#Are Biosignatures Even Real?

The standard definition, codified in the astrobiology literature and most prominently operationalized by Schwieterman et al. (2018), is that a biosignature is any substance, structure, or pattern whose origin requires a biological agent — or more precisely, whose presence is better explained by biology than by any known abiotic process. The community revised this definition in 2023 to add the phrase "with all plausible abiotic explanations having been considered and eliminated." That revision is load-bearing. It implicitly acknowledges that no biosignature is interpretable in isolation.

There are multiple classes of biosignatures. Atmospheric gaseous biosignatures — the ones accessible to remote spectroscopy — are the most discussed because they're the ones JWST can in principle detect. These include molecular oxygen (O₂) and its photochemical proxy ozone (O₃), methane (CH₄), nitrous oxide (N₂O), and more recently, biogenic sulfur gases like dimethyl sulfide (DMS) and dimethyl disulfide (DMDS). There are also surface biosignatures — the vegetation red edge, pigmentation signals — and temporal biosignatures, where the time-varying flux from a planet encodes biological cycling.

The key thing to understand about gaseous biosignatures is that they are not stable properties of a planet. They are fluxes. Methane has a photochemical lifetime in an oxic atmosphere of roughly twelve years; it vanishes from a spectrum unless it is being continuously produced. N₂O has a lifetime of around 150 years. O₂, by contrast, has a geological lifetime of approximately two million years — long enough that its presence doesn't require continuous biological production at any given moment, which is part of why it's a weaker indicator than it looks. A biosphere that stopped producing oxygen tomorrow would leave an oxygen-rich atmosphere for another two million years. You are not detecting life; you are detecting the integrated history of a planetary system.

The flagship biosignature target for most of the past three decades has been molecular oxygen. The argument for O₂ as a biosignature is structurally simple: on Earth, photosynthesis is the overwhelmingly dominant source of atmospheric O₂, and the only known abiotic mechanisms for producing O₂ in large quantities operate on early, pre-biological worlds through water photolysis and hydrogen escape. Therefore, a substantial O₂ abundance in the atmosphere of a planet past its early evolution should indicate active biological production.

This argument survived mostly intact until the mid-2010s, when a cascade of modeling results identified multiple abiotic mechanisms capable of generating Earth-like O₂ abundances for planets in habitable zones. Luger and Barnes (2015) demonstrated that the extended pre-main sequence luminosity phase of late M-dwarf stars could drive enough XUV-flux-induced water photolysis and hydrogen escape that a planet accumulating hundreds or thousands of bars of abiotic O₂ before life ever had a chance to emerge was a real possibility — not a pathological edge case.

What makes these false positive mechanisms particularly damaging is that they tend to be strongest precisely where we're most likely to look. The planets most accessible to JWST transmission spectroscopy are sub-Neptunes and rocky planets orbiting M-dwarf stars. But M-dwarfs are exactly where abiotic O₂ generation is most efficient. The observational selection effect and the false positive exposure are correlated in the worst possible way.

The oxygen-methane disequilibrium pair has been the field's answer to this problem. On Earth, atmospheric O₂ and CH₄ cannot coexist at thermodynamic equilibrium; they react. Their simultaneous presence requires continuous biological production of both. This thermodynamic argument is more robust than either gas individually. But it is not clean. Felton et al. (2022) showed that atmospheric exchange in tightly-packed systems like TRAPPIST-1 can in principle generate false positives even for the disequilibrium pair under specific flux conditions.

There is a deeper systematic error beneath all of this that the community has been slow to confront. In 2022, Julien de Wit and Prajwal Niraula at MIT's Disruptive Planets Lab published a paper in Nature Astronomy demonstrating that the opacity databases underpinning every atmospheric retrieval — the molecular cross-section tables that tell you how each gas absorbs light at each wavelength — contain errors large enough to matter at JWST precision. When eight perturbed versions of standard opacity models were tested against synthetic JWST-quality spectra, the resulting atmospheric property estimates varied by an order of magnitude: a temperature discrepancy of 300 K versus 600 K, chemical abundances that differed between 5% and 25% of atmospheric layers. Critically, multiple incorrect models still produced statistically good fits. As de Wit put it: "There is a scientifically significant difference between a compound like water being present at 5 percent versus 25 percent." This is not a calibration issue that better instruments will solve. It is a theoretical problem — and it means the spectral fingerprints we are searching for may not be as sharp as the literature assumes.

In April 2025, a Cambridge group led by Nikku Madhusudhan published new JWST MIRI data on K2-18b, a sub-Neptune 124 light-years away in the habitable zone of an M-dwarf. They detected spectral features consistent with dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) at three-sigma significance. On Earth, DMS is produced almost exclusively by marine phytoplankton. The headlines wrote themselves.

The scientific community's response was precisely calibrated. The three-sigma threshold is not a detection — it corresponds to a false positive probability of 0.3%, which sounds small but is meaningless without a base rate for how many such observations you're conducting simultaneously and how many abiotic mechanisms you've ruled out. Sara Seager and colleagues at MIT, in a concurrent analysis, concluded the K2-18b DMS detection failed all three of their evaluation criteria: signal-to-noise insufficient for attribution, unambiguous spectral assignment not established, and physical consistency with the planet's known environment not demonstrated.

K2-18b is not an Earth analogue. It has 8.6 times Earth's mass and 2.6 times Earth's radius. It is classified as a Hycean world — a hypothesized class of planets with a global ocean beneath a hydrogen-rich atmosphere. This classification matters because the context shapes what a biosignature should look like, and we don't yet have the theoretical machinery to specify that. Seager's own work on hydrogen worlds underscores how wide this gap is: in 2020, her MIT group demonstrated in Nature Astronomy that both E. coli and yeast survive and grow in 100% hydrogen gas atmospheres for extended periods. Life, it turns out, can tolerate hydrogen-rich environments — which means the class of atmospheres plausibly capable of harboring biology is substantially broader than the Earth-centric catalog implies. Hydrogen-dominated atmospheres are also approximately 14 times more vertically extended than Earth's at equivalent surface pressure, making them in principle more accessible to transmission spectroscopy. The problem is not that we were wrong to look at K2-18b. The problem is that looking at it without a physics-grounded framework for what life would leave behind in that specific environment is an exercise in wishful pattern-matching.

As Seager's analysis concluded, JWST has moved the field away from the possibility of finding a definitive single-molecule "silver bullet" biosignature and toward a regime requiring parallel interpretations — suites of constraints that are collectively consistent or inconsistent with biological activity. That is methodologically correct. It is also, honestly, much harder.

The field has developed sophisticated tools for thinking about false positives — abiotic processes that mimic biology. There is a parallel problem that receives far less attention: false negatives, where biology is present but the biosignature is suppressed or invisible.

The canonical false negative case is Earth's own history. Oxygenic photosynthesis evolved in cyanobacteria approximately 2.7 billion years ago. Atmospheric oxygen didn't accumulate significantly until the Great Oxidation Event roughly 2.4 billion years ago — a lag of 300 million years. During that interval, the geological oxygen sinks effectively neutralized the biological flux. An observer with JWST technology watching Earth during the Archean would have seen a world with active photosynthesis and no detectable oxygen biosignature.

This is not a hypothetical. It is what happened. Earth spent the majority of its inhabited history looking, from a remote spectroscopic standpoint, like a dead or anoxic world. A Bayesian observer reasoning from the absence of O₂ and O₃ would have assigned low probability to the presence of a biosphere. They would have been wrong.

There is also the subsurface biosphere problem. If life exists primarily underground — in hydrothermal systems, beneath ice sheets, within rock-hosted microbial communities — it may produce essentially no atmospheric biosignature regardless of its metabolic activity. Europa and Enceladus are plausible candidates for exactly this scenario within our own solar system.

Here is the structural deficit in the field that I find most striking and least addressed: the biosignature community has built a sophisticated framework for what gases to look for and how to interpret their presence or absence, but has underinvested in understanding how the basic physical parameters of a planet reshape the biosignature landscape from first principles.

Consider surface gravity. The atmospheric scale height — the vertical distance over which atmospheric pressure drops by a factor of e — scales inversely with surface gravity. A planet with surface gravity three times Earth's has an atmosphere compressed into roughly one-third the vertical extent at equivalent surface pressure. This has direct consequences for transmission spectroscopy: the signal strength in a transit observation is proportional to the scale height times the number of scale heights over which an absorbing gas is present. Published work by Rauer et al. explicitly compares 1g and 3g super-Earth scenarios and finds that gravity substantially modifies the spectral appearance of biosignature gases — not merely in signal strength but in the photochemical pathways that produce and destroy them.

Surface pressure is an independent variable that the community frequently treats as fixed at 1 bar by analogy to Earth, when the actual range for habitable planets plausibly spans orders of magnitude. Pradhan et al. (2025) explicitly modeled biosignature detectability across 0.5 to 4 bar surface pressures and found significant variation in the identifiability of key species.

A parallel systematic error operates at the level of stellar characterization — and this is where de Wit's group at MIT has done some of the most operationally important work. Their 2024 JWST atmospheric characterization roadmap, published in Nature Astronomy with René Doyon, identified stellar contamination as the dominant source of systematic error in current JWST transmission spectra. M-dwarf stars like TRAPPIST-1 have heterogeneous surfaces — starspots, faculae, magnetically active regions — whose spectral contributions can swamp the planetary atmospheric signal in a transit observation. De Wit noted that the earliest JWST spectral data on terrestrial planets "don't really make any sense" in purely planetary terms, because the stellar component hadn't been adequately removed. His group's proposed approach — optimizing transit scheduling to capture multiple transits within five-hour windows, directly characterizing the stellar spectrum independently, and filtering contamination before extracting planetary signals — can reduce the observation time required by approximately 50% and represents the minimum methodological bar for any credible atmospheric claim on an M-dwarf planet. The majority of published atmospheric analyses have not cleared this bar.

The relationship between gravity, pressure, atmospheric retention, stellar environment, and biosignature expression is not adequately captured by any current observational or theoretical framework. We do not have a unified model that treats these as a coupled system, calibrated against what different biospheres might produce as a function of where they sit in that parameter space.

The emerging alternative to single-molecule silver bullet biosignatures is what some in the community are calling a "context-sensitive systems approach" — characterizing a planet's star, orbital architecture, atmospheric bulk composition, mass, radius, and thermal history first, then asking what biosignature expression a biosphere would have within those constraints. This is the right direction.

One productive thread is the concept of "peribiosignatures" — biosignatures that are more discriminating at the edges of parameter space, where abiotic false positive mechanisms are least efficient. A biosignature detected in a planetary environment where the abiotic machinery for producing it is theoretically suppressed carries more evidential weight than the same biosignature in an environment where abiotic production is plausible.

Another thread is thermodynamic disequilibrium as a global property rather than a pairwise gas detection. Krissansen-Totton et al. have argued that the simultaneous presence of N₂, CH₄, CO₂, and liquid water represents a disequilibrium ensemble that is extremely difficult to maintain abiotically and highly suggestive of active biological cycling. This framing is more robust, but it also requires more observational constraints per planet — constraints that are beyond current capabilities for most targets.

The most operationally tractable near-term approach may be neither of these. In 2023, de Wit and Amaury Triaud, with Sara Seager and other collaborators, proposed in Nature Astronomy that atmospheric carbon depletion — measured relative to planetary neighbors in the same system — is a detectable biosignature accessible to JWST now. The biogeochemical logic is this: Earth's oceans have sequestered carbon over geological time equivalent to the entirety of Venus's atmospheric CO₂ reservoir. A terrestrial planet whose atmospheric CO₂ is anomalously depleted relative to its sibling planets — worlds that formed from the same nebular material, around the same star, at the same epoch — is exhibiting a depletion that abiotic chemistry struggles to explain. The combination of low CO₂ and a detected ozone signal, the team argued, would be extremely difficult to produce without biology. As Triaud put it: "If we see ozone, chances are pretty high that it's connected to carbon dioxide being consumed by life."

This approach resolves two chronic failure modes simultaneously. It does not require detecting trace biogenic gases at low abundance against an uncertain stellar background. And it is comparative rather than absolute, which makes it substantially less sensitive to the poorly constrained opacity and stellar contamination systematics that de Wit's group has identified as the field's core theoretical liabilities. De Wit has projected that applied to TRAPPIST-1 — a seven-planet system 41 light-years away with three planets potentially in the habitable zone — this strategy could determine habitability or its absence within a decade.

The first JWST results for TRAPPIST-1e, published by Glidden et al. in 2025 (MIT EAPS / Kavli, with Seager as co-author), begin to build the scaffolding this approach requires. Multiple JWST transit observations ruled out hydrogen-dominated and Venus/Mars-like CO₂-rich atmospheres for TRAPPIST-1e. A warm, nitrogen-rich atmosphere remains consistent with the data. A surface liquid ocean is not yet excluded. This is constraint, not detection — but constraint applied across a multi-planet system is precisely what the comparative carbon framework needs to operate. As the atmospheric envelope of plausible compositions narrows for each planet in the TRAPPIST-1 system, the comparative anomaly, if one exists, becomes interpretable.

There is a fourth thread that is underexplored: the population-level statistical approach. If we observe many planets of similar type, we might use the statistical distribution of atmospheric properties across that population to identify outliers whose chemistry cannot be explained by the median abiotic model. This is probably the domain of the Habitable Worlds Observatory in the 2040s rather than JWST today.

What I think the field is slowly converging on, though it hasn't stated it this cleanly, is that biosignatures are not molecular identities. They are conditional probabilities. The presence of DMS, or O₂, or the O₂/CH₄ disequilibrium pair, is not itself the evidence for life — it is an observation that updates a prior, and the magnitude of that update depends entirely on how well we understand the abiotic production and destruction landscape for that molecule in that specific planetary physical regime.

The troubling implication is that we may have the observational technology before we have the theoretical framework mature enough to interpret it. JWST is already producing spectra for which the community cannot agree on interpretation — not because the data are bad, but because the models are underdetermined, the opacity databases are imprecise, and the stellar contamination signal has not been cleanly separated from the planetary one. K2-18b is the first high-profile example of this. It will not be the last. The honest reading of de Wit and Niraula's opacity work is that a non-trivial fraction of atmospheric detections published in the last several years may be reporting properties of the model rather than properties of the planet.

The Earth-centric catalog of O₂, CH₄, N₂O, and DMS as biosignature gases is a list of what one specific biosphere in one specific gravitational and atmospheric regime produces. The question of what other gravity regimes would produce — what biosignature phenomenology looks like across the full distribution of rocky, habitable-zone planets, including the hydrogen-rich worlds Seager's lab showed can sustain life — is a theoretical problem that is almost entirely open.

That is the frontier. Not whether we can detect molecules in exoplanet atmospheres — we can, and JWST has proven it. But whether we know enough about the physics governing how biospheres interact with planetary environments across the full parameter space to tell the difference between a planet that is inhabited and one that looks like it might be. The first problem is engineering. The second is science. And the second is where we are behind.

References available on request. Key works include: de Wit & Niraula et al., Nature Astronomy 2022 (opacity); de Wit, Triaud, Seager et al., Nature Astronomy 2023 (carbon depletion); de Wit & Doyon et al., Nature Astronomy 2024 (JWST roadmap); Glidden, Seager et al., ApJL 2025 (TRAPPIST-1e); Seager et al., Nature Astronomy 2020 (hydrogen atmospheres). All interpretive conclusions are the author's own.