By Daniel Apai, Associate Dean for Research and Professor of Astronomy and Planetary Sciences, University of Arizona. Originally published at The Conversation.
A team of astronomers announced on April 16, 2025, that in the process of studying a planet around another star, they had found evidence for an unexpected atmospheric gas. On Earth, that gas – called dimethyl sulfide – is mostly produced by living organisms.
In April 2024, the James Webb Space Telescope stared at the host star of the planet K2-18b for nearly six hours. During that time, the orbiting planet passed in front of the star. Starlight filtered through its atmosphere, carrying the fingerprints of atmospheric molecules to the telescope.
JWST’s cameras can detect molecules in the atmosphere of a planet by looking at light that passed through that atmosphere. European Space Agency
By comparing those fingerprints to 20 different molecules that they would potentially expect to observe in the atmosphere, the astronomers concluded that the most probable match was a gas that, on Earth, is a good indicator of life.
I am an astronomer and astrobiologist who studies planets around other stars and their atmospheres. In my work, I try to understand which nearby planets may be suitable for life.
K2-18b, a Mysterious World
To understand what this discovery means, let’s start with the bizarre world it was found in. The planet’s name is K2-18b, meaning it is the first planet in the 18th planetary system found by the extended NASA Kepler mission, K2. Astronomers assign the “b” label to the first planet in the system, not “a,” to avoid possible confusion with the star.
K2-18b is a little over 120 light-years from Earth – on a galactic scale, this world is practically in our backyard.
Although astronomers know very little about K2-18b, we do know that it is very unlike Earth. To start, it is about eight times more massive than Earth, and it has a volume that’s about 18 times larger. This means that it’s only about half as dense as Earth. In other words, it must have a lot of water, which isn’t very dense, or a very big atmosphere, which is even less dense.
Astronomers think that this world could either be a smaller version of our solar system’s ice giant Neptune, called a mini-Neptune, or perhaps a rocky planet with no water but a massive hydrogen atmosphere, called a gas dwarf.
Another option, as University of Cambridge astronomer Nikku Madhusudhan recently proposed, is that the planet is a “hycean world.”
That term means hydrogen-over-ocean, since astronomers predict that hycean worlds are planets with global oceans many times deeper than Earth’s oceans, and without any continents. These oceans are covered by massive hydrogen atmospheres that are thousands of miles high.
Astronomers do not know yet for certain that hycean worlds exist, but models for what those would look like match the limited data JWST and other telescopes have collected on K2-18b.
This is where the story becomes exciting. Mini-Neptunes and gas dwarfs are unlikely to be hospitable for life, because they probably don’t have liquid water, and their interior surfaces have enormous pressures. But a hycean planet would have a large and likely temperate ocean. So could the oceans of hycean worlds be habitable – or even inhabited?
Detecting DMS
In 2023, Madhusudhan and his colleagues used the James Webb Space Telescope’s short-wavelength infrared camera to inspect starlight that filtered through K2-18b’s atmosphere for the first time.
They found evidence for the presence of two simple carbon-bearing molecules – carbon monoxide and methane – and showed that the planet’s upper atmosphere lacked water vapor. This atmospheric composition supported, but did not prove, the idea that K2-18b could be a hycean world. In a hycean world, water would be trapped in the deeper and warmer atmosphere, closer to the oceans than the upper atmosphere probed by JWST observations.
Intriguingly, the data also showed an additional, very weak signal. The team found that this weak signal matched a gas called dimethyl sulfide, or DMS. On Earth, DMS is produced in large quantities by marine algae. It has very few, if any, nonbiological sources.
This signal made the initial detection exciting: on a planet that may have a massive ocean, there is likely a gas that is, on Earth, emitted by biological organisms.
K2-18b could have a deep ocean spanning the planet, and a hydrogen atmosphere. Amanda Smith, Nikku Madhusudhan (University of Cambridge), CC BY-SA
Scientists had a mixed response to this initial announcement. While the findings were exciting, some astronomers pointed out that the DMS signal seen was weak and that the hycean nature of K2-18b is very uncertain.
To address these concerns, Mashusudhan’s team turned JWST back to K2-18b a year later. This time, they used another camera on JWST that looks for another range of wavelengths of light. The new results – announced on April 16, 2025 – supported their initial findings.
These new data show a stronger – but still relatively weak – signal that the team attributes to DMS or a very similar molecule. The fact that the DMS signal showed up on another camera during another set of observations made the interpretation of DMS in the atmosphere stronger.
Madhusudhan’s team also presented a very detailed analysis of the uncertainties in the data and interpretation. In real-life measurements, there are always some uncertainties. They found that these uncertainties are unlikely to account for the signal in the data, further supporting the DMS interpretation. As an astronomer, I find that analysis exciting.
Is life out there?
Does this mean that scientists have found life on another world? Perhaps – but we still cannot be sure.
First, does K2-18b really have an ocean deep beneath its thick atmosphere? Astronomers should test this.
Second, is the signal seen in two cameras two years apart really from dimethyl sulfide? Scientists will need more sensitive measurements and more observations of the planet’s atmosphere to be sure.
Third, if it is indeed DMS, does this mean that there is life? This may be the most difficult question to answer. Life itself is not detectable with existing technology. Astronomers will need to evaluate and exclude all other potential options to build their confidence in this possibility.
The new measurements may lead researchers toward a historic discovery. However, important uncertainties remain. Astrobiologists will need a much deeper understanding of K2-18b and similar worlds before they can be confident in the presence of DMS and its interpretation as a signature of life.
Scientists around the world are already scrutinizing the published study and will work on new tests of the findings, since independent verification is at the heart of science.
Moving forward, K2-18b is going to be an important target for JWST, the world’s most sensitive telescope. JWST may soon observe other potential hycean worlds to see if the signal appears in the atmospheres of those planets, too.
With more data, these tentative conclusions may not stand the test of time. But for now, just the prospect that astronomers may have detected gasses emitted by an alien ecosystem that bubbled up in a dark, blue-hued alien ocean is an incredibly fascinating possibility.
Regardless of the true nature of K2-18b, the new results show how using the JWST to survey other worlds for clues of alien life will guarantee that the next years will be thrilling for astrobiologists.
I’d be more surprised if there were no life elsewhere in the universe. We live on a living planet – the result of a 3 billion-year plus living geoforming activity which is still ongoing. There is solid evidence of life on Mars, even the moons of Jupiter, and it seems to me that living organisms are universal, even in the most apparently (to our limited ape intellects) inhospitable environments.
Further, considering the “lifecycle” of the billions of Suns which glitter in the night sky, is it too far to imagine that the Sun, our one source of planetary energy, that drives all the biological processes on our planet, is also a living entity? Feel the love, feel the energy, gratitude and joy in our living universe!
‘is it too far to imagine that the Sun, our one source of planetary energy, that drives all the biological processes on our planet, is also a living entity?’
Gee, I hope not-
https://www.youtube.com/watch?v=ZkIQTX71GnU (2:34 mins)
K2-18b is a little over 120 light-years from Earth – on a galactic scale, this world is practically in our backyard.
With current technology — and with just about any imaginable technology — that is a big back yard, unattainably big. Contact will be impossible.
The discovery of a planet that has some life on it that is off-gassing is significant, if that is what the data indeed show us.
What is significant is that there may be life elsewhere in the galaxy. I have always had a tendency to put a couple of low values in the Drake equation. With a couple of low values, the likelihood of life in the Milky Way goes down to a few very faraway planets.
There is a lesson to be drawn from our solitude. We seem to be unwilling to be good stewards to this planet, in spite of the evidence that is a wonderful place, even if not unique.
The theological implications are going to cause some freakouts among the Tartuffians. Tanto peggio.
The Drake equation is [aaaargh. I am having trouble with the typography — the second character of each variable is supposed to be a subscript]:
N= R*∙fp ∙ne ∙fl∙fi∙fc ∙L
where
• N = the number of civilizations in the Milky Way galaxy with which communication might be possible (i.e. which are on the current past light cone);
and
• R∗ = the average rate of star formation in our Galaxy.
• fp = the fraction of those stars that have planets.
• ne = the average number of planets that can potentially support life per star that has planets.
• fl = the fraction of planets that could support life that actually develop life at some point.
• fi = the fraction of planets with life that go on to develop intelligent life (civilizations).
• fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
• L = the length of time for which such civilizations release detectable signals into space.[6][7]
This form of the equation first appeared in Frank Drake’s 1965 paper.
“Contact will be impossible.”
Well, yes, physical contact. Traversing interstellar space physically would take several generations which seems a practical impossibility.
It is possible, however, that other modes of contact exist through the ether. Perhaps we are already in contact with alien life but perceive it in other terms.
Russia has a new propulsion system:
https://oilprice.com/Energy/Energy-General/New-Propulsion-Technology-Could-Send-Spaceships-to-Mars-in-a-Month.html
If you have constant propulsion in space where there is no friction, you can get up to a very high speed. Also, with time dilation effects at speeds approaching light, time slows down for the travelers. There are some significant problems of dealing with zero g on physical health, and radiation exposure is significant, but its not clear that its anymore dicey then Captain Cooke’s expedition.
What if Musk finally gets to Mars to claim that planet as his own fiefdom – only to discover a thriving Russian tourist resort asking him if he would like to check in. Payment only in Rubles.
Rev Kev: What if Musk finally gets to Mars to claim that planet as his own fiefdom
Not going to happen.
Musk’s whole story is fairly ludicrous. See below.
I absolutely agree with you about Musk and his obsession with Mars. Sending astronauts there right now would just be an unpleasant death sentence. And yet he seems to have convinced Trump to have the US aim for Mars as that is what Trump declared the other day. Crazy.
KD: …radiation exposure is significant, but its not clear that its anymore dicey then Captain Cooke’s expedition.
Exactly. These aren’t new ideas. Something like this or a nuclear drive has always been the way to go. Indeed, back in 1973, for its Mars program NASA already had the Nuclear Engine for Rocket Vehicle Application (NERVA), a nuclear thermal rocket, tested and set for construction when the Nixon administration, post-Apollo and with the gas shock, closed it down.
https://en.wikipedia.org/wiki/NERVA
With a nuclear rocket running at constant 1G acceleration, it would be theoretically possible to reach Mars in as little as 3.5 days. This includes the necessary midpoint turnaround to decelerate. And, yes, the 1G thrust would provide the astronauts with 1G Earth gravity, which on a longer journey would prevent the health problems of prolonged weightlessness. On a much longer flight, with time dilation, 1G constant thrust would allow a 20-year duration as experienced by the astronauts to cross the Galaxy.
Conversely, any chemical rocket like Musk’s Starship is always ultimately a losing proposition for going anywhere in space beyond the Moon, since the fuel necessary just to launch and reach Earth escape velocity requires something like Apollo, and taking more fuel beyond that means a yet bigger chemical rocket, and etcetera.
In fact, Musk’s whole story about Starship and Mars is fairly ludicrous. I’ve wondered whether he’s truly that ignorant, being that he’s a university dropout — he dropped out of Stanford,on the second day of what would have been his Applied Physics and Materials Science Masters, and Silicon Valley scuttlebutt is that he wasn’t up it intellectually — or knows the truth but is telling a PR story, P. T. Barnum-style.
Essentially, according to Musk’s story:
[1] Once one Starship has reached Earth orbit with its cargo of 1,000 people, then another empty Starship carrying fuel will launch and rendezvous in orbit with it and refuel it. After which…
[2] The Starship carrying the 1,000 folks will set off for Mars, a trip that’s theoretically possible with a chemical rocket thanks to Newton’s First Law of Motion — an object in motion stays in motion at a constant velocity unless acted upon by an external force — but will take 9-12 months depending on the distance between Mars and Earth at that point.
[3] Upon Mars arrival, the amount of radiation received during a 9-12 month flight through Solar space outside Earth’s magnetosphere, will be such that those still left alive by that point will die shortly after their arrival from radiation sickness and tumors.
[4] Nevertheless, in Musk’s story they’re going to begin colonizing operations and prospecting down on Mars’s surface for the elements of fuel that will magically be there and extractable so they can refuel Starship because it’s used the last of its fuel maneuvering into its Mars injection points.
[5] And to bring this back to Life in Space, Mars is the nearest planet to Earth, sure. But long ago, when it lost its magnetosphere, almost all its atmosphere was stripped away by the solar wind and it’s too small to have enough gravity to retain sufficient atmosphere if we tried terraforming it. So the Martian surface has nearly the same levels of cosmic and solar radiation as interplanetary space. Any human beings there would die within months.
NERVA was not continuous thrust, just 25% higher impulse than chemical leaving low Earth and Mars orbits. Design was to cut transit time by 10% (down to 6 months to Mars, 11 back to Earth via Venus). Not judged sufficiently worthwhile even during later development after 17 hours of powered tests because the design needed to evolve to a gas core to work for Mars. It would have been a viable upper stage to shuttle time critical cargo between LEO and lunar orbit.
Thanks. I knew it was far from constant thrust, but Greg Benford painted it in a more flattering light than you have.
Hmm, I was pessimistic in that tests were favorable enough for the team to propose a second generation NERVA for Mars with twice chemical impulse to cut transit time to 4 months, burning for an hr at Earth departure. A third generation gas core design would have cut transit to 90 days, but the tradeoff is always mass sent vs transit time.
It seems if earthlings manage to travel at one third the speed of light it would take 370 years to get there (rounded off according to one source). I don’t think we’ll be able to go that fast for that long, and that far at that velocity without getting a crack in the windshield? By that time China and Russia will need plenty of resources just to cope with weird weather and rising seas. I wonder if all the resources they’re capable of processing without messing things up further…or if they’ll be driven to consume an amount close to what they’re using now? If greenwashing gets’em, the answer would probably be yes. I’m sure many reading here have come across the claim that if any wormholes could be found or created, they couldn’t last long enough.
It is at least a nice coincidence that the distance prevents colonialism out there. Things may not have been set up this way for that reason, but it seems like a good one.
divadab writes upthread: it seems to me that living organisms are universal
No. Living organisms almost certainly cannot be universal. Almost the single thing we do know with some assurance is that life is very likely restricted to very specific regions and time periods within any galaxy. (Well, unless we get into imagining science-fictional energy beings, and then how do such standing energy fields come into existence and maintain their necessary stability?). Here’s why:
[1.] Galactic Habitable Zones
Just like planets have habitable zones around stars, galaxies have regions where conditions are more favorable for life. Too close to the galactic center, and intense radiation — all galactic centers have massive black holes with the seething energies released by the accretion disks around them — and gravitational disturbances make life unlikely.
Too far out, and maybe there aren’t enough heavy elements to form rocky planets. Life as we know it is built out of elements heavier than hydrogen and helium. Stars in a galaxy’s outer regions tend to be metal-poor, meaning fewer planets with the necessary ingredients for life.
[2.] Supernova Frequency.
Galactic areas with supernovae are hazardous for life. Supernovae release deadly radiation and can strip away planetary atmospheres within hundreds of light-years’ range, making long-term habitability difficult.
[3.] Orbital Stability
A planet’s orbit needs to be stable over long periods. If a planet is in a region with strong gravitational interactions—like near a dense star cluster—it’s likely to undergo chaotic orbital shifts that disrupt its climate, making biologic evolution improbable.
[4.] Cosmic Rays and Gamma-Ray Bursts
High-energy cosmic events, like gamma-ray bursts, can sterilize entire regions of a galaxy. If a planet is unlucky enough to be in the path of one, life will be wiped out.
[5.] The Odds *Against* Life May Be Practically Infinite
Here we are on Earth. But we’re a single case, and the transition from simple molecules to self-replicating life forms is incredibly intricate. We know that formation of proteins, RNA, and cellular structures requires precise chemical interactions, which are statistically improbable to occur spontaneously. How improbable?Some calculations suggest that the odds of even the simplest cell forming randomly are astronomically low—maybe 1 in 10^650, which is effectively a mathematical impossibility.
Interestingly, the odds of a Boltzmann Brain —
https://en.wikipedia.org/wiki/Boltzmann_brain
— coming into existence may arguably be higher. If one extends that line of thought just one step further, if a Boltzmann Brain is more probable than us, then why not a Boltzmann Brain with the capabilities to imagine us into existence? God, in other words.
gollygee, Michaelmas,
after a whole painweek, you almost broke my brain with that link.
now im stuck in a wikwander,lol…Measurement Problem, then who knows?
nice distraction from the by now usual chaos magick that has apparently taken over our shared reality.
amfortas the hippie. To add to Michaelmas’s marvelous conditions, I’m going to translate a joke written by Enrico Bertuccioli, published in today’s paper:
An incredible discovery by scientists: Planet K2-18b has the same name as one of Elon Musk’s sons.
If that planet is eight times more massive than the earth, can you imagine what the gravity must be like there? Here on Earth we have a helluva job getting a rocket to escape our gravity well but that planet would have it much worse. Ain’t nobody there getting off that planet.
Gravity might be about 1.3g (12m/(s * s)) . It’s a function not only of mass, but of the radius.
Takeoff into space will not necessarily be impossible, but it will be vastly more expensive to place surveillance satellites into the orbit.
Thanks for doing those sums. That’s not a lot different to Earth standard so would be doable. Expensive in terms of resources but doable. I would have guessed it to be about three or four gs at least.
If it’s all water that’s another problem – we don’t tend to set up space launch sites in the ocean.
It’s a rather odd world in another sense, in that it orbits a small (red dwarf) star with a very short orbital period, around 33 days. This is not (necessarily) because that setup is especially common, but because large planets in close, short period orbits are by far the easiest for us to detect. There are almost certainly countless other planets including many Earth-like ones that we can’t observe or haven’t observed yet.
Do we assume in the above that “life” means the kind of cellular biology we see on our planet? Iiuc, complex systems exhibiting intentional behavior can in principle be built by evolutionary process from other components. If we’re talking about alien life then I’d be interested in all those options too. But I guess for that search we’ve no idea what to look for with a telescope.
My understanding – reinforced by the OP – is that ‘Astrobiologists’ look for chemical compounds which are out of place. In the OP, it’s dimethyl sulfide, which on Earth is [almost?] only created by algae.
The most obvious example of this would be finding a[nother] planet with lotsa free O2 floating around. That’s unlikely, because Oxygen is *really* slutty – it will hook up with damn near anything, generally releasing a bunch of heat in the process (see: Fire, but also Rust). Earth’s atmosphere had *very* little free O2 until Cyanobacteria (or their precursors?) invented photosynthesis and spent a Billion years sucking up CO2 and farting out O2.
So, yes, most ‘Astrobiologists’ are keenly aware that they need to avoid assuming that Life can only happen along chemical pathways similar to what evolved here, which is why they aren’t just looking for O2.
(Note: scare-quotes around ‘Astrobiologists’ because it’s a new field, and they’re attempting to ‘study’ something which has not yet been proven to exist!)
Oh well.
…Joshua Krissansen-Totton, an astrobiologist at the University of Washington, said he worried that American astrobiologists may not be able to follow up on the latest results on K2-18b. The Trump administration is reportedly planning to cut NASA’s science budget in half, eliminating future space telescope and other astrobiology projects. If that happens, Dr. Krissansen-Totton said, “the search for life elsewhere would basically stop.”
https://www.nytimes.com/2025/04/16/science/astronomy-exoplanets-habitable-k218b.html
No surprise.
For the last decade, I’ve thought that China will probably put people back on the Moon before the US does.
As for Mars, if anyone goes there, it’ll be Chinese taikonauts first.
Mars will truly be the Red Planet then.
Although sending an interstellar ship that can communicate back is infeasible with current and near-future technology, off-board laser acceleration of a small integrated circuit “ship,” which, if there is a more advanced civilization at the other end, is feasible with work begun. Of course, many would need to be launched to ensure some would survive interstellar dust. There needs to be someone at the receiving system to send a chip back. Presumably, from nearby systems with planets in Goldilocks zones, we would be (we aren’t) continuously watching for a we-launched signal.
https://www.centauri-dreams.org/2022/02/15/going-interstellar-with-a-laser-powered-rocket/
That said, within a decade or so, AI explorers will probably set off on a mission of replication at the destination system.
The question, though, is why other civilizations’ AI systems are not already exploiting our systems’ resources for replication? The most likely explanation is in the Drake Equation variable “L,” exemplified by the US willing to risk thermonuclear war with Russia for a chance (and a small one at that) of geopolitical gains.