Detecting life is, in a sense, two problems wearing one coat. First, find a signal that life could produce. Second — the harder one — rule out everything else that could produce the same signal without life. The second problem is where most of the actual science lives, and it’s the reason no detection has ever been confirmed.

What Counts as a Biosignature

A biosignature is any measurable feature that indicates biological activity. They come in a few flavors. Atmospheric biosignatures are gases — the kind a telescope can read in a planet’s spectrum. Surface biosignatures include things like the “red edge,” a sharp jump in reflectance at the wavelength where Earth’s vegetation reflects infrared light. Chemical biosignatures, examined in a physical sample, include molecular patterns and isotope ratios that life tends to skew.

The most discussed remote biosignature is the combination of oxygen and methane. These two gases react with each other and shouldn’t both persist in an atmosphere at meaningful levels — unless something replenishes them continuously. On Earth, biology does exactly that: photosynthesis floods the air with oxygen, while microbes and other processes release methane. Find both together on a distant world, and you have something that’s genuinely hard to explain without life.

The False Positive Problem

“Hard to explain” is not “impossible to explain,” and that gap is where careers are made and reputations lost.

Take oxygen on its own. For years it was treated as a near-certain sign of life. Then researchers worked out several ways a planet could build up atmospheric oxygen with no biology at all. Around a red dwarf, intense ultraviolet light can split water vapor and let the lighter hydrogen escape to space, leaving oxygen behind. A planet that lost a primordial ocean this way could look oxygen-rich and be utterly dead. Victoria Meadows and others have mapped these abiotic pathways in detail, precisely so a future detection isn’t mistaken for life.

The same caution applies to methane (volcanic and geological sources exist) and to nearly every individual candidate gas. This is why the oxygen-methane combination is valued over either gas alone — stacking conditions that are individually fakeable but jointly very difficult to fake. Even then, the standard is context: what’s the star, what’s the geology, what else is in the atmosphere?

Reading an Atmosphere From Light-Years Away

The practical method for exoplanets is transmission spectroscopy. When a planet crosses in front of its star, a sliver of starlight filters through the planet’s atmosphere on its way to us. Molecules in that atmosphere absorb specific wavelengths, stamping the spectrum with their fingerprint. The James Webb Space Telescope can resolve these fingerprints for large molecules — CO2, water, methane — on planets hundreds of light-years away.

For a small rocky planet in a habitable zone, though, the signal is brutally faint. It can take dozens of transits stacked together to pull a reliable atmospheric measurement out of the noise — years of telescope time on a single target. And there’s a complication: the star itself isn’t a clean light source. Starspots and bright regions on the stellar surface can imprint features that masquerade as atmospheric signals. Untangling the planet’s atmosphere from the star’s surface activity is one of the hardest parts of the analysis, and a few early claimed detections turned out to be the star, not the planet.

The Sample-in-Hand Methods

For targets inside the solar system, detection can get more direct. A mass spectrometer on a rover or a probe can measure the actual molecules in a rock or a plume. Here the prized signatures are subtler than gases.

One is chirality. Many biological molecules come in two mirror-image forms, but life on Earth overwhelmingly uses just one — left-handed amino acids, right-handed sugars. Non-biological chemistry tends to produce a roughly even mix. A strong imbalance in a sample would be a powerful clue. Another is isotope ratios: living things preferentially incorporate lighter isotopes of carbon, leaving a measurable bias. A third is molecular complexity itself — long, specific, ordered molecules that random chemistry struggles to assemble.

The Confirmation Problem

In 2021, a group of scientists including NASA’s James Green proposed a formal scale for reporting evidence of life — a way to communicate confidence levels so a tentative hint isn’t reported as a discovery. The proposal was a direct response to a recurring pattern: an exciting result, a media frenzy, and then a slow walk-back as scrutiny found an alternative explanation.

The frustrating truth is that a single measurement will almost never be enough. A genuine claim of extraterrestrial life would need independent confirmation, ruled-out alternatives, and ideally a second method pointing the same way. For an exoplanet, that might mean a biosignature gas detected by one telescope, confirmed by another, with the planet’s geology and host star understood well enough to exclude abiotic sources. That’s a tall order, and it’s supposed to be.

Putting a Number on Confidence: The CoLD Scale

The field has taken its hard-won caution and tried to formalize it. In 2021, a team led by NASA’s James Green proposed the Confidence of Life Detection scale, or CoLD — a framework for communicating exactly how strong any claimed evidence of life actually is. Modeled loosely on the Torino scale used to rate asteroid impact risk, it runs through seven levels, from a first tentative signal up to robust, independently confirmed detection.

What makes CoLD useful is that it builds the discipline of doubt directly into the reporting. Early levels require ruling out contamination and obvious non-biological sources. Crucially, the scale includes two separate false-positive checks: ruling out the abiotic explanations known beforehand, and then ruling out new abiotic explanations proposed after the discovery — because skeptics will always invent fresh ways for a result to be mundane, and a real detection has to survive those too. The scale’s explicit purpose is to stop a low-confidence hint from being reported as a discovery, the recurring failure mode that has burned the field before. It’s an attempt to give scientists, journalists, and the public a shared vocabulary for confidence, so that “possible sign of life” and “confirmed life” are never again collapsed into the same headline.

Agnostic Biosignatures: Looking for Life We Can’t Predict

Every method described so far shares a hidden assumption: that alien life resembles ours closely enough to leave familiar traces. It might not. This worry has driven a search for “agnostic” biosignatures — markers of life that don’t depend on it using DNA, the same amino acids, or even carbon chemistry the way Earth life does.

The most developed idea is to detect complexity itself. Living systems build large, highly specific, intricately ordered molecules in abundance — structures that random chemistry essentially never produces in quantity, because the number of possible arrangements is astronomical and life selects a tiny, repeated subset. One framework, sometimes called assembly theory, attempts to quantify how many steps it would take to build a given molecule; above a certain threshold of complexity, the only known explanation is something life-like directing the assembly. A detector measuring molecular complexity in a sample could, in principle, flag biology without knowing anything about its specific chemistry. It’s an attempt to ask not “is this Earth life?” but “is this the kind of order only life produces?”

Where Technosignatures Fit In

Biosignatures detect life; technosignatures detect life that builds things. They sit at the far end of the same detection spectrum, and they have one practical advantage — technology can be far more conspicuous than biology. A microbe’s atmospheric trace is faint and ambiguous. A powerful, narrow-band radio transmission, an industrial pollutant with no natural source, or the waste heat of a vast energy-harvesting structure would be much harder to mistake for geology.

The methods differ accordingly. Technosignature searches look for narrow-band radio or laser pulses that nature doesn’t produce, for atmospheric chemicals like industrial chlorofluorocarbons that have no abiotic source, or for thermal anomalies suggesting megastructures. They share the biosignature field’s central discipline, though: the burden is always to rule out the mundane explanation first. Most candidate signals turn out to be human interference or natural phenomena. The standard of proof is the same whether you’re hunting a microbe’s breath or a civilization’s broadcast — exhaust every ordinary explanation before reaching for the extraordinary one.

The Method Is Doubt, Applied Relentlessly

If there’s a single thread connecting every technique here, it’s structured skepticism. Find a signal, then attack it. Propose an abiotic explanation and try to make it work. Demand a second method, a second instrument, a second look. The methods of life detection are, at bottom, methods for not fooling yourself — and the field’s hardest-won lesson is that a discovery this large will only ever be announced once, so it had better survive every challenge the discoverers can throw at it before anyone else does.

Why Getting It Right Matters More Than Getting It First

There’s a quiet pressure in any high-stakes science to be the one who announces the breakthrough. Astrobiology has learned to resist it. A confirmed detection of life beyond Earth would be reported around the world within minutes and scrutinized for decades; a mistaken one would damage the field’s credibility for just as long. So the methods are built to favor being right over being first — every candidate signal treated as guilty of being mundane until proven otherwise. It’s a slower path, and an unglamorous one, but it’s the only path that produces an answer the rest of science, and the rest of the world, will actually believe.

SETIworld tracks the methods used to detect life beyond Earth — and the high bar required to confirm it. Join the portal to follow each new claim and challenge.

References

• Schwieterman et al., Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life, Astrobiology 2018 doi.org/10.1089/ast.2017.1729
• Meadows, Reflections on O2 as a Biosignature in Exoplanetary Atmospheres, Astrobiology 2017
• Catling et al., Exoplanet Biosignatures: A Framework for Their Assessment, Astrobiology 2018
• Seager, Bains & Petkowski, Toward a List of Molecules as Potential Biosignature Gases, Astrobiology 2016
• Walker et al., Exoplanet Biosignatures: Future Directions, Astrobiology 2018
• Green et al., Call for a framework for reporting evidence for life beyond Earth, Nature 2021