The truth about life on other planets and what it means for Earth

Today, scientists have announced the detection of signs of a gas, dimethyl sulfide (DMS), on an exoplanet named K2-18b, a "Hycean" world hundreds of trillions of miles away orbiting a distant star. On Earth, DMS is predominantly produced by simple marine organisms, raising tantalizing questions about the biological activity on this distant, water-rich world. The lead scientist of the team behind this groundbreaking detection, Professor Nikku Madhusudhan of the Institute of Astronomy at Cambridge University, believes humanity is on the precipice of answering one of its most fundamental questions: "This is basically as big as it gets in terms of fundamental questions, and we may be on the verge of answering that question." This potential discovery, however, immediately sparks further profound inquiries: if life is indeed found on another world, how will this redefine our understanding of ourselves and our species?

Our ancestors have always been captivated by the heavens, weaving intricate narratives of beings that might inhabit the skies. In the early 20th Century, erroneous telescopic observations led astronomers to believe they saw straight-line features, or "canals," on the Martian surface. This fueled widespread speculation that our nearest planetary neighbor might host an advanced civilization, giving birth to a rich genre of pulp science fiction featuring flying saucers and the iconic "little green men." This era also coincided with Western governments’ anxieties about the spread of communism, which often saw extraterrestrial visitors portrayed as menacing invaders, bringing peril rather than hope, as famously depicted in H.G. Wells’ "The War of the Worlds."

Decades later, what has been described as "the strongest evidence yet" of life beyond Earth has emerged, not from the familiar landscapes of Mars or Venus, but from an exoplanet far beyond our solar system. The challenge of locating alien life lies precisely in knowing where to direct our gaze. For a long time, NASA’s primary focus in the search for extraterrestrial life was Mars, driven by its relative proximity and early hints of past water. However, this focus began to broaden significantly in 1992 with the initial discovery of exoplanets orbiting a pulsar, followed by the groundbreaking detection of 51 Pegasi b in 1995, the first exoplanet found orbiting a sun-like star. While astronomers had long theorized about other worlds beyond our solar system, these discoveries provided the first irrefutable proof. Since then, nearly 6,000 exoplanets have been confirmed, with thousands more awaiting confirmation.

Many of these distant worlds are gas giants, akin to Jupiter and Saturn in our own solar system, or they are situated in environments too extreme – either searingly hot or frigidly cold – to sustain liquid water, which is widely considered essential for life as we know it. Yet, a significant number of exoplanets reside within what astronomers term "The Goldilocks Zone," or habitable zone, where conditions are "just right" for liquid water to exist on the surface. Professor Madhusudhan estimates that there could be thousands of such potentially life-supporting worlds within our Milky Way galaxy alone, a mere fraction of the estimated 100 billion planets NASA suggests populate our galaxy.

As the pace of exoplanet discovery accelerated, scientists embarked on the breathtakingly ambitious endeavor of developing instruments capable of analyzing the chemical composition of their atmospheres. Their goal was audacious: to capture the minuscule amount of starlight that filters through the atmospheres of these faraway worlds during a transit, then dissect this light for the spectral "chemical fingerprints" of molecules. Specifically, they sought "biosignatures" – gases that, on Earth, are exclusively or predominantly produced by living organisms and are often out of thermodynamic equilibrium in the atmosphere, suggesting biological activity.

Remarkably, they succeeded. Instruments for both ground- and space-based telescopes were designed and deployed. The NASA James Webb Space Telescope (JWST), launched in 2021, represents the zenith of this technological ambition. As the most powerful space telescope ever constructed, its deployment ignited immense excitement that the elusive search for life was finally within humanity’s grasp. The JWST’s unparalleled infrared sensitivity and advanced spectrographs, like NIRSpec and MIRI, were instrumental in detecting DMS, along with methane and carbon dioxide, in K2-18b’s atmosphere, suggesting a dynamic, potentially water-rich "Hycean" environment.

However, even the JWST has its limitations. It struggles to detect exoplanets as small as Earth or those orbiting too closely to their parent stars due to the overwhelming glare. To overcome these hurdles, NASA is planning the Habitable Worlds Observatory (HWO), slated for the 2030s. This next-generation space telescope will employ a sophisticated coronagraph – essentially a high-tech internal sunshield – to block out the star’s light, enabling it to directly image and analyze the atmospheres of Earth-sized planets within their habitable zones. Concurrently, the European Southern Observatory (ESO)’s Extremely Large Telescope (ELT) is set to come online later this decade. Located in the pristine, crystal-clear skies of the Chilean desert, the ELT boasts a colossal 39-meter diameter mirror, making it the largest optical telescope ever built and offering unprecedented detail in planetary atmospheric analysis.

Despite these incredible technological leaps, Professor Madhusudhan anticipates that even with enough data within two years to categorically demonstrate the presence of biosignatures around K2-18b, it will not immediately lead to widespread celebrations. Instead, it will likely initiate a rigorous scientific debate, with researchers attempting to find plausible non-biological explanations for the detected biosignatures. The scientific method demands such scrutiny.

Yet, as more data accumulates from an increasing number of exoplanet atmospheres, and as chemists exhaust all alternative abiotic explanations for these biosignatures, the scientific consensus will gradually but inexorably shift. Professor Catherine Heymans, Scotland’s Astronomer Royal at Edinburgh University, believes this incremental accumulation of evidence will be key. "With more time on telescopes, astronomers will get a clearer vision of the chemical compositions of these atmospheres. You won’t know that it’s definitely life. But I think the more data that’s built up, and that if you see this in multiple different systems, not just this one particular planet, it gives us more confidence." This process might mirror the emergence of the World Wide Web – a series of incremental technological breakthroughs whose profound, transformative impact was not fully recognized until much later. Similarly, humanity may one day realize that the most enormous scientific, cultural, and social transformation in its history – the confirmation of other life in the Universe – dawned not with a sudden flash, but through a gradual, undeniable accumulation of evidence.

A much more definitive discovery, one that could significantly limit scientific pushback, would be the detection of life within our own solar system using robotic spacecraft equipped with portable laboratories. Any off-world microorganism could be directly analyzed, possibly even returned to Earth, providing irrefutable, prima facie evidence. The scientific case for the possibility of extant or past life in our solar system has strengthened considerably in recent years, fueled by data from various spacecraft, prompting several new missions.

The European Space Agency’s (ESA) ExoMars Rosalind Franklin rover, planned for launch in 2028, will drill deep beneath the Martian surface to search for signs of past and potentially present microbial life, where it would be shielded from harsh surface radiation. Given Mars’ extreme conditions, the discovery of fossilized past life is considered the more probable outcome. China’s Tianwen-3 mission, also targeting a 2028 launch, aims to collect Martian samples and return them to Earth by 2031 for detailed analysis. Meanwhile, NASA and ESA have spacecraft en route to the icy moons of Jupiter and Saturn, where vast subsurface oceans are believed to exist beneath their frozen crusts. NASA’s Europa Clipper and ESA’s JUICE mission are designed to investigate Europa and Ganymede, respectively, to determine their habitability potential, laying the groundwork for future missions that could directly search for life.

But the most exotic target might be Titan, Saturn’s largest moon. NASA’s Dragonfly mission, scheduled to launch in 2034, is a revolutionary rotorcraft designed to land and explore Titan’s surface. This alien world boasts lakes and rivers of liquid methane and ethane, a dense, nitrogen-rich atmosphere, and complex organic chemistry, giving it an eerie orange haze and evoking imagery from "Lucy in the Sky with Diamonds" with its "marmalade skies." Along with liquid water, these carbon-rich chemicals are considered essential ingredients for the genesis of life. Professor Michele Dougherty of Imperial College, London, a leading planetary scientist, expresses profound optimism about the prospects of finding life on these icy moons. "I’d be very surprised if there wasn’t," she states, beaming. "Three things are required: a heat source, liquid water and organic (carbon-based) chemicals. If we have those three ingredients, the chances that life is able to form rises really steeply."

The implications of discovering even simple life forms are immense, though it offers no guarantee that more complex or intelligent life forms are widespread. Professor Madhusudhan believes that, if confirmed, simple life should be "pretty common" across the galaxy. However, he cautions, "But going from that simple life to complex life is a big step, and that is an open question. How that step happens? What are the conditions that govern that? We don’t know that. And then going from there to intelligent life is another big step."

Dr. Robert Massey, Deputy Executive Director of the Royal Astronomical Society, concurs that the emergence of intelligent life is far less likely than simple life. "When we see the emergence of life on Earth, it was so complex. It took such a long time for multi-cellular life to emerge and then evolve into diverse life forms," he explains. "The big question is whether there was something about the Earth that made that evolution possible. Do we need exactly the same conditions, our size, our oceans and land masses for that to happen on other worlds or will that happen regardless?" Dr. Massey posits that the discovery of even simple alien life would represent the latest chapter in the gradual diminution of humanity’s perceived "specialness" in the cosmos, a historical progression from the Copernican revolution which displaced Earth from the center of the Universe.

Conversely, Professor Dougherty views such a discovery, particularly within our own solar system, as profoundly beneficial for both science and the human spirit. "The discovery of even simple life will allow us a better understanding about how we might have evolved way back those millions of aeons ago when we first evolved. And so, for me, it’s helping us find our place in the Universe," she asserts. "If we know there is life, elsewhere in our solar system and potentially beyond, [this] would somehow be comforting to me, knowing that we’re a fabric of something larger will make us bigger."

Never before have scientists dedicated such intense effort and utilized such incredible tools in the search for life beyond Earth. Many experts in the field believe it is no longer a question of if but when they will discover alien life. Far from instilling fear, Professor Madhusudhan believes this discovery will usher in an era of hope. "When we would look at the sky, we would see not just physical objects, stars and planets, we would see a living sky. The societal ramifications of that are immense. It will be a huge transformational change in the way we look at ourselves in the cosmic scene." He concludes with a powerful vision: "It will fundamentally change the human psyche in how we view ourselves and each other, and any barriers, linguistic, political, geographical, will dissolve, as we realize we are all one. And that will bring us closer. It will be another step in our evolution."