives in the atmosphere for only a few hundred years before sunlight and chemical reactions destroy it. That means something must currently be venting it into the Martian atmosphere to replace what is lost.

That something could be life. On Earth, life (particularly bacteria) is the biggest source of methane. But Martian methane could also have been produced by life billions of years ago, then been trapped inside ice and rock until now. Or it might not have a biological origin at all. On Earth, geological methane also escapes from deep inside our planet.

“If life does exist on Mars today, the most plausible place to find it is underground,” says Gault. “The surface is simply too harsh, whereas the subsurface offers more stable temperatures and protection from radiation.” So life on Mars remains an open question, but the debate is now skewed towards the past. It is far more likely that living things called Mars home billions of years ago than that they survive there today. Ultimately, the most decisive answers may come from samples returned to Earth, where laboratories far more sophisticated than any rover can analyse Martian rocks for signs of ancient life.

But there are other places around the Sun where life could be hiding now.

The Great Oxygenation Event (GOE)

Oxygen is one of the defining features of life on Earth today. We breathe it in, plants release it. And yet there was a time when oxygen was a minor component of the atmosphere. The Earth was changed forever by the Great Oxygenation Event, which occurred during the Paleoproterozoic era around 2.4 billion years ago.

Before this, although nitrogen was still the main constituent of the atmosphere, levels of carbon dioxide and methane were higher than today. The sky may have been a hazy orange and oxygen made up just 0.0001 per cent of the atmosphere.

Then microbes in the ocean called cyanobacteria began producing oxygen through photosynthesis. These microbes formed layered colonies called stromatolites, which are preserved in ancient rocks.

As oxygen accumulated in the atmosphere, it triggered a mass extinction of the existing microbial life forms. But it also led to a reduction in methane and the formation of the ozone layer. This new protective blanket shielded the surface from the Sun’s harmful ultraviolet radiation. It was a step that would ultimately allow life to become multicellular and move onto land.

Timeline of Life on Earth

4.56 BILLION YEARS AGO

Formation of the Earth Our planet forms from rocky, metallic debris left over from the birth of the Sun.

4.2 TO 3.9 BILLION YEARS AGO

LUCA: Last Universal Common Ancestor The most recent ancestor shared by all modern life, showing that life began quickly after Earth formed.

3.8 TO 3.4 BILLION YEARS AGO

The first photosynthesis Fossil evidence dates to 1.75 billion years ago, but the process likely began much earlier.

2.1 BILLION YEARS AGO

Multicellularity Fossilised tracks in Gabon suggest early multicellular organisms.

538–520 MILLION YEARS AGO

Cambrian explosion A rapid diversification of life, with many new body shapes appearing.

450 MILLION YEARS AGO

Terrestrial colonisation Life spreads onto land, aided by biological innovations such as lungs and eggs.

Icy moons

Astrobiologists often subscribe to a simple mantra: follow the water. Usually, finding liquid water means looking close to a star where there is enough heat to keep it from freezing. However, tidal heating offers a way to maintain liquid water far from the Sun.

Jupiter and Saturn, the two giant planets of our Solar System, are swarming with moons, hosting almost four hundred between them. When it comes to the search for life, two moons stand out: Jupiter’s Europa and Saturn’s Enceladus.

Both are covered in ice, but are thought to harbour subsurface oceans of liquid water. These moons are relentlessly stretched and squeezed by the gravity of their host planets, keeping their interiors warm enough to melt some of the ice.

The boundary between the warm, rocky seafloor and the water is particularly noteworthy. Many biologists suspect that life on Earth started on the ocean floor before later colonising the land. Hydrothermal vents known as ‘black smokers’ could have provided the chemistry and energy necessary to create biology.

If Europa has these vents too, perhaps life ignited there as well. However, a study earlier this year concluded that Europa’s ocean bottom may be geologically inactive.

Instead, Martins favours Enceladus. “Enceladus is particularly exciting because the tiger-stripe fractures near the south pole connect the subsurface ocean with the surface,” she says. “The geysers erupting from those cracks allow us to sample material from the ocean without drilling through kilometres of ice.”

Measurements of Enceladus’s plumes taken by NASA’s Cassini spacecraft have already revealed salts, silica nanograins and molecular hydrogen, all tantalising hints that water is interacting with warm rock deep below the ice. Martins is part of a team pressing for a dedicated Enceladus mission, but it could be decades away. In the meantime, our best chances of finding signs of life might lie beyond the Solar System, among the stars twinkling in the night sky.

Alien worlds

We have known for centuries that stars are distant versions of our own Sun. During that time, astronomers have long suspected that planets would exist around those stars too, but it took until 1995 to find the first world circling a Sun-like star. Astronomers call them exoplanets, and more than 6,000 have now been discovered.

However, when it comes to the search for life, not all planets are equal. Those highest on our hit list must have conditions suitable for liquid water. Although life could be possible without water, given that every living thing on Earth needs it, this suggests it is an excellent place to start.

Despite being abundant on Earth, liquid water is fickle. It needs the right temperature and pressure; otherwise it turns into ice or water vapour. For this reason, astronomers often talk of the habitable zone: the region around a star where conditions are suitable for liquid water.

Astronomers have found plenty of planets occupying this sweet spot – enough to estimate that around fifteen per cent of all stars in our Milky Way Galaxy host a habitable- zone planet.

However, habitable does not mean inhabited. It is a starting point for our searches, not the end point. Fortunately, there are ways to go further. One of the most important is transmission spectroscopy.

Spectroscopy was the technique used by Venus Express to unpick the contents of Earth’s atmosphere from afar. It involves splitting an object’s light into a spectrum, similar to the way a raindrop splits sunlight to create a rainbow. However, this colourful spectrum often has missing colours, which appear as dark bands. These gaps are the result of different elements and molecules absorbing specific wavelengths of light, leaving telltale signatures. These spectra look a lot like barcodes. In effect, that is what they are: scanning them reveals the constituents of the object.

When it comes to spectroscopy of exoplanets, astronomers take advantage of an event called a transit. This occurs when a planet passes directly between us and its star. The star’s light filters through the planet’s atmosphere; molecules in that atmosphere absorb specific wavelengths, and the altered light continues on towards Earth.

But this planetary signal is meagre compared to that of its star. “Imagine you are looking at the skyline of London at night, where all the skyscrapers are lit up. That is the star,” says Wang. “What we are trying to detect is a single light coming on in one room in one building and noticing that change.”

By comparing the star’s spectrum before and during the transit, astronomers can isolate the dark bands that belong only to the planet itself.

In this way, they can determine what makes up the planet’s atmosphere.

Like Venus Express’ chemical inventory of Earth, astronomers are looking for gases associated with life – so-called biosignatures. Examples include oxygen, ozone and methane.

In isolation, these markers may not betray biology. But if some of them are seen together, it might be a strong clue. “On Earth, the coexistence of oxygen and methane would be almost a smoking gun for life because, under normal chemistry, those two gases destroy each other,” says Wang.

Many of these absorption features lie beyond the visible part of the spectrum, particularly in the infrared. Traditionally, this has been a difficult part of the spectrum to observe from Earth because our own atmosphere absorbs the same wavelengths. This is why the James Webb Space Telescope has been a game-changer. Its great distance from Earth – more than four times farther away than the Moon – coupled with its giant 6.5-metre hexagonal mirror, means it can observe infrared light from exoplanetary systems unobstructed.

Webb has already produced the most detailed molecular spectra of exoplanet atmospheres ever obtained. Its highlights include the detection of water in the atmosphere of WASP-96b and the first definitive detection of carbon dioxide in an exoplanet’s atmosphere (WASP-39b).

But if there is one exoplanet that underscores the difficulties astronomers and astrobiologists still face, it is K2-18b.

Biosignatures Explained

Oxygen is highly reactive and does not remain in the atmosphere for long without being replenished. Yet Earth’s atmosphere maintains a steady level of around 21 per cent because life continually replaces it.

Finding oxygen – or methane – in the atmosphere of a distant exoplanet could hint at biological activity. These are known as biosignatures. While either gas could have a non-biological origin, detecting both together is considered one of the strongest indicators of life, as it is difficult to explain their coexistence through abiotic processes.

Plant life also reflects light strongly at the red end of the visible spectrum and into the near-infrared. A feature known as the ‘red edge’ could therefore signal vegetation.

Another possible signal is the ‘green edge’ associated with cyanobacteria, which reflect green light in shallow water or on rock surfaces.

However, such signals may be difficult to detect, requiring large-scale biological coverage and favourable observing conditions. They are therefore often considered secondary biosignatures.

Unfamiliar worlds

K2-18b sits in its star’s habitable zone, yet it is unlike anything in our own Solar System.

About 2.6 times wider than Earth (8.6 times as massive), it belongs to a class of planets known as sub-Neptunes – worlds that fall somewhere between rocky planets and gas giants.

So far, Webb has detected methane and carbon dioxide in K2-18b’s atmosphere, along with strong evidence for water vapour. But spectroscopy mostly reveals conditions in the upper atmosphere, not what lies deeper down. That is where the ambiguity begins.

Filling in the missing pieces requires detailed atmospheric and interior modelling, with different models producing very different portraits of the planet. One possibility is that K2-18b is a so-called Hycean world – a portmanteau of hydrogen and ocean. In this scenario, a global sea lies beneath a thick, hydrogen-rich atmosphere.

In such an environment, the immense atmospheric pressure could keep water liquid at temperatures that would normally cause it to boil away. Some researchers have suggested that microbial life might survive in these alien oceans, perhaps floating in the upper layers where light and nutrients are available.

That would place Hycean planets like K2-18b firmly on the astrobiology map. But the same spectral data can also be explained if the planet has no ocean at all. Remove the water, and the astrobiological allure fades.

Future missions

While Webb is currently at the cutting edge of our search for life elsewhere in the Universe, the astrobiology community is already planning the next missions.

Europe’s PLATO mission, due to launch later this year, will play a key role in discovering and characterising Earth-sized planets around nearby Sun-like stars, providing prime targets for future atmospheric studies.

Within the next year or so, we should also see the Nancy Grace Roman Space Telescope head to the same distant orbit as Webb. Roman will transform our ability to directly image exoplanets. Planets are often billions of times fainter than their stars, so photographing them is like trying to take a picture of a firefly in the glare of a lighthouse.

Roman will be equipped with an instrument called a coronagraph. This is effectively a metal disc that creates an artificial eclipse and blocks out the star’s light. The technology will allow astronomers to photograph worlds and dusty discs around nearby stars with a level of detail up to a thousand times better than current observatories.

However, Roman will only be able to do this for large, Jupiter-sized planets and not small, rocky worlds like our own. To probe smaller worlds, we will need the next generation of missions. Two of the most important are Blue Skies Space Ltd’s Twinkle, currently slated for launch in 2027, and ESA’s Ariel, due in 2031. They will be able to probe the atmospheres of rocky planets larger than Earth, known as super-Earths.

“Due to their size, the signal we detect from super-Earths is stronger [than from Earth-sized worlds],” says Wang. “In many ways, they are the easiest planets for us to study when searching for atmospheric biosignatures.”

But if we want to probe truly Earth-sized planets, we will have to wait until the first half of the 2040s and the launch of NASA’s Habitable Worlds Observatory (HWO). The mission builds on decades of design studies for earlier concepts such as HabEx, LUVOIR, and the proposed Darwin and Terrestrial Planet Finder missions. Its goal is simple: to directly image Earth-like exoplanets and examine them for signs of life.

The aim is to probe 25 of the most promising habitable-zone planets, performing highly detailed analyses of their atmospheres and potential biosignatures. We have come a long way since the first planet around a Sun-like star was discovered just over 30 years ago. We are approaching the point where planetary biology becomes an observable science. For the first time, the question of whether we are alone in the Universe feels less like philosophy and more like something astronomy may soon be able to test.

Colin Stuart is an astronomy author and speaker. Join his free astronomy club at colinstuart.substack.com.