James Webb May Have Found Alien Life — 99.7% Signal Explained | Neil deGrasse Tyson

Added: 2026-05-12

[00:00:00]Ladies and gentlemen, imagine for a moment that you are holding your phone scrolling through the news when suddenly you see a headline that makes your heart skip a beat. The James Webb Space Telescope may have detected signs of alien life. Not maybe, not possibly, but with a staggering 99.7% confidence level. Now, I know what you're thinking. You've heard these stories before, right? But this time something is different. This time we're not talking about ambiguous signals or maybes buried in data. We're talking about a specific molecule floating in the atmosphere of a world 120 light years away. A molecule that on Earth is only produced by living organisms. Take a deep breath because what I'm about to share with you in the next few minutes could fundamentally change how we see our place in this vast, mysterious cosmos. This isn't science fiction anymore. This is science fact unfolding before our very eyes and you're here at exactly the right moment to understand what it all means. So, silence your phones, lean in close because we're about to journey to a planet called K2-18b and what we found there might just answer humanity's oldest question, are we alone? The molecule at the center of this astronomical sensation is called dimethyl sulfide or DMS for short. Now, before your eyes glaze over at the chemistry terminology, let me tell you why this particular molecule has scientists barely able to contain their excitement. On Earth, DMS is exclusively produced by biological processes, primarily by phytoplankton in our oceans. These microscopic marine organisms release DMS as a byproduct of their metabolism. We've never found a non-biological geological process that creates this compound naturally. Think about that for a second. Every single time we've encountered DMS on our planet, life has been the source. The James Webb Space Telescope detected spectral signatures consistent with DM in the atmosphere of an exoplanet designated Leo, approximately 120 light years from Earth. When starlight passes through this distant planet's atmosphere on its way to our telescopes, different molecules absorb specific wavelengths of light, creating a unique fingerprint.

[00:02:02]The fingerprint Webb detected matches dimethyl sulfide. This isn't like finding methane, which can be produced by volcanoes or other geological activity. This is finding a molecule that, based on everything we understand about chemistry and biology, screams biological origin. The detection has been analyzed, peer-reviewed, and confirmed with that remarkable 99.7% confidence level, meaning there's only a 0.3% chance this is instrumental error or misidentification.

[00:02:28]Let's talk about what that 99.7% confidence level actually means, because numbers can be deceiving, and we need to understand the scientific rigor behind this claim.

[00:02:39]In scientific research, particularly in astronomy and physics, we use something called sigma levels to express confidence. A 99.7% confidence corresponds to a three-sigma detection. To put this in perspective, in particle physics, scientists typically require a five-sigma confidence level, 99.999943%, before declaring a definitive discovery.

[00:03:04]That's the standard that was used when announcing the Higgs boson, for example.

[00:03:08]So, why are scientists so excited about a three-sigma detection? Because in astronomy, especially when observing exoplanet atmospheres, three-sigma is remarkably strong. We're looking at incredibly faint signals. Light that has traveled for over a century through the void of space, passed through an alien atmosphere thinner than gossamer, and then been captured by our instruments.

[00:03:29]The fact that the signal is this strong, this clear, is extraordinary. However, and this is crucial, that 99.7% confidence doesn't mean there's a 99.7% chance of alien life.

[00:03:43]It means we're 99.7% confident we're actually detecting DMS and not seeing a false positive or instrument artifact.

[00:03:54]The interpretation of what that DMS means is a separate question entirely, one that requires additional observations, alternative explanations to be ruled out, and corroborating evidence from multiple independent measurements.

[00:04:09]Itself is a fascinating world that deserves our attention because it's unlike anything in our own solar system.

[00:04:16]This planet is classified as a sub-Neptune, meaning it's larger than Earth but smaller than Neptune with a radius about 2.6 times that of our home world and a mass roughly 8.6 times Earth's mass.

[00:04:31]It orbits a red dwarf star called K2-18, completing one orbit every 33 days.

[00:04:40]Here's where it gets truly interesting.

[00:04:42]K2-18b sits squarely in its star's habitable zone, that Goldilocks region where temperatures allow liquid water to exist on a planet's surface.

[00:04:52]But scientists believe K2-18b might be what's called a Hycean world, a relatively new category of potentially habitable planet.

[00:05:01]The term Hycean combines hydrogen and ocean, describing a planet with a hydrogen-rich atmosphere overlying a massive liquid water ocean. Imagine an ocean world possibly covered entirely in water hundreds of kilometers deep beneath a thick hydrogen atmosphere. The temperatures and pressures on such a world would be extreme by Earth standards, yet they might still permit liquid water and crucially might still allow for life, albeit life very different from what we know.

[00:05:32]The existence of Hycean worlds dramatically expands the number of potentially habitable planets in our galaxy because sub-Neptunes are actually the most common type of exoplanet we've discovered.

[00:05:44]If life can exist on Hycean worlds, the universe might be far more populated than we ever imagined.

[00:05:52]Understanding biosignature gases is fundamental to appreciating the magnitude of this discovery.

[00:05:59]A biosignature is any substance, element, molecule, or phenomenon that provides scientific evidence of past or present life. When we're searching for life on distant exoplanets, we can't send rovers or land probes, at least not with our current technology.

[00:06:15]Instead, we must rely on remote sensing, analyzing the light from these worlds to detect the chemical composition of their atmospheres.

[00:06:25]Scientists have identified several gases that could indicate biological activity.

[00:06:30]Oxygen combined with methane creates a chemical disequilibrium that's hard to explain without life. Phosphine at sufficient concentrations might indicate biological processes, and dimethyl sulfide, our star molecule, is considered one of the most robust biosignatures because we have no known non-biological production pathway.

[00:06:50]However, the search for biosignatures is complicated by the possibility of false positives. We must be absolutely certain that what we're seeing can't be explained by geology, photochemistry, or other non-biological processes. This is why the discovery on K2-18b is being treated with both excitement and caution.

[00:07:13]Alongside the potential DMS detection, Webb also found methane and carbon dioxide in K2-18b's atmosphere, along with a notable depletion of ammonia.

[00:07:23]This combination is actually consistent with a Hycean world harboring a liquid water ocean beneath its hydrogen atmosphere. The multiple lines of evidence, the chemical environment, the presence of water-related molecules, and now potentially DMS, create a compelling picture, but one that requires extraordinary evidence before we can make extraordinary claims.

[00:07:47]The James Webb Space Telescope represents a quantum leap in our ability to study the universe, and understanding its capabilities helps us appreciate why this discovery is only possible now.

[00:07:58]Launched on Christmas Day 2021 and positioned at a point in space called L2, about 1.5 million kilometers from Earth, Web is the most powerful space telescope ever constructed. Its primary mirror is 6.5 m across, made of 18 hexagonal segments coated in gold, giving it about six times the light-collecting area of its predecessor, the Hubble Space Telescope.

[00:08:25]But size isn't Web's only advantage. It observes primarily in the infrared spectrum, wavelengths of light longer than what our eyes can see.

[00:08:35]This is crucial for exoplanet science because molecules in planetary atmospheres have distinctive absorption features in the infrared.

[00:08:43]When an exoplanet transits in front of its star from our perspective, a tiny fraction of the star's light filters through the planet's atmosphere before reaching us.

[00:08:53]Web's instruments can split this light into its component wavelengths, creating a spectrum that reveals the chemical composition of that distant atmosphere.

[00:09:03]The precision required is mind-boggling.

[00:09:05]We're measuring changes in brightness of sometimes less than 1% looking for absorption features that might represent only a few parts per million of atmospheric composition in light that left its source over a century ago.

[00:09:20]Web's ability to do this is why we're now having conversations about biosignatures on exoplanets that would have been impossible just a few years ago.

[00:09:31]This potential detection fundamentally changes the trajectory of astrobiology and our search for life beyond Earth, but it also raises profound questions about what comes next.

[00:09:44]If the DMS detection on K2-18b is confirmed through additional observations and we can rule out alternative non-biological explanations, we would have our first genuine evidence that life exists beyond Earth.

[00:09:59]Not microbial fossils on Mars, not amino acids on asteroids, but active living metabolism-driven on another world orbiting another star.

[00:10:11]The implications are staggering, touching everything from philosophy and theology to planetary science and biochemistry. How did life arise on K2-18b?

[00:10:21]Did it originate there independently, suggesting life emerges readily when conditions are right, or could panspermia have seeded multiple worlds?

[00:10:31]What kind of organisms might produce DMS in such an alien environment? In an ocean potentially beneath a crushing hydrogen atmosphere? The immediate scientific priority is confirmation.

[00:10:45]Webb will observe K2-18b again, as will other telescopes. Scientists will work to model whether any non-biological processes could produce the observed DMS signal. They'll look for additional biosignatures and study the overall atmospheric chemistry in greater detail.

[00:11:02]They'll also turn their attention to other Hycean candidates, other sub-Neptunes in habitable zones, to see if DMS is common or rare.

[00:11:11]Beyond the science, this discovery, if confirmed, represents a pivotal moment in human history, the moment we learn that life has emerged elsewhere, that the universe is not sterile, that we are part of a cosmos teeming with biology.

[00:11:24]It's a humbling and exhilarating prospect.

[00:11:28]Thank you so much for taking this incredible journey with me today through the cosmos and into one of sciences most thrilling frontiers.

[00:11:37]I hope I've managed to convey not just the facts and the figures, but the sheer wonder and magnitude of what we're witnessing in our time.

[00:11:47]We live in an extraordinary era where questions that once belonged only to philosophy and imagination are becoming answerable through observation and evidence.

[00:11:58]As you leave here today and go about your lives, I encourage you to look up at the night sky with fresh eyes.

[00:12:05]Somewhere out there, 120 light years away in the constellation Leo, orbits a world that might just harbor life. And we have the privilege of being the generation that discovered it. Keep asking questions, stay curious, support science and exploration, and remember that the universe is far stranger and more wonderful than we can possibly imagine. Good luck to all of you in your own journeys of discovery, wherever they may lead. May you always retain that sense of wonder, that spark of curiosity that makes us uniquely human.

[00:12:39]Thank you for your attention, your enthusiasm, and for caring about these profound questions.

[00:12:45]Now, go forth and share this excitement with others because discoveries like this belong to all of humanity.

[00:12:51]The cosmos is calling and we are finally learning to listen. Thank you and clear skies to you all.