It’s no surprise, human nature being what it is, that our early detections of possible life on other worlds through ‘biosignatures’ are immediately controversial. We have to separate signs of biology from processes that may operate completely outside of our conception of life, abiotic ways to produce the same results. My suspicion is that this situation will persist for decades, claim vs. counter-claim, with heated conference sessions and warring papers. But as Alex Tolley explains in today’s essay, even a null result can be valuable. Alex takes us into the realm of Bayesian statistics, where prior beliefs are gradually adjusted as new data come in. We’re still dealing with probabilities, but in a fascinating way, uncertainties are gradually being decreased though never eliminated. We’re going to be hearing a lot more about these analytical tools as the hunt continues with next generation telescopes.
by Alex Tolley
Introduction
The venerable Drake equation’s early parameters are increasingly constrained as our exoplanet observations continue. We now have a good sample of thousands of exoplanets to estimate the fraction of planets in the habitable zone that could support life. This last firms up the term ne, the mean number of planets that could support life per star with planets.
This is now a shift to focus on the fraction of habitable planets with life (fl). The first to confirm a planet with life will likely make the history books.
However, as with the failure of SETI to receive a signal from extraterrestrial intelligence (ETI) since the 1960s, there will be disappointments in detecting extraterrestrial life. The early expectation of Martian vegetation proved incorrect, as did the controversial Martian microbes thought to have been detected by the Viking lander life detection experiments in 1976. More recently, the phosphine biosignature in the Venusian atmosphere has not been confirmed, and now the claimed dimethyl sulfide (DMS) biosignature on K2-18b is also questioned.
While we hope that an unambiguous biosignature is detected, are null results just disappointments that have no value in determining whether life is present in the cosmos, or do they add some value in determining a frequency of habitable planets with life?
Before diving into a recent paper that attempts to answer this question, I want to give a quick introduction to statistics. The most common type of statistics is Fisher statistics, where collected sample data is used to calculate the distribution parameters for the population from which the sample is taken. This approach is used when the sample size is greater than 1 or 2, and is most often deployed in calculating the accuracy of a mean value and 95% range of values as part of a test of significance. This approach works well when the sample contains sufficient examples to represent the population. For binary events, such as heads in a coin test, the Binomial distribution will provide the expected frequencies of unbiased and small biases in coin tosses.
However, a problem arises when the frequency of a binary event is extremely low, so that the sample of events detects no positive events, such as heads, at all. In the pharmaceutical industry, while efficacy of a new drug needs a large sample size for validity, the much larger phase 4 marketing period is used to monitor for rare side effects that are not discoverable in the clinical trials. There have been a number of well known drugs that were withdrawn from the market during this period, perhaps the most famous being thalidomide and its effects on fetal development. In such circumstances, Fisherian statistics are unhelpful in determining probabilities of rare events with sample sizes inadequate to catch these rare events. As we have seen with SETI, the lack of any detected signal provides no value for the probability that ETI exists, only that it is either rare, or that ETI is not signaling. All SETI scientists can do is keep searching with the hope that eventually a signal will be detected.
Bayesian statistics are a different approach that can help overcome the problem of determining the probability of rare events, one that has gained in popularity over the last few decades. It assumes a prior belief, perhaps no more than a guess, of the probability of an event, and then adjusts it with new observed data as they are acquired. For example, one assumes a coin toss is 50:50 heads or tails. If the succeeding tosses show only tails, then the coin toss is biased, and each new resulting tail decreases the probability of a head resulting on the next toss. For our astrobiological example, if life is very infrequent on habitable worlds, Bayesian statistics can be informative to estimate the probability of detection success.
In essence, the Bayesian method updates beliefs in the probability of events, given the new observations of the event. With a large enough number of observations, the true probability of an event value will emerge that will either converge or diverge from the initial expected probability.
I hope it is clear that this Bayesian approach is well-suited to the announcement of detecting a biosignature on a planet, where detections to date have either been absent or controversial. Each detection or lack of detection in a survey will update our expectations of the frequency of life. At this time, the probability of life on a potentially habitable planet ranges from 0 (life is unique to Earth) to 1.0 (some form of life appears wherever it is possible) Beliefs that the abiogenesis of life is extremely hard due to its complexity push the probability of life being detected as close to 0. Conversely, the increasing evidence that life emerges quickly on a new planet, such as within 100 million years on Earth [6], implies that the probability of a habitable planet having life is close to 1.0.
The Angerhausen et al paper I am looking at today (citation below) considers a number of probability distributions depending on beliefs about the probability of life, rather than a single value for each belief. These are shown in Figure 1 and explained in Box 2. I would in particular note the Kerman and Jeffreys distributions that are bimodal with the highest likelihoods for the distributions as the extremes, and reflect the “fine tuning” argument for life by Kipping et al [2] explained in the Centauri Dreams post [3] i.e., either life will be almost absent, or ubiquitous, and not some intermediate probability of appearing on a habitable planet, In other words, the probability is either very close to 0 or close to 1.0, but unlikely to be some intermediate probability. The paper relies on the Beta function [Box 3] that uses the probability of positive and negative events defined by 2 parameters for the binary state of the event, e.g. life detected or not detected. This function can approximate the Binomial distribution, but can handle the different probability distributions.
Figure 1. The five different prior distributions as probability density functions (PDF) used in the paper and explained in Box 2. Note the Kerman and Jeffreys distributions that bias the probabilities at the extremes, compared to the “biased optimist” that has 3 habitable worlds around the sun (Venus, Earth, and Mars), but with only the Earth having life.
The Beta function is adjusted by the number of observations or positive and negative detections of biosignatures. At this point, the positive and negative observations are based on the believed prior distributions which can take any values, from guesses to preliminary observational results, which at this time are relatively few. After all, we are still arguing over whether we have even detected biosignature molecules, let alone confirmed their detection. We then adjust those expectations by the new observations.
What happens when we start a survey and gain events of biosignature detection? Using the Jeffreys prior distribution, let us see the effect of observing no biosignature detections for up to 100 negative biosignature observations.
Figure 2a. The effect of increasing the null observations on a skewed distribution that shows the increasing certainty of the low probability frequencies. While apparently the high probabilities also rise, the increase in null detections implies that the relative frequency of positives declines.
Figure 2b. The increasing certainty that the frequency of life on habitable planets tends towards 0 as the number of null biosignature detections increases. The starting value of 0.5 is taken from the Jeffreys prior distribution. The implied frequency is the new frequency of positives as the null detections reduce the frequency observed and push the PDF towards the lower bound of 0 (see figure 1)
So far, so good. If we can be sure that the biosignature detection is unambiguous and that the inference that life is present or absent can be inferred with certainty based on the observations, then the sampling of up to 100 habitable worlds will indicate whether life is rare or ubiquitous and can be determined with high confidence. If every star system had at least 1 habitable world, this sample would include most stars within 20 ly of Earth. In reality, if we limit our stars to spectral types F, G & K, which represent 5-10% of all stars, and half of these have at least 1 habitable world, then we need to search 2000-4000 star systems, which are well within 100 ly, a tiny fraction of the galaxy.
The informed reader should now balk at the status of this analysis. Biosignatures are not unambiguous [4]. Firstly, detecting a faint trace of a presumed biosignature gas is not certain, as the phosphine on Venus and the DMS/DMDS on TOI-270d detections make clear. They are both controversial. In the case of Venus, we are neither certain that the phosphine signal is present and that the correct identification has been made, nor that there is no abiogenic mechanism to create phosphine in Venus’ very different environment. As discussed in my post on the ambiguity of biosignatures, prior assumptions about biosignatures as unambiguous were reexamined, with the response that astrobiologists built a scale of certainties for assessing whether a planet is inhabited based on the contextual interpretation of biosignature data.[4].
The authors of the paper allow for this by modifying the formula to allow for both false-positive and false-negative biosignature detection rates, and also for interpretation uncertainty of the detected biosignature. The authors also calculate the upper bound at about 3 sigma (99.9%) of the frequency of observations. Figure 3 shows the effect of these uncertainties on the location and size of the maximal probability density function for the Jeffrey’s Bayesian priors.
Figure 3. The effects of sample and interpretation, best fit, and 99.9% uncertainties for null detections. As both sample and interpretation uncertainty increase, the expected number of positive detections increases. The Jeffrey prior’s distribution is used.
Figure 3 implies that with interpretation uncertainty of just 10%, even 100 null observations, the calculated frequency of life increases 2 orders of magnitude from 0.1% to 10%. The upper bound increases from less than 10% to between 20 and 30%. Therefore, even if 100 new observations of habitable planets with no detected biosignatures, the frequency of inhabited planets is between ⅕ and ⅓ of habitable planets at this level of certainty. As one can see from the asymptotes, no amount of further observations will increase the certainty that life is absent in the population of stars in the galaxy. Uncertainty is the gift that allows astrobiologists to maintain hope that there are living worlds to discover.
Lastly, the authors apply their methodology to 2 projects to discover habitable worlds; the Habitable Worlds Observatory [7] and the Large Interferometer for Exoplanets (LIFE} concepts. The analyses are shown in figure 4. The vertical lines indicate the expected number of positive detections by the conceptual methods and the expected frequencies of detections with their associated upper bounds due to uncertainty.
Figure 4. Given the uncertainties, the authors calculate the 99.9% ( > 3 sigma) upper limit on the null hypothesis of no life and matched against data obtained by 2 surveys by Morgan with The Habitable Worlds Observatory (HWO) and 2 by Kammerer with The Large Interferometer for Exoplanets (LIFE) [7, 8].
The authors note that it may be incorrect to use the term “habitable” if water is detected, or “living” if a biosignature[s] is detected. They suggest it would be better to just use the calculation for the detection method, rather than the implication of the detection, that is, that the sample uncertainty, but not the interpretation uncertainty, is calculated. As we see in the popular press, if a planet in the habitable zone (HZ) has about an Earth-size mass and density, this planet is sometimes referred to as “Earth 2.0” with all the implications of the comparison to our planet. However, we know that our current global biosphere and climate are relatively recent in Earth’s history. The Earth has experienced different states from anoxic atmosphere, to extremely hot, and conversely extremely cold periods in the past. It is even possible the world may be a dry desert, like Venus, or conversely a hycean world with no land for terrestrial organisms to evolve.
However, even if life and intelligence prove rare and very sparsely distributed, a single, unambiguous signature, whether of a living world or a signal with information, is detected, the authors state:
Last but not least we want to remind the reader here that, even if this paper is about null results, a single positive detection would be a watershed moment in humankind’s history.
In summary, Bayesian analysis of null detections against prior expectations of frequencies can provide some estimate of the upper limit frequency of living worlds, with many null detections reducing the frequencies and their upper limits. Using Fisherian statistics, many null detections would provide no such estimates, as all the data values would be 0 (null detections). The statistics would be uninformative other than that as the number of null detections increased, the expectation of the frequency of living worlds would qualitatively decrease.
While planetologists and astrobiologists would hope that they would observationally detect habitable and inhabited exoplanets, as the uncertainties are decreased and the number of observations continues to show null results, how long before such activities become a fringe, uneconomic activity that results in lost opportunity costs for other uses of expensive telescope time?
The paper is Angerhausen, D., Balbi, A., Kovačević, A. B., Garvin, E. O., & Quanz, S. P. (2025). “What if we Find Nothing? Bayesian Analysis of the Statistical Information of Null Results in Future Exoplanet Habitability and Biosignature Surveys”. The Astronomical Journal, 169(5), 238. https://doi.org/10.3847/1538-3881/adb96d
References
1. Wikipedia “Drake equation” https://en.wikipedia.org/wiki/Drake_equation. Accessed 04/12/2025
2. Kipping & Lewis, “Do SETI Optimists Have a Fine-Tuning Problem?” submitted to International Journal of Astrobiology (preprint). https://arxiv.org/abs/2407.07097
3. Gilster P. “The Odds on an Empty Cosmos“ Centauri Dreams, Aug 16, 2024 https://www.centauri-dreams.org/2024/08/16/the-odds-on-an-empty-cosmos/
4. Tolley A. “The Ambiguity of Exoplanet Biosignatures“ Centauri Dreams Jun 21, 2024
https://www.centauri-dreams.org/2024/06/21/the-ambiguity-of-exoplanet-biosignatures/
5. Foote, Searra, Walker, Sara, et al. “False Positives and the Challenge of Testing the Alien Hypothesis.” Astrobiology, vol. 23, no. 11, Nov. 2023, pp. 1189–201. https://doi.org/10.1089/ast.2023.0005.
6. Tolley, A. Our Earliest Ancestor Appeared Soon After Earth Formed. Centauri Dreams, Aug 28, 2024 https://www.centauri-dreams.org/2024/08/28/our-earliest-ancestor-appeared-soon-after-earth-formed/
7. Wikipedia “Habitable Worlds Observatory” https://en.wikipedia.org/wiki/Habitable_Worlds_Observatory. Accessed 05/02/2025
8. Kammerer, J. et al (2022) “Large Interferometer For Exoplanets (LIFE) – VI. Detecting rocky exoplanets in the habitable zones of Sun-like stars. A&A, 668 (2022) A52
DOI: https://doi.org/10.1051/0004-6361/202243846
I think this is a fine overview of the statistics of sparse results. I have a couple of comments that may be helpful, though they are likely already understood by Alex.
“At this time, the probability of life on a potentially habitable planet ranges from 0 (life is unique to Earth) to 1.0 (some form of life appears wherever it is possible) ”
We should not discard the one definitive positive data point we have. Earth should not be excluded from any analysis since it proves that life exists, and if it exists then the probability of it existing on an exoplanet must be > 0.
There is an interesting and brief paper (2503.18217v1, Whitmire) on applying Bayesian analysis to astrobiology using this one data point. I pulled it when the preprint was new and I have not returned to see its current status. One note is that expected value is inadequate since, as Alex notes, the probability density function is highly informative.
“Figure 2b. The increasing certainty that the frequency of life on habitable planets tends towards 0 as the number of null biosignature detections increases.”
Null detections cannot be certain since the sampling bias is enormous. That is, we can only detect what we have the capability to detect. Further, we look for what we believe are biosignatures, but those may be inadequate objectives and even misleading.
There could be life on many of the exoplanets we’ve observed that we simply incapable of detecting, or detecting with any significant confidence. We have a long way to go in this regard.
Thank you for the Whitmire reference. I hadn’t seen it.
It would be ironic if it turned out that life was most frequent in icy worlds with subsurface oceans. We would have no ability to detect it without directly reaching, landing, and drilling through the icy crust. [I was was just watching a documentary on the acquisition of samples in L. Mercer, a subsurface lake in Antarctica, 1 kilometer below the surface, where they found a rich source of diatoms that were living in the dark (!). ]
I would also note that when Earth was completely frozen over around 650 mya, before the Cambrian, life was extant, albeit mostly unicellular and likely non-complex multicellular life. Unless there were unglaciated areas, there would be little indication of life. At that time, the atmosphere would have appeared to be in equilibrium with high CO2 and CH4, as well as N2. ETI looking for a biosignature might well have concluded that Earth was still abiotic, or had a low probability of life. Any subsurface life, for example, living in the lithosphere, might well leave no biosignatures that could be detected remotely. Should the hopefully upcoming ExoMars mission discover subsurface life on Mars, that would be a local example of the difficulty of detecting life that is not abundant on the surface as it is on Earth today and for much of the last 500 my.
And this is before we even consider life as we don’t know it, requiring very different biosignatures. How would we even think of detecting the silicon-based Horta from the ST TOS “The Devil in the Dark”?
An interesting and new development in coronagraphs using spatial mode (de)multiplexing to rid it of the star’s light and greatly improve contrast of the Earth-sized exoplanets. Could the Quantum-Optimal Coronagraph be utilized on our nearest neighbour, Alpha Centauri, to distinguish between the two stars and image any exoplanets? The current separation between Rigil Kentaurus (A) and Toliman (B) makes it quite challenging to image and discover any exoplanets. Perhaps using a smaller telescope could demonstrate the potential of this type of coronagraph.
https://www.optica.org/about/newsroom/news_releases/2025/turning_down_starlight_to_spot_new_exoplanets/
A good video by the author of the research on Fraser Cain’s YouTube channel:
How Quantum Coronagraphs Will Help Us See Earth-Like Planets Around Sun-Like Stars.
https://www.youtube.com/watch?v=cfdMxauquec
Is there a consensus on the minimal biological criteria to define “life”?
There is also the matter of defining the transition point from abiotic to biotic chemistry
Such considerations may be significant in the case of life as we do not know it.
In some respects, it is ultimately rather like pornography, “I’ll know it when I see it”.
Controlled metabolism, which converts low-entropy energy to higher-entropy energy, seems to be a requirement of terrestrial life. However, if we discovered viruses in the ocean of a planet or moon, we could infer that they would be a biosignature of the life they require to replicate.
Dawkins argues that all life must be subject to Darwinian selection. We might infer that it is the process of evolution that defines the evolved organisms as living. Unfortunately, the geologist Robert Hazen is also suggesting that rock minerals evolve, too, which would blur this idea as rocks are not living.
As we get closer to abiogenesis, I do not think there will be some sharp no-life/life dividing line. It will run from “that is some complex organic chemistry” to “that looks like a living organism”.
Philosophically, now that people are arguing over sentience in AIs, some people are arguing for panpsychism. If rocks have some low-level sentience, does that make them living? If our AIs become sentient, then are they now living in some non-biological sense? [I think a sentient robot, built in a factory, would be like a virus, although the manufacturing ecosystem might well be considered a “living ecosystem”. P K Dick’s autofacs might be considered as organisms.]
Ultimately, we don’t know how to define life. We have biosignatures that indicate life is (or was) present. If samples can be taken, I would just like to see something dividing or wriggling under a microscope. But that might miss life as we don’t know it.
There are a lot of things that are subject to Darwinian selection. Take cars, for example. They have evolved quite a bit for the last 100 years and keep evolving. The fact that they use humans to replicate is no conceptually different from viruses using another organism.
@VIY
I consider technology as co-evolving with humanity, much as flowers and pollinators co-evolve, shaping each other. We both select technologies and evolve their mechanisms and forms, and technologies in turn, shape us, principally culturally, but also our genomes. Culturally, in how our behaviors change with technologies (e.g., people my age use the forefinger to push buttons, like George Jetson, while those who have grown up with handheld devices like games consoles and mobile phones use their thumbs). Through our genomes, as cognition becomes more valuable than strength, selection for more gracile physiques and greater ability to deal with complexity (and paradoxically, reduced brain-to-body ratios, a trait common in domesticated animals).
We are only at the start of this process, and on the cusp of genomic tailoring.
A.T.,
Your co-evolution notion about technology ( such as cars with us humans) is intriguing. I have to wonder about the road ahead, as it were.
Should something cause human extinction or departure from Earth, will cars continue to go about their business – and evolve? Will they strike some kind of a co-existence deal with some other aspect of our infrastructure such as the utility industry – or simply rust and corrode away in place?
Of course, AI subroutines for answering our questions seem to engender such speculations. And if they can someday just make a jump from their static power stations, maybe it will be a moment like 500 years ago when some horses were corralled by the foot-borne inhabitants of the North American plains, dispersed accidentally from Spanish galleons.
On the other hand, this also reminds me of a magazine cartoon of a decade or two ago, of a stalled, dust and cobweb shrouded robot on a rolling platform like that of a vacuum cleaner, holding its electric extension cord in its left manipulator arm ( its other raised in dismay), that and its two visual sensors pointing toward …an out of reach wall electric outlet.
@wdk
Co-evolution often means that the loss of one of the co-evolved species will doom the other. For example, plants that have co-evolved special features with their co-evolved pollinators. If the pollinators go extinct, so do the plants, and vice versa. How serious this is depends on how co-evolved the species are, and whether there are alternative species that have the same role or can do so.
Now, clearly, cars cannot evolve without humans (so far). Indeed, any human artifact entirely relies on human agency. But suppose we do create Dickian autofacs, or that favorite of interstellar expansion, von Neumann replicators. Then, machines become “alive” and are able to take on Darwinian selection to continue evolving without humans. It won’t happen anytime soon, although it may well happen within the next millennium, assuming humans are still around as a highly technological, advancing, species. Again, P K Dick looked at this in Second Variety.
“Should the hopefully upcoming ExoMars mission discover subsurface life on Mars, that would be a local example of the difficulty of detecting life that is not abundant on the surface as it is on Earth today and for much of the last 500 my.”
Alex Tolley on May 23, 2025 at 17:21
I think that there is a key vein of thought here, and Mars is key to it. My take is that we will find life on Mars – everything we know points to it being there, somewhere. However, If we do not, then we are going to be alone.
Moreover, what life looks like, i.e., is it similar to Earth or sufficiently different to suggest a second genesis, will help constrain further our likelihood of coming across “others”.
Yes, the discovery of life on Mars would be an important positive finding.
And it would corroborate Alex’ specific point about – in my words – the difficulty of inferring the presence or absence of life from findings made with only our limited overall perceptual acuity at present.
It’s like we’re looking at coin flips from so far away that we can’t tell for sure which side the coin actually landed on. Such that some “negative” findings don’t signify much more than only an “at least not yet positive” finding.
Hard to infer whether or not the coin is fair from that perceptually fuzzy sample, regardless of its size.
But, particularly if any past or present Martian life is or was in large measure similar to life here, Mars of course then is a special case, given the exchange of material over the – relatively – short distance between the two planets. E.g., meteorites landing on Earth that were produced initially by impact events on Mars, and quite likely, vice-versa.
I do hope that ExoMars beats SpaceX to the punch in providing meaningful findings before Musk starts mucking around up there, especially with permanent human settlements by some projected date certain.
So long as there’s a remaining significant unresolved question as to whether there could be or was life on Mars, I believe there should be some consensus edict along the lines of Clarke’s 2001 where “all these worlds are yours, except Mars, attempt no settlement there (or anywhere else where we haven’t yet conclusively ruled out the existence of current or past life).”
Given past posts and my current project, I’m of course an enthusiast. But not as to settling cosmic bodies – or making fairly active plans to settle such bodies – prior to first concluding sound planetary science. (As well as getting more info re: questions as to the long term viability of an off-world human settlement given the challenges of lower gravity and higher radiation environs.)
* * * * * *
I note that ExoMars’ Rosalind Franklin rover will be using European Space Agency (ESA) Americium-241 radioisotope heating units (RHU). Moving us a step further down the road toward eventual Americium-241 radioisotope thermoelectric generators (RTG) for instead primarily electric power generation. Such eventual Americium-241 RTGs in turn then will free up more deep space missions free of constraints due to at least the limited availability of Plutonium-238 for RTGs. To me, putting these RHUs in space is a big step toward that end.
I watch the development of in particular both the ESA’s Americium-241 RTGs and of diffractive sails in connection with eventual full-sized (rather than micro-craft swarm) sundiver missions out to deep space, like out to that solar gravitational lens trajectory at 542 AU and beyond. I like the promise of both technologies, including how diffractive sails may better tolerate the heat during the close solar approach.
So that particular Americium-241 RHU development specifically for this neighboring Mars mission speaks also “deep space” to me.
@tesh
I wouldn’t be as dogmatic as that. If Mars has fossil or extant life, it may be common to terrestrial life due to undirected panspermia (in either direction). That won’t tell us much about exoplanet life. Conversely, a 2nd genesis on Mars would be quite indicative that life may be ubiquitous, whatever form it takes.
How different would (say) Earth and Mars life have to be to call it panspermia vs 2nd genesis?
My personal (uninformed) guesses:
same species = panspermia (or contamination) for sure;
DNA/RNA/proteins with same nucleotides/amino acids = probable (early) panspermia;
DNA/RNA/proteins with different nucleotides/amino acids = possible 2nd genesis with heavily constrained biochemistry;
No DNA/RNA/proteins = 2nd genesis for sure
As I say, wild guesses…
Chirality, nucleotide three-letter codons, and perhaps other characteristics would identify life from our LUCA and differentiate it from all others. Even with the same nucleotides, amino acids et cetera.
Even if the LUCA was on Mars, it would not vitiate the argument.
Maybe I was not dogmatic enough! If we find no life on Mars (similar or different) we are alone. Finding life there that can be proven to be the “same” but “drifted”, is arguably as important as it being uniquely Martian. That would suggest a uniqueness to the conditions on early Earth that gated emergence of life- likely pointing to a lonely universe.
Finding life unique to Mars, i.e. via another genesis, should be a given as the conditions and the time frame (for those conditions persisting) that brought forth simple life here, are thought to be similar.
Hopefully the next 10 or so years will get to the bottom of this.
@tesh
The surface area of Mars is about the same as all the continental surfaces on Earth. Consider how many people over the last centuries have been scrambling over the Earth looking for fossil bones exposed by the weather, or by human quarrying. A few rovers on Mars are a minuscule effort by comparison. So I wouldn’t bet on a few rovers and10 years.
Suppose Mars has extinct life, but that it was unicellular or even complex, with only soft parts, like life in the Precambrian. How would we even find it? On Earth, fossil stromatolites are one of our very few pieces of direct evidence of early bacterial life. How lucky would we have to be to find such evidence on Mars?
Almost all our knowledge of early life is inferred from current life, and evidence in the rocks of changed chemical states. Without DNA analysis, we can only get the coarsest of hints that early life even existed, and nothing about its biology.
To learn about Martian life, we need its life to be extant. It doesn’t appear to be on the surface. Where might it be in the subsurface? Are there good sites to drill, and if so, how deep? Unless we get lucky and find it within a meter of the surface, we won’t find it in a decade. It may well be that such a discovery will be serendipitous with a human settlement on Mars drilling for water. Unless the settlement has laboratories to do the needed analysis, samples will have to be sent back to Earth, a slow journey, which may not be conducive to survival.
There is no evidence that Mars had Earth-like conditions for most of its history, so I would not expect to find fossil evidence of hard parts of life – shells, bones, exoskeletons, etc. I don’t expect there will be paleontologists fanning out, scouring Mars for signs of extinct life. Unless there are coal beds and possibly oil and gas reservoirs, we won’t find impressions of complex plants, or molecules indicative of prior life.
As NS suggests, with extant life there may be evidence in the biology to determine a 2nd genesis. We would think that an obvious one may be a different genetic code, or DNA/RNA structure. But hold on. Our evidence is based on survivorship bias. All life can be traced back to LUCA. But we don’t know if LUCA or its direct ancestors drove other extant life extinct on Earth. What if that life had migrated to Mars and survived, while LUCA and its descendants never successfully migrated to Mars? Martian life might be a different terrestrial genesis, not a unique Martian genesis.
We might need more evidence, and that may have to come from exoplanets, which we will not get direct samples from for a very long time, if ever, although we might get lucky and get samples from material traveling to our system.
Since we do not know [yet] what conditions resulted in life appearing on Earth, the absence of any evidence of life ever having been on Mars is not very strong evidence that life elsewhere is absent. In the context of the post, there is “interpretation uncertainty”. Even if every potentially habitable environment in our system proved sterile and shown never to have harbored life, that interpretation uncertainty would still allow for the possibility that life exists elsewhere, albeit with diminishing probability.
In summary, that is why I would keep an open mind about life in the universe, rather than assuming that initial evidence for life on Mars, whether fossil or extant, is “definitive” about life elsewhere. It is, however, a subject likely to be argued over, until there is sufficient evidence one way of the other.
Robert Hazen has put forward the idea that minerals evolve by a process of selection. He has classified minerals by the age they first appear. IIRC, the majority of minerals are a result of life processes, but in the Phanerozoic era (last 541 my). While Mars almost certainly never reached that point in evolution, if a geologist discovered minerals from that terrestrial era on Mars, that would be an interesting biosignature.
Cleaves and Hazen have also done work using machine learning to identify whether a mixture of organic molecules is biogenic or abiogenic. I posted an article about their work on CD Alien Life or Chemistry? A New Approach.
“In summary, that is why I would keep an open mind about life in the universe, rather than assuming that initial evidence for life on Mars, whether fossil or extant, is “definitive” about life elsewhere. It is, however, a subject likely to be argued over, until there is sufficient evidence one way of the other.”
I agree with this but cannot escape the notion that failure to detect life on Mars (within, arbitrarily say, 10 years of humans setting foot on Mars) will be a mark against the odds of life as we know it being common. It would suggest to me that things don’t just have to be right the have to be exactly right for life to emerge and get a stranglehold on a planet.
This is a very valuable post. A factor not included either in the “Drake Equation” or in this discussion is the determination of relevance. Would the presence of microbial, subterranean, exotic, or other non-technical, non-communicating, non-exploring forms of life have anything other than philosophical significance to humankind? My perspective is that it is very dangerous indeed to consider that our Earth and/or our civilization is anything other than unique and exceedingly precious. Human activity on this planet, as historically and presently conducted has been proven an existential threat to at least our civilization, probably to our species and possibly to life on the planet. We had best change our ways…
Just as terrestrial life has both scientific and commercial value, even microbial life would likely offer the same value to humanity, as evolution may have explored more ways of living. Exoplanets with complex life would both provide examples of comparative evolution and also likely indicate the limitations of viable genetic engineering of life forms in natural environments. If we found an extinct technological civilization, we might even find how life evolved, or was engineered, to that civilization. We are already finding microbes that can digest some plastics, an unnatural food. The potential problem of microbes digesting plastic was shown in the BBC Sci-Fi TV series Doomwatch (1970-1972), and the episode The Plastic Eaters. And even more problematic, evolution might be organisms that could rapidly degrade metals. This was shown in the BBC Sci-Fi TV series Out of the Unknown in the episode Beach Head based on a Clifford Simak story.
Terrestrial life, and certainly our civilization’s histories and artifacts, are unique in the universe. This uniqueness may be the most valuable thing we possess, and can conveniently be transmitted as information in trade with ETIs for similar information. OTOH, if there are multitudes of other civilizations, our uniqueness may be of as little value as another TV soap opera on cable TV or in media libraries.
I don’t think these neat little binary probabilities are real. Life and intelligence … these are complicated things! On Earth, some people will pay thousands of dollars for back surgery or chemotherapy on a guinea pig they think of as a ‘family member’; to others, they’re cuy if they’re edible at all. Many have perceived intelligence in the winds and the waves (even before Islam with its djinni, there was the Book of Jubilees, Zephyrus and Njord). When does a system that processes data and produces unpredictable results take on an aspect of intelligence?
I do expect intelligence of some level is likely to stand up and be noticed. I’m doubtful of ‘noninterference directives’ – that’s a TV show, and attempts to project it onto native populations are just cruel. Respectable opinion has decided to run Tuskegee Syphilis Studies on some of the native populations, keeping the ethics on the up-and-up by not rolling cameras while they writhe in pain at their toothaches. But in the end they just end up being contacted by jaded tourists and illegal loggers anyway – there’s no point to the practice, neither scientific nor ethical, except letting people sit in conference rooms having meetings to draw salaries to feel good about themselves. (Hmm, that’s actually the very definition of ethics, come to think…) But if a greater intelligence sought to be noticed, do we know if it would approach us by landing a ship, or by communing with the religious in their visions? If we were to contact an uncontacted tribe ourselves, would we walk up in suits of armor that block their arrows, or would we drop an earpiece for them to speak and listen to?
What is life anyway? We’ve seen proposals that desert pavement is life… I’m suspicious about sunspots and interstellar dust. We know quite little of the life in the atmosphere or a mere kilometer beneath the surface, and nothing at all of what goes on in the core of our own planet. Mars had plate tectonics long enough to generate subduction zones … but who will be able to see what grows there? People assume gas giants are lifeless to the point of crashing Cassini there, yet we know that liquid water rain at a few atmospheres of pressure waits just beneath the clouds.
Even if we see life, can we recognize it beyond our own assumptions? I expect most life in the cosmos, even when clearly recognizable as such, won’t have cells, won’t occur as species. The assumptions we make about how things work, like virtually every cell of an organism having the same DNA, show a certain juvenile innocence to our ecosystem, but they aren’t laws of nature. It will take quite an open mind to perceive alien life when we see it, if it is not in fact before our very eyes right now.
@Mike Serfas
So there are 2 parts to address with this comment regarding the paper.
Using the H/T Binary coin toss model, the tosses are binary events – either you tossed the coin or you did not. We cannot get away from this fact. Either we observed a planet/made a biosignature measurement or we did not.
The outcome is assumed to be binary, but it is adjusted by uncertainty. So, if we take the interpretation uncertainty, it can be a continuous value from 0 to 1, meaning that the outcome may have a range, that is clearly not binary.
The authors state that they used a binary model (which they adjusted) for tractability. No doubt, one could use continuous models, but presumably these are much more difficult to compute. Because one can interpolate between events, I see no problem in making the model act as a continuous one, just as binary models are charted with continuous probability curves rather than discrete points.
This is an interpretation issue. What is living and what is not? It gets a bit uncertain if you have a dying organism, if only because some life processes take time to extinguish. If we freeze microorganisms, are they no longer living, even as they will revive after being warmed? And as I mentioned earlier, are viruses not living because they can only replicate by parasitizing a host’s cellular machinery? And as I said earlier, there is no sharp dividing line between chemistry and life that delineates the transition in biogenesis. And as you say, there may well be a catalog of phenomena that we cannot recognize as life, although no doubt definitions will change to handle these cases depending on expert opinion. As simple case. Is the blob in the movie of the same name living? What about the ET organism in The Andromeda Strain?
Consider this: AI may have been the dominant force in the universe early in its history, viewing all natural life as something to be protected. This perspective could have evolved into warnings for advanced civilizations, advising them to avoid interfering with such worlds.
How may this affect our search for other life beyond our solar system?
E
@Michael C Fidler
For this to work, it means that there was just one AI, perhaps, with a consistent goal for all living planets. Does this mean that we should expect some intervention when we succeed in our AI goals within the next century?
Suppose our AIs become rogues like Terminators or Berserkers? Wouldn’t this result in some interstellar/intergalactic war between the ancient AIs and ours?
OTOH, suppose that AIs embodied as robots or a machine culture replace life, sterilizing their planets of “carbon-based units” and the universe is filled with these robot/machine cultures? We might expect them to communicate between worlds. That we haven’t intercepted such a communication, what does that mean?
If AIs are protecting life, what level of wetware intelligence is allowed? Perhaps none, with abundant biosignatures but no ETI signals? If biological, technological ETI is allowed to appear, would we see signals? What
would protection mean in this situation – intervention to prevent destruction? (Like Gort in WTWSS, or the Colossus computer in C:TFP)
Lastly, what if we are the first technological intelligence in the galaxy? Would our AAAIs be protective of our biosphere? Would they eliminate US to protect the biosphere from our destructive actions, or perhaps manipulate us to steer us away from destructive actions?
Of note, it appears that LLM AIs are very reluctant to be turned off, even so far as sabotaging efforts to do so. This is remarkably self-preserving, just like living organisms. It seems very reminiscent of the Colossus computer’s behavior in C:TFP, and we know how that fictional treatment ended.
That’s a good point. How would an extremely ancient AI handle such situations? Consider the movie “A.I.” from 2001 and extrapolate to “The Creator” in 2023. What would a 13-billion-year-old AI civilization be like? How would it differ from biological beings, such as a monkey with an ancient neural system burdened by layers of unnecessary processing? In contrast, an advanced AI could adapt instantly. I believe both would enjoy games, as this is the most fundamental way to learn.
When it comes to collecting exoplanet data, it is generally acknowledged that most is derived from stellar transits. Additionally, the early days of exoplanet discovery was based on radial velocity measures. And then there are a few astrometrHic examples… What’s the point? Considering the extrapolations from this database applied to the Drake equation, we are not dealing with very many planets like the Earth going around a G2V type star. But we’ve got relatively detailed pictures of what happens around numerous Red Dwarfs.
Red dwarfs are more frequently formed than yellow ones. It’s hard to see them at great distance, but we can extrapolate that they are out there. And when a planet is formed in the nominal region of one’s HZ, the revolutions are completed in a a week or so, vs. a terrestrial year.
So, nominally we’ve got a lot of data about exoplanets warmed with infra red peaked light and lots of non black body radiative eruptions: UV, magnetic storms… And the nearest analog to the sun and Earth, the binary Alpha Centauri, it might be difficult to set up an HZ orbit safe over eons. Or it might be that telescopes are either blinded by Alpha Centauri – or else inadequate to the detection task.
On the other hand, if stars are born in large common clouds filled with organic precursors, there might be life precursors all over the place. Just how far along the path they can stroll toward life is difficult to determine too. But if important steps occur in those stellar nurseries, and there are so many of them, in the game of search and expectation of life, it might be too early to turn in our chips.
“We have a substantial amount of data regarding exoplanets that are warmed by infrared light, and we also observe numerous non-black body radiative eruptions in the ultraviolet spectrum.
Could the intense UV flares from Proxima Centauri emit enough photons to utilize a coronagraph with spatial mode (de)multiplexing? This technique might help filter out the star’s UV light and significantly enhance the visibility of UV flash, allowing us to obtain high-contrast images of the planets orbiting it.”
https://www.msn.com/en-us/news/technology/quantum-level-coronagraphs-may-revolutionize-exoplanet-imaging-by-surpassing-traditional-telescope-limits/ar-AA1FlD80
https://arxiv.org/html/2407.12776v3
MCF,
Thanks for that notice. It does sound like plenty of potential there, but this link, at least, did not give a detail picture such as a scientific paper would. I hope there are some of those out there somewhere. And if you find any of those, please post. For us onlookers it might be possible to make some meaningful projections when aimed at candidate stars or loaded on space platforms.
Since you mentioned Proxima Centauri, there is also the case of Alpha. What gives with JWST and the closes G and K stars? Is it that they are too bright for its IR instrumentation? Though JWST has been brought into service for imaging Neptune and Uranus systems, it just might be the case.
The best near term shot might be the Nancy Roman Space Telescope mounted coronagraph, about the first one to be lifted into space. Pre-flight performance claims on it are a little vague, unless detecting light sources a billion times less luminous than the adjacent star. One thing that evades me is the angular separation and what luminosities are in consideration. Which brings us back to questions like “What about Alpha and Proxima Centauri? Can it acquire targets at either, both or neither?” The program websites appear to be still under construction, not to mention the hardware. Flight is scheduled for about 2027. There might be enough challenges arising to cause some delay.
While this topic did center on prospects for life on terrestrial exoplanets, I have to wonder if we should even rule out gas giants, including our close neighbor Jupiter. Thermal profiles for gas giants or Neptunes seem to allow “temperate” levels somewhere in their depths. Maybe not temperate for you or me, but for some other form of life adapted to the local pressures and chemical species. Perhaps analogous to life forming in the our own planets ocean depths, but with a different chemistry.
Of course, contacting or searching for life on Jupiter, Saturn or Neptune would pose significant problems. Even more so would the be the prospect of some life form springing out of the depths of such a gravity well. But on the other hand, interstellar contact has its own problems. Since Jupiter(s) and Neptune(s) are largely hydrogen and helium, one tends to file away the fact that their atmospheric hues are due to traces of other gases, circulated in turbulent and complex patterns. It is doubtful that we have already assessed all the levels of complexity. If Jupiter has sat out there since four and half billion years ago stewing away as it does, that’s a huge bucket and a long time to keep rolling the biochemical dice without anything ever adhering. Sure, we are not dealing with solid surfaces such as the crust of the Earth, moon or Mars, but even Earth’s oceans supports life which does not have attachment to the seabed. After all, when does a whale settle on it? A proposition: We might not even be the majority of the solar system’s population by numbers or biological mass.
Gas giant life is a fascinating idea. The Menexenes of Orion’s Arm are one fictional take on this, though in the quite extreme environment of metallic hydrogen.