How we'll actually detect alien life: biosignatures, technosignatures and assembly theory

The Short Version

For 2,500 years, are we alone? was argued rather than measured. Astrophysicist Adam Frank says that has changed: "we now actually have the capacity to answer that question," and he expects hard data limiting how common life is within 10 to 30 years [1]. Three kinds of test now exist. Biosignatures are gases and chemical patterns in a planet's atmosphere that rock and air can't make alone. Technosignatures are the traces of technology: industrial pollutants, city lights, solar panels, waste heat from megastructures, starship exhaust, even old hardware on the Moon [1]. Assembly theory, from chemist Lee Cronin and physicist Sara Walker, is a "life meter" that doesn't depend on Earth chemistry: find many identical copies of a molecule that takes more than about 15 steps to build and you have found the product of selection, meaning life [2][3]. Each method has blind spots, and the sources argue about how alien life might hide from all of them. Still, together they turn the old question into a set of measurements that telescopes, spectrometers and lab experiments can actually run.

Table of contents

What They Said

Adam Frank: the question is now empirical

Frank studies how star systems evolve and leads the first NASA grant to search for technosignatures on exoplanets [1]. His starting point is that one half of the ancient debate is already settled. Early in the 20th century, one theory held that planets formed only when two stars nearly collided. That would make planets so rare that you'd expect about one star in a trillion to have them. "Instead, every star in the night sky has planets," he says [1]. Exoplanet science has now filled in two of the Drake equation's seven factors: essentially every star has planets, and about one in five has a planet in the habitable zone [1].

Frank thinks this changes the odds a great deal. In a 2016 paper with Woody Sullivan, he set aside the unknown lifetime of civilizations and asked a narrower question: has any technological civilization besides ours ever arisen? Cosmic history has held about 10 billion trillion habitable-zone planets, each one an experiment. So humanity is the only civilization ever only if the chance per planet is below one in 10 to the 22 [1]. He calls this the pessimism line and describes it as "sort of the plank length of astrobiology": "As long as you're above 10 to the minus 22, then you actually, absolutely, it has occurred in the, in the, in the history" [1]. That doesn't put anyone nearby, but it shifts the burden of proof onto the pessimists [1]. For him, the biggest unknown left is abiogenesis: "The only open question to me is like, how hard is it for life to originate?" His answer to that is to go and look: "we don't know unless we look" [1].

Why atmospheres are the target

Frank explains why the habitable zone matters for detection even though it's too strict as a definition of where life could exist. Europa sits far outside the zone yet has a hundred-mile-deep ocean. But "I wouldn't be able to see life on Europa because it's under 10 miles of ice," he says. Habitable-zone planets are expected to have atmospheres, and "Atmospheres are the things we can characterize for across 10, 50 light years" [1]. The James Webb Space Telescope's real strength, he stresses, is spectra rather than pretty pictures. Its data on K2-18b, about 120 light years away, suggest a possible Hycean world: a hydrogen atmosphere over a warm ocean, with methane detected at five sigma [1].

Biosignatures: what rock and air won't do

His classic biosignature example is oxygen together with methane. The two react with each other and disappear quickly unless something keeps replacing them [1]. In general, he says, for a biosignature "I'm looking for things that a geosphere" can't make, things that "rock and air wouldn't produce on its own" [1].

Frank also sees the limits of this approach. Each known biosignature assumes life works the way it does on Earth. So the field is moving toward what he calls agnostic signatures, which ask a broader question: "What is it in general that life does that leaves an imprint?" [1] He gives three options:

• Chemical networks. Biology always produces "very different chemical networks who's reacting with who, then just rock and water," so the pattern of reactions itself could be a signature [1].
• Information content. Researchers could measure the information in the compounds in an atmosphere, where the finding might be "oh, there's too much information here. There must have been biology happening. It's not just rock" [1].
• Disequilibrium. "One way to define life is it is a system that is far from equilibrium," Frank says. A living thing stays out of equilibrium, but "as soon as it dies, it turns into, it goes back to equilibrium" [1]. The same logic scales up to a planet. Even without knowing which chemicals are involved or whether they have anything to do with life, scientists can use the Gibbs free energy to measure how far an atmosphere is from the state its chemicals would settle into if they all reacted away. In his words, "if you see an atmosphere that is wildly out of equilibrium, that indicates that there's something happening on that planet, biosphere or technosphere, that is pumping gases into the atmosphere, that is keeping the whole system from relaxing" [1].

He admits that spotting something that "doesn't look right" is "really difficult." It's an information-theory problem: "You really have to use information theory to say, like, what's the background?" Alien technology could also be "really weird stuff" that we don't think to look for, and "That's why there's this emphasis now on these agnostic signatures" [1].

Technosignatures: a library of alien industry

Frank prefers "technosignatures" to "SETI" because the search is for technology, not intelligence. Exoplanets also make it possible to look for passive signs of a civilization going about its business instead of waiting for a deliberate beacon [1]. His NASA-funded team is building the first library of technosignatures [1]. The list includes:

• Industrial pollution. His group published a paper showing that chlorofluorocarbons (CFCs) could be detected across interstellar distances. You "look at the light coming from a distant planet, pass the light through a spectrograph and see the spectral lines, the fingerprint, the spectral fingerprint of chlorofluorocarbons in an atmosphere." That would settle it, "Because there's no other way to make chlorofluorocarbons except through some kind of industrial process" [1]. He notes that this one is not agnostic, because we know to look for it only because we made CFCs ourselves [1].
• Solar panels. Large arrays of collectors would leave a characteristic reflection signature [1].
• City lights. Sodium lamps reflecting off the ground could be detected if the city covered the whole planet, like Coruscant in Star Wars [1].
• Radio. A study of how Earth would look to an observer with our own level of technology found that radio is still our most detectable signature [1].
• Megastructures and waste heat. Freeman Dyson proposed that advanced civilizations would capture their star's energy with a swarm of orbiting collectors, and he proposed it precisely because the waste heat would show up in infrared. Jason Wright's group has searched for this and found a few anomalous candidates, which they carefully call "not non-detections" [1]. A dense belt of satellites in geosynchronous orbit, a Clarke belt, is another possible sign [1].
• Propulsion plumes and mining dust. To find civilizations using a whole star's worth of energy (Kardashev type two), Frank has just proposed to NASA a search for the plumes of fusion-drive ships constantly accelerating between planets. Others suggest looking for dust from asteroid mining [1].
• Artifacts in our own backyard. His team calculated that micrometeorite gardening would take hundreds of millions to billions of years to erase a lander on the Moon. That makes it worth scanning lunar maps with machine learning for ancient alien hardware; a test run found an Apollo landing site straight away [1].

Frank also points to future instruments. He's thrilled by the proposed solar gravitational lens telescope, which would send telescopes about 500 times Earth's distance from the Sun to use the Sun's bending of spacetime as a giant lens. It could image an exoplanet at about 24-kilometer resolution, enough to see Manhattan [1].

Standards of evidence

Throughout, Frank insists on strict proof. He supports open scientific study of UFOs and UAPs, but says no data so far comes close to supporting claims of non-human technology. Scientists are "really mean to each other" about evidence, and if his own team announced a technosignature, they'd expect to be shredded: "You don't get a pass because it's a really cool topic" [1]. He describes what a real UAP study would need: calibrated ground detectors like those used for meteors, satellite sensors, and alert systems on aircraft that trigger data collection, with every instrument fully characterized and placed according to a rational strategy [1]. Given a trillion dollars, though, he'd spend it looking at alien planets: if you want to find Nebraskans, go to Nebraska [1].

Sara Walker: redefine life so you can measure it

Walker, an astrobiologist and theoretical physicist, starts one step earlier than Frank. Before we can detect life, she argues, we need a definition that holds up. She takes apart the standard astrobiology definition, life as a self-sustaining chemical system capable of Darwinian evolution, and says every word of it is wrong [2]. Nothing is self-sustaining: humans depend on societies. Life isn't inherently chemical: it shows up first in chemistry because chemistry is the first combinatorial space the universe can't exhaust. And Darwinian evolution happens to populations, not individuals [2]. Her replacement is "life is the process of how information structures matter over time and space" [2].

This matters for detection because it shifts attention from the organism to its history. Living things, Walker says, are among the biggest structures in the universe, but they are big in time rather than in space. "The universe is far larger in time than it is in space," and "the modern technosphere is the largest object in time in the universe that we know about" [2]. On this view, an alien signature is evidence that a long causal history has been compressed into matter.

Assembly theory's two observables

Assembly theory, which Walker developed with Lee Cronin, puts numbers on that idea. It starts from the observation that ordinary objects, from computers to pens, exist nowhere else in the known universe and needed a long evolutionary history to exist here [2]. It measures two things:

• Copy number: how many identical copies of an object exist.
• Assembly index: the minimum number of recursive steps needed to build the object from basic building blocks [2].

Cronin's team has shown that in organic chemistry, molecules above roughly 15 steps are found in high abundance only when life made them, though the threshold differs in other chemical spaces [2]. One property of the space helps: there is no long way home. The number of possible objects grows exponentially with assembly index, so real objects come into existence mostly along near-shortest paths close to what already exists [2].

Origin of life and first contact are the same problem

Walker works on life detection in the solar system and on biosignature planning for the Habitable Worlds Observatory [2]. She believes solving the origin of life and finding alien life are the same problem, predicting that "the first contact with alien life will actually be in an origin life experiment" [2]. Her own lab studies the transition. Among small molecules with fewer than about seven to 11 heavy (non-hydrogen) atoms, almost none are chiral (have a left- or right-handed form); above that threshold, almost all are. Her lab's work suggests that life's origin happens around the complexity at which chiral molecules become very likely, which sets off molecular recognition and self-reinforcing (autocatalytic) feedback [2]. She now thinks, for the first time in her career, that the origin of life is very abrupt rather than gradual [2]. She points to an example from Cronin's lab: a large molybdenum ring of perhaps around 150 atoms that templates its own production, the first non-organic autocatalytic set. Out of an exponentially large space of possibilities, the system collapses onto one mutually reinforcing structure [2].

The warning: a "great perceptual filter"

Walker adds a sobering caveat. As a biosphere packs more and more time into the same physical space, it may become so virtualized and abstract that it can't perceive other such structures. She calls this a "great perceptual filter," by analogy with the great filter [2]. She cites Paul Davies' idea that advanced intelligence might become a dark quantum computer in space, and Freeman Dyson's idea of analog life running exponentially slower to persist forever. She even speculates that sufficiently advanced technology might look like black holes, though she stresses she isn't sure she agrees with the idea [2]. Her grounding point is that humans didn't recognize microbes, which are everywhere and closely related to us, until we invented microscopes [2]. She also describes a student project that models alien signals on fireflies, based on a colleague's theory that firefly species evolve flash patterns that differ from each other as much as possible. The idea is that an alien beacon might stand out against a background of pulsars the same way [2].

Lee Cronin: build the life meter

Cronin, a chemist at the University of Glasgow, turns assembly theory into a working instrument. The Nature paper he wrote with Walker, "assembly theory explains and quantifies selection and evolution," set off a storm of criticism from evolutionary biologists, complexity theorists, physicists and chemists [3].

The core logic

Cronin's short version is that the history is in the objects [3]. The assembly index counts the minimum steps to rebuild an object, and anything already built can be reused at the cost of one step. With Lego, once you've made a particular offset piece, you can reuse that motif. For molecules, the building blocks are atoms and the steps are bonds [3]. Copy number is what makes the index a test for life. As Cronin puts it, "the ultimate randomness and ultimate complexity are indistinguishable until you can see a structure in the randomness" [3]. A single complex object could be a random fluke. But a million identical copies of a molecule with a hundred distinct parts could never arise by chance, even across an infinite set of universes [3]. Many identical copies of a complex object mean some kind of factory, meaning selection, produced them.

He describes four nested spaces. The assembly universe is where anything goes. The assembly possible is where physical laws like chemical bonding apply. The assembly contingent is where you can't reach complex objects until a causal chain has built their precursors. The assembly observed is what we actually see [3]. The key step is from possible to contingent: "memory" in the universe is a real physical process, such as a cell or a factory, that directs construction [3].

How to measure it in practice

Computing the index gets very hard for big molecules, so his lab found physical shortcuts. Mass spectrometry, which breaks molecules into fragments, infrared spectroscopy and NMR each correlate with the calculated index [3]. For a mission to Mars or anywhere else, Cronin would bring a high-resolution mass spectrometer. On Earth, he says, molecules above about 350 molecular weight or with more than 15 fragments are made only by life, and skeptics are free to raise the threshold [3]. He and Lex Fridman playfully design a Lex-and-Lee life meter. It has a nose that sucks up volatile chemicals, separates them by chromatography, and uses mass spectrometry and infrared to measure complexity on a scale from dead, through Earth life, up to alien intelligence [3]. Because planetary surfaces are harsh, he'd drill into soil that is billions of years old [3]. The big advantage, he argues, is that the method has no Earth-chemistry or Earth-biology bias, because "complexity in abundance is evidence of selection" [3].

The blind test

The method has already passed one test. Cronin's lab ran a set of samples: a peaty Scotch whisky from Islay, a lowland whisky, vodka, E. coli, beer yeast, taxol, and five blind samples from NASA, including fossils, Antarctic sediment and a meteorite. Biological samples produced molecules with an assembly index above 15, and abiotic ones stayed below. When the team told NASA which samples were living and which were dead, NASA confirmed they were right [3]. (The peatier the whisky, Cronin jokes, the higher the assembly index [3].) His lab has also rebuilt the tree of life from mass spectrometry alone, with no gene sequencing, by fingerprinting the most complex molecules in samples and comparing their joint assembly spaces. He hopes to apply this to old fossils whose DNA is gone [3].

He also describes watching complexity rise. A Petri dish of simple food with one E. coli cell starts with low total assembly, and as the bacteria eat and multiply, the amount of assembly rises in an S-curve. His lab is trying to get molecular networks to produce complexity the same way from raw chemicals [3].

Cronin's loneliness problem

Like Walker, Cronin sees a limit to how well any single test can work. He thinks life is probably common around other stars, but each planet's contingent history may be so different that our causal cones never overlap: "my biggest fear in a way is that life is everywhere but we become infinitely more lonely" [3]. His answer is to build a planet simulator, create alternative life in the lab, learn which kinds of worlds are most likely to overlap with ours, point telescopes there, and search incoming signals for copy-number structure [3].

Where They Agree, and Where They Don't

They agree the question is now testable. Frank says data is coming within decades [1]. Walker plans biosignature work for the Habitable Worlds Observatory [2]. Cronin has already run blind tests for NASA [3].

They agree Earth-specific signatures aren't enough. Frank's field is moving to "agnostic signatures" (chemical networks, excess information, disequilibrium) because the assumptions behind CFCs or oxygen-plus-methane may not hold elsewhere [1]. Cronin sells assembly theory as having no Earth-chemistry bias [3]. Walker's definition of life as information structuring matter is designed to cover life that isn't chemical at all [2]. Frank even lists excess information in atmospheric compounds as a candidate signature, which is close to assembly theory's idea that too much structure implies a factory [1][3].

They agree life and its setting can't be separated. Frank quotes Walker and David Grinspoon: "Life doesn't happen on a planet. It happens to a planet" [1]. Walker's lineages and technosphere, and Cronin's contingent assembly spaces, say something similar: what you find depends on a planet's whole history [2][3].

They differ on where to look and what to measure. Frank's methods are remote: spectra of distant atmospheres, infrared excess, reflected light, radio [1]. Assembly theory as described is mostly done with samples in hand: mass spectrometers on Mars or drilling into soil [3]. Cronin does connect it to telescopes, proposing to search incoming signals for copy-number structure [3], and Walker to biosignature planning [2]. But the sources don't show how an assembly index could be read from a spectrum light years away.

They differ on whether we'd recognize what we find. Frank is fairly confident that the right instruments, strict standards and libraries of signatures can work. He also warns that "alien minds could be so unbelievably different" that they might not recognize our questions [1]. Walker goes further with her "great perceptual filter," suggesting advanced life might be imperceptible in principle, though she holds the idea loosely [2]. Cronin's fear is that life is everywhere but so historically distinct that we're lonely anyway [3].

Assembly theory is contested. The Nature paper offended evolutionary biologists, some complexity theorists, physicists who believe initial conditions explain everything, and prebiotic chemists who wanted more chemistry [3]. A common charge is that it's just Kolmogorov complexity, a measure of how far a data set can be compressed. Cronin's reply is that Kolmogorov complexity assumes a computer and is about compression, while assembly theory infers from the object itself how deep it is in time and how big a factory was needed to make it [3]. An earlier assembly paper was rejected after six rounds of review, with one chemist accusing the team of fraud [3]. Frank's own caution applies here too: no method gets a pass because the topic is exciting [1].

Why It Works

Disequilibrium is the common thread. Living systems keep themselves far from equilibrium and slide back to it when they die [1]. A planet whose gases haven't relaxed, or a pair like oxygen and methane that should destroy each other, points to something continually pumping gases into the atmosphere [1]. Gibbs free energy puts a number on that gap without knowing what the chemicals are [1].

Industrial chemistry has no natural source. CFCs only come from industry, so their spectral fingerprint would be proof, not just a hint [1].

Physics doesn't let you hide waste heat. Frank notes that Kardashev's energy-use scale ignored the second law of thermodynamics: using energy produces waste heat [1]. That is exactly why Dyson proposed his swarms, since the heat would glow in infrared [1].

Copy number turns complexity into evidence. One complex molecule could be luck. Many identical copies can't be, because the space of possible molecules is enormous: Cronin's example is that making every three-dimensional arrangement of taxol's molecular formula, at a molecular weight of about 853, would fill about 1.5 universes [2]. So a high assembly index combined with high copy number means a process that remembers how to build things, which is what selection is [3]. Selection happens when objects persist in environments that destroy others, and it gets interesting when they are continually made and destroyed, as when a molecule helps catalyze its own formation [3]. Given two self-copying molecules in a destructive environment, the one needing five steps outcompetes the one needing ten, which is why the shortest path matters [3].

The threshold is empirical, and it's shown up more than once. The roughly 15-step cutoff comes from measurements, confirmed in the blind NASA samples [2][3]. Walker's chirality result, with almost nothing chiral below about seven to 11 heavy atoms and almost everything chiral above, points to a similar complexity threshold for when life starts [2].

Where It Applies, and Where It Doesn't

• Atmospheric biosignatures apply to planets with atmospheres, which mostly means habitable-zone worlds. They miss subsurface oceans like Europa's, sealed under ice [1].
• Earth-analog signatures (CFCs, sodium lamps, oxygen with methane) are strong but narrow. They only find life or technology that resembles ours, which is why the field is adding agnostic tests [1].
• Some technosignatures need extreme scale. City lights are detectable only for a planet-covering city [1]. Dyson swarms and fusion plumes assume Kardashev type-two energy use [1]. Frank also warns that we live in a great acceleration and may wrongly assume technology always advances [1].
• Our own records are thin. Frank and Gavin Schmidt showed that even a 10,000-year industrial civilization would leave little obvious trace after 100 million years, and only a targeted search of isotopes in rock layers might reveal it [1]. A planet's past civilizations could be invisible to casual inspection.
• Assembly theory's threshold is domain-specific. Fifteen steps holds for organic chemistry, but the number differs in other spaces [2]. What counts as "identical" also depends on your detector: mass spectrometry can't tell left- and right-handed molecules apart [3]. Samples need to be in hand, or drilled out of protected soil [3].
• The search so far is tiny. Jason Wright's team found that if the search space were an ocean, SETI has looked at a hot tub's worth of water [1]. Silence so far means almost nothing.
• Recognition may fail entirely if Walker's perceptual filter or Cronin's non-overlapping causal cones are real [2][3].

What To Do With It

1. Read atmospheres for chemistry that shouldn't be there. Treat any gas mix that rock and air can't sustain, such as oxygen together with methane, as a lead to follow up [1].
2. Measure disequilibrium, not just specific molecules. Use Gibbs free energy to score how far an atmosphere sits from chemical equilibrium, whatever the chemicals are [1].
3. Build and use a technosignature library. Check exoplanet spectra and light for CFCs, solar-panel reflections, city lighting, radio, infrared waste heat and fusion-drive plumes, as Frank's NASA team is cataloguing [1].
4. Search close to home. Scan lunar maps with machine learning for artifacts that micrometeorites would take hundreds of millions of years or more to erase [1].
5. Bring a mass spectrometer when sampling other worlds. Drill below harsh surfaces, fragment the molecules, and look for many identical copies of molecules above an assembly index of about 15 (or above about 350 molecular weight or 15 fragments), raising the bar if skeptics want [3].
6. Validate blind. Follow Cronin's NASA test: mix living, dead, fossil and meteorite samples, classify them without knowing which is which, then check the answers [3].
7. Treat origin-of-life labs as detection labs. Walker expects first contact in an origin-of-life experiment, and Cronin wants planet simulators that show which alien worlds are worth pointing telescopes at [2][3].
8. Demand calibrated, characterized data. Hold every claim, including your own, to the standard Frank describes, with fully characterized instruments, and expect to be shredded [1].

Sources

1. Alien Civilizations and the Search for Extraterrestrial Life · Lex Fridman Podcast
2. Sara Walker: Physics of Life, Time, Complexity, and Aliens · Lex Fridman Podcast
3. Lee Cronin: Controversial Nature Paper on Evolution of Life and Universe · Lex Fridman Podcast