The search for life on planets beyond our Solar System is too often depicted as a binary process. One day, so the thinking goes, we’ll be able to directly image an Earth-mass exoplanet whose atmosphere we can then analyze for biosignatures. Then we’ll know if there is life there or not. If only the situation were that simple! As Alex Tolley explains in his latest essay, we’re far more likely to run into results that are so ambiguous that the question of life will take decades to resolve. Read on as Alex delves into the intricacies of life detection in the absence of instruments on a planetary surface.

by Alex Tolley

“People tend to believe that their perceptions are veridical representations of the world, but also commonly report perceiving what they want to see or hear.” [17]

Evolution has likely selected us to see dangerous things whether they are there or not. Survival favors avoiding a rustling bush that may hide a saber-toothed cat. We see what we are told to see, from gods in the sky that may become etched as a group of bright stars in the sky. The post-Enlightenment world has not eradicated those motivated perceptions, as the history of astronomy and astrobiology demonstrates.

There have been some famous misperceptions of life and ETI in the past. Starting with Giovanni Schiaparelli’s perceptions of channels (canali) on Mars, followed by Lowell’s creation of a Martian civilization from whole cloth based on his interpretation of canali as canals. As a consensus seemed to be building that plants existed on Mars, in 1957 and later in 1959 Sinton claimed that he had detected absorption bands from Mars indicating organic matter probably pointing to plant life. These “Sinton bands” later proved to be the detection of deuterium in the Earth’s atmosphere.

I note in passing that Dr. Wernher Von Braun assumed that the Martian atmosphere had a surface pressure 1/12th of Earth’s, based on a few telescopic and spectrographic observations and calculations. This assumption was used in his The Mars Project [23] and Project Mars: A Technical Tale [24], to design the winged landers that were depicted in the movie “The Conquest of Space”. How wrong those assumptions proved!

Briefly, in 1967, regular radio pulses discovered by astrophysicist Jocelyn Bell were thought to be a possible ET beacon, perhaps influenced by the interest in Frank Drake’s initial Project Ozma search for radio signals that was started in 1960. They were quickly identified as emitted by a pulsar, a new degenerate stellar type.

Not to be outdone, nn 1978, Fred Hoyle and Chandra Wickramasinghe published their popular science book – Lifecloud: The Origin of Life in the Universe [5]. In the chapter “Planets of Life”, they made the inference that the spectra they observed around stars most closely matched cellulose (a macromolecule of simple hexose sugars, composed of the common elements carbon, hydrogen, and oxygen, and the major material of plants). This became the basis of their claims of the ubiquity of life, panspermia, and cometary delivery of viruses to Earth. Again, this assertion of the ubiquity of life proved to be incorrect based on faulty logical inference, which was in turn based on the incorrect interpretation of cellulose as the molecule identified from the light. It remains a cautionary tale.

Figure 1. A) Illusory lines between objects are interpreted as canals. – Source [16] B) The assumption that lichen-like plants were abundant and produced teh dark areas on Mars. Source [16] C) The spectral fit that convinced Hoyle and Wickramasinghe that they had detected cellulose around other stars. – Source [5] D) View of a purported “fossil” in the famous Mars meteorite, Allan Hills 84001. Doubters argue that the feature is too small to be a sign of Mars life. (Image credit: NASA).

The latest possible misinterpretation may be the results of the Hephaistos Project that claimed 7 stars had anomalous longer wavelength intensities that might indicate a technosignature of a Dyson sphere or swarm [19]. Almost immediately a natural explanation appeared suggesting a data contamination issue with a distant galaxy in the same line of sight.

These historical observational misinterpretations are worth bearing in mind, as the odds are we’ll find life beyond our Solar System, if only because of the vast number of planets that have conditions that could bear life on their surfaces, perhaps even an “Earth 2.0” We will be restricted to data from the electromagnetic (em) spectrum with no hope of acquiring the ground truth from probes sent to those systems.

Returning to the early search for life, the limitations of 1960s technology optical astronomy were highlighted in the US publication of Intelligent Life in the Universe by Iosif Samuilovich Shklovsky, with additional content by Sagan [4], as well as by earlier papers by Sagan [3]. Carl Sagan noted that hypothetical Martian astronomers would not be able to confirm the detection of life and intelligence on Earth using the many terrestrial techniques available at the time, and highlighted some of the issues, including the resolution needed to detect human artifacts, and the ambiguity of spectral data. He asserted that Martian astronomers required ground truth, i.e. a probe in Earth’s orbit or a lander.

From the paper [3]

Moreover, with Earth at 1 km resolution “no seasonal variations in the contrast of vegetation could be detected…. [I]t is estimated that better than 35 m resolution, with global coverage, would be needed to detect life on a hypothetical Earth with no intelligent life.

The best telescopic resolution based on a hypothetical solar gravitational line telescope (SGL) would be far too low to detect life without ambiguity. A simulated image is shown in Figure 2 below.

Figure 2. A 1024×1024 pixel image simulation of an exoplanet at a distance of up to 30 parsecs, imaged with a possible solar gravitational lens telescope, has a surface resolution of a few 10s of km per pixel, Credit: Turyshev et al., “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission,” Final Report NASA Innovative Advanced Concepts Phase II. – Source [26]

Given the limitations outlined by Sagan and Shklovskii, how might we detect that exoplanet life soon? In an influential 1967 paper, “Life Detection by Atmospheric Analysis”, Hitchcock and Lovelock argued that a living planet would create disequilibria in the atmospheric gases of the planet [1].

“Living systems maintain themselves in a state of relatively low entropy at the expense of their nonliving environments. We may assume that this general property is common to all life in the solar system. On this assumption, evidence of a large chemical free energy gradient between surface matter and the atmosphere in contact with it is evidence of life. Furthermore, any planetary biota which interacts with its atmosphere will drive that atmosphere to a state of disequilibrium which, if recognized, would also constitute direct evidence of life, provided the extent of the disequilibrium is significantly greater than abiological processes would permit. It is shown that the existence of life on Earth can be inferred from knowledge of the major and trace components of the atmosphere, even in the absence of any knowledge of the nature or extent of the dominant life forms. Knowledge of the composition of the Martian atmosphere may similarly reveal the presence of life there.”

This proxy has become the core approach for searching for biosignatures of exoplanets, as we are rapidly evolving the technology to detect gas mixtures via transmission spectroscopy of these worlds.

We have no hope of sending probes to those worlds within the foreseeable future, and the speed of light limits when we could even receive information from a probe that landed on such a planet. Therefore, unlike our system, where samples can be both locally analyzed or returned to Earth, only remote observation is currently possible for exoplanets.

For terrestrial worlds like our contemporary Earth, the signature would be the presence of both oxygen (O2) from oxygenic photosynthesis and methane (CH4) from methanogenic bacteria or archaea. So strong is this idea, that my first post on Centauri Dreams was to review a paper that discussed what the atmospheric biosignature would be for an early Earth before photosynthesis had made O2 the 2nd most common gas in the atmosphere, a period that encompassed most of Earth’s history [2].

However, there are increasing concerns from the astrobiology community about this approach because of possible false positives. For example, O2 could be entirely generated by photolysis of water, and even CH4 might be sufficiently produced by geologic means to maintain that disequilibrium, creating false positives or at best ambiguity of the spectral analysis as a biosignature.

Astrobiologists have continued to explore other possible biosignatures, usually with terrestrial life as the template. For example, Sara Seager published a catalog of small, detectable molecules that included some that are only made by life forms. This included phosphine (AKA phosphane, PH3). In 2021, Greaves reported that PH3 had been discovered in the spectra of Venus’s atmosphere [27]. As PH3 is principally produced by life on Earth, this set off a flurry of observations, experiments, and even a soon-to-be-launched probe to the temperate zone of the Venusian atmosphere. Unfortunately, in this case, confirmation of the signal was not made. It was also suggested that the very similar spectral signature of sulfur dioxide was the culprit. The original observation remains controversial, but at least we will eventually get the ground truth we need. But as we will see later, there is a theoretical abiotic route to PH3 production via Venusian volcanic emissions which adds ambiguity to the finding as a biosignature.

In 2018, Sara Walker published a long paper on the issue of dealing with false positives for any biosignature [7]. Much of the paper relied on the use of Bayesian statistical methods, although a potential flaw was the issue of assigning the prior probabilities. Much of the paper dealt with data that would need some sort of sample, even ground truth, such as a sample taken by an in situ probe, as originally suggested by Sagan and Shklovskii. Purely electromagnetic spectrum (em) data would exclude these sample analyses.

A 2021 paper by Green et al [8] suggested that the detection of life should be viewed on a scale of increasing certainty, rather than a binary true or false determination. In other words, ambiguity was to be encompassed:

“The Community Workshop Report argues (with reasonable grounds) that the first detection of an extraterrestrial biosignature will likely be ambiguous and require significant follow-on work.”

They suggested a new Confidence of Life Detection scale (CoLD), to indicate reliability based on NASA’s Technology Readiness Level (TRL) approach: Confidence is increased with confirmation and as abiotic causes are ruled out. A modified chart is shown in Figure 3.

Figure 3. CoLD scale. Printed with permission in the Vickers et al paper.

At about the same time, NASA sponsored a biosignature workshop [9] to assess and report on life detection to address the recognized problems of reliability of life detection as an immature science. Their effort was especially targeted at communicating a possible life detection. It should be noted that NASA did a very poor job of this in the past, notably the press conferences on the “Martian microbes” from the ALH84001 meteorite in 2006, and the arsenic-based bacteria in 2010, billed as a possible different alien biology that could exist on another world [28]. Nasa clearly wanted to avoid such premature announcements and tread a more cautious approach. [In an ironic twist, researchers have discovered arsenic metabolism in some deep sea marine microbes [29].

This has gained importance because of the increasingly attention-grabbing approach of articles on the discovery of exoplanets that resemble Earth, often using the “Earth 2.0” label. It is highly unlikely any exoplanet with similar dimensions and orbits in the habitable zone (HZ) is a terrestrial-type verdant world suitable for eventual colonization if or when our starships can reach them.

The workshop produced this Standards of Evidence scale published in 2022:

Table 1. Standards of Evidence Life Detection scale, produced by a community wide Effort [9] Credit: Walker et al [14]

In 2023, Smith, Harrison, and Mathis published an extensive critique of the reliability of biosignatures for life detection. In their essay [10] they state in the abstract that:

“Our limited access to otherworlds suggests this observation is more likely to reflect out-of-equilibrium gases than a writhing octopus. Yet, anything short of a writhing octopus will raise skepticism about what has been detected.”

In the introduction, they state that atmospheric gas disequilibria are byproducts of life on Earth, and not unique, for example, abiotic production of O2 in the atmosphere. The following includes a critique of the Krissansen-Totton et al paper that I sourced in my first Centauri Dreams post:

“Often these models don’t rely on any underlying theory of life, and instead consider specific sources and sinks of chemical species, and rules of their interactions. Astrobiologists label these sources, sinks, or transformations as being due to life or nonlife, tautologically defined by the fluxes they influence. For example, defining life via biotic fluxes of methane from methanogenesis [6,7] and defining abiotic fluxes via rates of serpentinization and impacts.”


“This leads to the conclusion that most exoplanet biosignatures are futile if our goal is to detect life outside the solar system with confidence.”

Figure 4 below shows the 4 approaches they suggest to firm up the strength of biosignatures.

1. Biological research on terrestrial life,

2. Looking for life with probes sent to planets in our system

3. Experimental work on abiogenesis and molecular outcomes

4. SETI via technosignatures

Figure 4. The 4 approaches to explore biosignatures. Credit: Smith, Harrison & Mathis.

They conclude:

“As astrobiologists we believe the search for life beyond Earth is one of the most pressing scientific questions of our time. But if we as a community can’t decide how to formalize our ideas into testable hypotheses to motivate specific measurements or observational goals, we are taking valuable observational time and resources away from other disciplines and communities that have clearly articulated goals and theories. It’s one thing to grope around in the dark, or explore uncharted territory, but do so at the cost of other scientific endeavors become increasingly difficult to justify. One of the most significant unification of biological phenomena–Darwin’s theory of natural selection–emerged only after Darwin went on exploratory missions around the world and documented observations. It’s possible the data required to develop a theory of life that can make predictions about living worlds simply has not been documented sufficiently. But if that’s the case we should stop aiming to detect something we cannot understand, and instead ask what kinds of exploration are needed to help us formalize such a theory.”

One month later, Vickers, Peter, et al. published “Confidence of Life Detection: The Problem of Unconceived Alternatives.” [11]. This paper aimed to demolish the idea of using Bayesian probabilities as there was little hope of even conceiving of novel ways life may arise, nor the abiotic mimics of possible signatures.

From the abstract:

“It is argued that, for most conceivable potential biosignatures, we currently have not explored the relevant possibility space very thoroughly at all. Not only does this severely limit the circumstances in which we could reasonably be confident in our detection of extraterrestrial life, it also poses a significant challenge to any attempt to quantify our degree of (un)certainty.”

From the introduction:

“(…) the problem of unconceived abiotic explanations for phenomena of interest……., we stress that articulating our uncertainty requires an assessment of the extent to which we have explored the relevant possibility space. It is argued that, for most conceivable potential biosignatures, we currently have not explored the relevant possibility space very thoroughly at all.”

From section 2 – The challenges of known and unknown false positives:

“As Meadows et al. (2022, p. 26) note, “[I]f the scope of possible abiotic explanations is known to be poorly explored, it suggests we cannot adequately reject abiotic mechanisms.” Conversely, if it is known to be thoroughly explored, we probably can reject abiotic mechanisms.”

In effect, they are reiterating the problems of inference. For example, it was thought since Roman times that all swans were white as no examples of differently colored swans had been seen in the European, Asian, and African continents. This remained the case until black swans were discovered in Australia.

Today the more popular phrase that covers the issue of inference is “Absence of Evidence is not Evidence of Absence”.

They critique Green’s CoLD scale and suggest that the Intergovernmental Panel on Climate Change (IPCC) approach using a 2D scale of scientific consensus and strength of evidence may be more suitable.

Figure 5. IPCC framework for climate as a biosignature confidence scale. Credit: Vickers et al.

As Vickers was sowing doubts about biosignature detection reliability and how best to handle the uncertainties, Sara Walker and collaborators published “False Positives and the Challenge of Testing the Alien Hypothesis” [12] repeating their earlier argument for Bayesian methods, and the need for definitive biosignatures to avoid false positives. Those definitive biosignatures may be based on the Assembly Theory of the composition of organic molecules [13]. I would include the complementary approach, even though it does allow for false positives [14]. However, these approaches require samples that will not be available for exoplanets. Walker ends with the suggestion that the approach of levels (or ladders) of certainty such as the Confidence of Life Detection (CoLD) scale, and the community Standards of Evidence Life Detection scale should be used because we need more data to understand planetary types and life, and that data will improve the probabilities of evaluating the specific probability of life on a planet, especially with the rapidly increasing number of exoplanets.

Whatever the pros and cons of different approaches, it appears that continuing research and cataloging of exoplanets will help narrow down the uncertainties of life detection. Ideally, several orthogonal approaches can be used to triangulate the probability that the biosignature is a true positive.

Sagan was right in that we need the ground truth of close observation and samples to validate electromagnetic data. While ground truth can eventually be acquired for planets in our solar system, we don’t have that for exoplanets, nor will we have that for the foreseeable future, unless that low probability SETI radio or optical signal is detected. In time, with a catalog of exoplanet data, it might be possible to collect enough examples to determine if there are abiotic mimics of different gas disequilibria, or other phenomena like the chlorophyll “red edge”. But we cannot know with certainty, and any abiotic mimic reduces the confidence of biotic interpretations.

Therefore, biosignatures from exoplanets will remain uncertain indications of life. We cannot escape from this. It will be up to the community and the responsible media to make this clear. [And good luck with the media].

As if this issue were not relevant, Payne and Kalteneggar just published a paper indicating that the O2 + CH4 signature is stronger in the last 100-300m than today, principally due to the greater partial pressure of O2 in the atmosphere during that period [15]. It was covered by the press as “Earth Was More Attractive to Aliens Back When Dinosaurs Roamed” [21]. C’est la vie.


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