Are We Alone? (And How Would We Know?)

The one idea

In 2023, the James Webb Space Telescope read the chemistry of the atmosphere of a planet circling another star, 120 light-years away. It found methane and carbon dioxide, plus a faint, contested hint of a third gas that on Earth is made almost entirely by ocean life. That is genuinely the closest we have ever come to a planet that might be inhabited. It is also not a discovery of life. The entire session lives in the distance between “we detected a gas living things can produce” and “we detected living things,” because every candidate biosignature has an innocent, lifeless explanation that has to be ruled out first, and ruling it out is the hard part.

The science

We cannot photograph the surface of a planet 120 light-years away. It’s a point of light lost in the glare of its star. But we can read its air. When a planet passes in front of its star (a “transit”), a sliver of starlight filters through the planet’s atmosphere on its way to us. Each gas absorbs specific colors of light, leaving a barcode of dark notches in the spectrum. JWST is sensitive enough to read that barcode. For the planet K2-18b, a world about 8 times Earth’s mass orbiting a small red star, it picked out methane (CH₄) and carbon dioxide (CO₂), plus the absence of certain other gases. That combination led some astronomers to argue the planet might have a liquid-water ocean under a hydrogen-rich sky.

Then came the disputed part, a tentative signal of dimethyl sulfide (DMS), a gas that on Earth is produced overwhelmingly by marine algae. If real, it would be a biosignature. But the signal is weak, it sits near the edge of what the instruments can resolve, and other research teams reanalyzing the same data have not been able to confirm it. Some find no significant DMS at all, and DMS has even been detected on a lifeless comet, which means non-biological chemistry can make it too. So the honest current status is intriguing chemical signatures, not a confirmed sign of life. The central, unglamorous challenge of the whole field is exactly this, telling a true biosignature apart from ordinary geochemistry, where lifeless rock, light, and gas conspire to mimic the fingerprints of biology. (Book line-refs. v10 ~L8459–8484, the “Search for Life Beyond Earth / Exoplanet Atmospheres” passage. Mainstream science as of 2025–2026 has if anything cooled on the DMS claim, which we follow here rather than the more hopeful early framing.)

What this changes about how you picture reality

For almost all of human history “are we alone?” was a question you answered with a story, because there was no way to actually look. We are the first humans who can point an instrument at another world and measure its air. That alone is staggering. The universe contains more planets than there are grains of sand on every beach on Earth, and we have started chemically sampling them one at a time. But the deeper shift is about what honesty under uncertainty looks like. The discipline here isn’t the excitement of “we found life.” It’s the patience to say we found a gas that life can make, and now we have to spend years trying to prove it wasn’t life that made it. The awe isn’t in jumping to the answer. It’s in being a species that finally has the tools to ask, and the integrity to not lie to ourselves about what the data says yet. The cosmos didn’t get smaller when the DMS signal got shaky. It got more real.

Two ways to see it

Put two genuine perspectives in front of the room. Not believer versus skeptic as a cartoon, but two honest scientific instincts that both live inside the evidence.

• The conservative read. Assume rock until proven otherwise. Walk through, in a clip or a read-aloud, how a planetary chemist defaults. Every gas we found has a boring explanation that must be eliminated first. Methane and CO₂ can come from geology. The DMS signal is below the threshold most scientists trust. On this view the right words are not yet, and that isn’t pessimism. It’s the burden of proof that protects us from a false “we’re not alone.”
• The exploratory read. This is what the first real detection would look like. A confirmed biosignature won’t arrive as a clean headline. It will arrive exactly like this, faint and contested and re-analyzed and argued over for years, because that’s how every major discovery enters science. On this view K2-18b is less the answer than a dress rehearsal for the methods, instruments, and arguments we’ll need when the real one comes.

There’s an optional third anchor, the transit-spectroscopy explainer visual itself (star → planet crosses → filtered light → barcode spectrum), as a neutral grounding in how we know anything at all before the two voices go up.

Discussion questions

1. What’s the difference, for you, between “we found a gas that life can produce” and “we found life”? Where exactly does that line sit, and who gets to decide we’ve crossed it?
2. If the strongest evidence we ever get is faint and disputed, never a clean, certain headline, could we actually accept it? What would it take to convince you personally?
3. Scientists reported the DMS hint and flagged that it’s tentative and might be wrong. Does broadcasting uncertainty make you trust them more, or less? Why?
4. The universe has more planets than grains of sand on every beach. Does that make “Earth is the only place with life” feel reasonable or absurd to you, and is that a feeling or an argument?
5. Imagine the signal is confirmed next year and a planet 120 light-years away has life we’ll likely never visit or contact. Does that change anything about how you live on this one?
6. Why do you think “are we alone?” pulls at people harder than most science questions? What is the question really asking underneath?

Closing question

How do you feel about this science and its understanding of reality?

Take it further

• The Book, v10, “The Search for Life Beyond Earth / Exoplanet Atmospheres,” approx. lines L8459–8484 (the “more planets than grains of sand” passage, K2-18b methane and CO₂, and the tentative, disputed DMS). Nearby you’ll also find the Jocelyn Bell Burnell “Little Green Men” pulsar story (~L8264), a clean historical case of almost calling something aliens and being right to wait.
• External, the original claim. Madhusudhan et al. (2023), “Carbon-bearing Molecules in a Possible Hydrogen-rich Atmosphere of a Habitable-zone Sub-Neptune,” The Astrophysical Journal Letters. This is the JWST K2-18b methane/CO₂ detection and the first DMS hint.
• External, the pushback, which matters for honesty. Several 2024–2025 reanalyses, including work led by groups reprocessing the same JWST data, found the DMS signal does not rise to statistical significance, and noted DMS has been seen on a comet, so lifeless chemistry can make it. Mark this as actively contested. Present K2-18b as a candidate or unconfirmed, never as a found biosignature.
• For facilitators. NASA’s biosignatures framework and the idea of a detection ladder (signal → confirmed → contextualized → community-accepted) is a useful structure if the room wants to know what would actually count.

Visual notes

• Primary anchor. The exoplanet transit spectroscopy explainer. Animate the chain of a small star, a planet crossing its face, starlight filtering through the planet’s thin ring of atmosphere, and the resulting spectrum with dark absorption notches labeled CH₄ / CO₂ / (DMS?). This is the how-do-we-even-know-anything visual and it should open the room.
• Secondary. A K2-18b atmosphere readout, a clean depiction of the actual barcode spectrum, with the DMS feature visibly faint and flagged as disputed. Do not render it as a confident peak. The honesty of the visual matters as much as the talk.
• Scale beat. The “more planets than grains of sand” visual, a beach and sand transitioning out to galaxies, to seat the gut-level scale before the discussion turns to a single planet.
• The room should mostly be looking at the spectrum, not an artist’s painting of a watery alien ocean. Keep the imagined-surface art minimal and clearly labeled “artist’s impression,” so the visuals reinforce that we measured air and did not see a world.