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False Positives in the Search for Extraterrestrial Life

I’m still smarting about having to cancel my travel plans for the Tennessee Valley Interstellar Workshop in Chattanooga, particularly since the Tau Zero Foundation was one of the sponsors of the event. But fortunately, I do have people offering to write up the workshop for Centauri Dreams, so we’ll have some coverage and photos soon. Onward…

Hunting for Biosignatures

We only have two years before the James Webb Space Telescope is scheduled to launch. Assuming all goes well, JWST should help ease us into the era of biosignature detection, as we look for the characteristic signs of living organisms in the atmospheres of their worlds. But just how definitive are such signatures? A new paper from the University of Washington digs into potential false positives and offers specifics on the signatures that could fool us.

One way to study biosignatures is by transit spectroscopy, using data gathered from starlight as it passes through a planet’s atmosphere during a transit. This allows us to analyze the features of light that flag particular atmospheric constituents. Would a strong oxygen signature necessarily indicate life? You would think so, because the oxygen in Earth’s atmosphere, O2, is unstable over geological timeframes and would gradually be depleted through reactions with volcanic gases and oxidation at the surface.

On our planet, then, oxygen needs a replenishing source, which is provided through photosynthesis as plants and algae tap sunlight for energy. But Edward Schwieterman and team argue that while abiotic oxygen will not build up to significant levels on Earth, it can certainly arise in a number of different planetary settings. That fact should give us pause.

Working under the aegis of the university’s Virtual Planet Laboratory, Schwieterman sees oxygen as a potential ‘biosignature impostor.’ The abiotic creation of oxygen, particularly around the kind of low mass stars that are likely to be our first targets for this kind of work, can happen when ultraviolet light from the star splits carbon dioxide molecules, allowing some of the oxygen atoms to form O2. We get oxygen without any requirement for biology to sustain it.


Image: New research from the University of Washington-based Virtual Planetary Laboratory will help astronomers better identify and rule out “false positives” in the ongoing search for life. Shown is an artist’s rendering of Kepler 62E, about 1,200 light-years away in the constellation Lyra. Credit: NASA

Usefully, Schwieterman’s computer models show that this process will also produce detectable amounts of carbon monoxide. Thus the presence of carbon dioxide and carbon monoxide would raise doubts about the simultaneous detection of oxygen in that planetary atmosphere. Abiotic processes rather than life might well be the agent. We can use this to our advantage:

…in the cases presented here, the spectral discriminators against abiotic O2/O3 are more detectable with a hypothetical JWST observation than the O2 or O3 signatures themselves. In our example spectra, neither O2 nor O3 would be directly detectable with just 10 transits, but the abiotic discriminators CO/CO2 and O4 could be. This provides an opportunity to maximize the utility of observing time if the ultimate goal is to characterize planets where true biosignatures are obtainable. If spectral indicators for biosignature impostors are detected with reasonable confidence, the community may wish to reallocate the remaining time to other promising targets, rather than integrate further.

The mention of O4 above is a reference to a second kind of biosignature impostor, where light from the primary star breaks down atmospheric water on the rocky planet under investigation. In this case, we get large amounts of oxygen as the hydrogen escapes, and might expect to find short-lived pairs of oxygen molecules that become O4 molecules, producing a distinctive signature of their own. A higher percentage of oxygen is produced than Earth has ever had in its atmosphere, as Schwieterman notes:

“Certain O4 features are potentially detectable in transit spectroscopy, and many more could be seen in reflected light. Seeing a large O4 signature could tip you off that this atmosphere has far too much oxygen to be biologically produced.”

Detecting this particular ‘biosignature impostor’ is best done through direct imaging, while carbon dioxide and carbon monoxide signatures are more readily identified through transit spectroscopy. The paper looks at simulations of direct imaging spectra:

This simple test case demonstrates that if the high-O2 atmospheres proposed by Luger & Barnes (2015) exist, the O4 absorption band strength in those planetary spectra would rival or exceed that of the monomer O2 bands. These spectra are qualitatively different than modern-Earth’s spectrum, even in the 0.3-1.0 µm range, with a different shape, broader O2 features, and additional features from O4. These are all signs of a much higher O2 abundance than the Earth’s atmosphere – self-regulated by negative feedbacks – has ever achieved.

Schwieterman and team believe the first potentially habitable planets whose atmospheres we study will be orbiting M-class dwarf stars, the kind of planets most likely to show both kinds of biosignature impostors discussed above. Thus we’ll be faced with the need to interpret potential biosignatures that could be confusing without these criteria. Says Schwieterman:

“The potential discovery of life beyond our solar system is of such a huge magnitude and consequence, we really need to be sure we’ve got it right — that when we interpret the light from these exoplanets we know exactly what we’re looking for, and what could fool us.”

The paper is Schwieterman et al., “Identifying Planetary Biosignature Impostors: Spectral Features of CO and O4 Resulting from Abiotic O2/O3 Production,” Astrophysical Journal Letters Vo.. 819, No. 1 (25 February 2016). Abstract / preprint. A University of Washington news release is also available.


Comments on this entry are closed.

  • Alex Tolley March 2, 2016, 14:28

    OTOH, if we see CH4 as well as O2, wouldn’t that be a positive biosignature?
    Whilst the oxygen biosignature could be a false positive, we are also going to face false negatives, as Earth had low oxygen levels for most of life’s history.
    This also assumes that life will be very much “as we know it” with photosynthesis releasing oxygen type biochemistry.

    Looking for life below the surface on Mars, or even in icy moons, is very much a project that wants to go beyond the obvious terrestrial biosignature approach which would indicate these worlds must be lifeless.

    • Andrew LePage March 2, 2016, 15:34

      > if we see CH4 as well as O2, wouldn’t that be a positive biosignature?

      Not necessarily. Given our limited experience studying planetary environments, the possibility that the methane and oxygen could both have abiotic origins can not be definitively eliminated at this time. Or the methane and oxygen might not necessarily be in the same atmosphere but present in two or more separate bodies spatially unresolved by our instruments (e.g. a planet and one or more of its moons). We need to learn a lot more about exoplanetary environments *and* exobiologies before we can start identifying reliable biosignatures.


      • Alex Tolley March 2, 2016, 21:21

        How long can abiotic methane and oxygen exist in the same atmosphere?

        Good point about the signature coming from separate worlds. What might be the odds of this?

        • Andrew LePage March 3, 2016, 15:25

          > How long can abiotic methane and oxygen exist in the same atmosphere?

          As long as they are both being produced at the same time (obviously in different chemical environments on the same world), they can both be present perpetually.

          >Good point about the signature coming from separate worlds. What might be the odds of this?

          Until we learn a lot more about exoplanets and their moons, there is no way to know for sure. But it is not an unreasonable scenario and therefor the simultaneous detection of methane and oxygen can not be used as an unambiguous “biosignature”.

    • Robert March 2, 2016, 15:37

      Until we know of other kinds of life chemistry or get under the surface of expoplanets we are stuck with looking for life as we know it. But I am intrigued by Gold’s Deep Hot Biosphere concept.

      • Alex Tolley March 2, 2016, 21:19

        The deeper we find micro organisms in te lithosphere, the more the theory looks interesting. However the evidence that it is the cause of [some?] oil and coal reserves still seems dubious.

        • Robert March 2, 2016, 22:41

          Tommy Gold was really a clever guy which adds more credibility to the idea for me but time and data will tell. If true, we would be sitting on a virtual infinite supply of oil just when we learn we cannot use it in the long run without serious environmental complications. Fortunately, there will be alternatives.

          • Alex Tolley March 3, 2016, 12:16

            IIRC, Gold’s hypothesis is that primordial methane seeps up from the mantle and is transformed by micro organisms. First oil, then later coal is produced. The problem with this hypothesis is that coal contains fossil plants, which means that the source of coal is probably from the surface , not from deep methane.

            The experiment to drill for oil in igneous rocks did not find more than a trace of oil, so the hypothesis was neither proven nor falsified.

            Gold was involved in a number of interesting hypotheses, many of which were wrong. In this case, the idea of organisms deep in the crust is true, but their involvement in coal and oil formation is likely false.

            • Rob Henry March 3, 2016, 17:06

              Paradoxically, those plant fossils are Gold’s strongest evidence. It has always been a mystery as to how they retain their shape while all but carbon is crushed out of them. Another mystery that he goes into (and that I first read of as a kid) it that some tree fossils seem to clearly continue over two separate layers of coal. These have been put in the too hard basket long ago, but Gold revives them.

  • Andrew Palfreyman March 2, 2016, 16:18

    It is somewhat irksome to see three rover missions to Mars go by, none of which have excavation capabilities and direct life/organics detection capabilities.

    • Alex Tolley March 2, 2016, 21:17

      The one rover that was looking for life – The UK’s Beagle – unfortunately crashed and was destroyed. I believe the next US Mars rover will be able to excavate, but what its life detection instrumentation is, I haven’t looked at yet.

      I’d like to see a decent microscope added so that either micro organisms or microfossils could be looked at directly, without requiring sample return.

      • Robert March 2, 2016, 22:44

        I believe direct confirmation of microbial life on Mars would be too controversial at this point so NASA is not really pushing that science as hard.

  • Geoffrey Hillend March 2, 2016, 17:35

    @Alex Tolley. Yes with CH4 and oxygen, there has to be life. Add carbon dioxide and carbon monoxide and you have Earths spectroscopic biosignature and an industrial, technological civilization. Carbon monoxide is in automobile exhaust. The oxidation process in automobile engines and large furnaces such as coal power plants produce NOx compounds such as nitric oxide NO, nitrous oxide N2O and nitrogendioxide NO2.

  • H. Floyd March 2, 2016, 20:03

    Best wishes for speedy recovery (in health and tech), with selfishly mixed sympathies for plans gone awry: we got another fascinating post at least a day sooner.

    Hawkingite METIskeptics, I wonder if humans would ever be able to mimic negating biosignatures, sort of impostor-impostors. Prey or predator, evolution seems to like species that camouflage.

  • Roy Clymer March 2, 2016, 23:17

    Early Earth had plentiful CO2 and water but negligible O2. But from what is said above it could/should have. Seems hard to believe that splitting of CO2 and H2O could produce more O2 than is produced by life on earth now.

  • Ashley Baldwin March 3, 2016, 2:22

    All said and done the sooner we start imaging and using transit spectroscopy the better.” Theory without observation is sterile, observation without theory is pointless” as George Bernard Shaw said presciently . We are seeing that play out at present while we wait for JWST and WFIRST. Theory without observation also seems capable of tying itself up in knots too and as recent observation of young circumplanetary disks by ALMA has just recently shown ( to say nothing of the architecture of planetary systems ) , has often be shown to be wrong .

  • Mark Zambelli March 3, 2016, 11:35

    I wonder, would the current levels of human-made chemicals, such as CFCs, in our atmosphere be enough to detect, or would that require lengthy integration times?

    • Alex Tolley March 3, 2016, 14:04

      Interesting idea. However in the search for life, CFCs are but an almost invisible period in time for life on Earth, and in the future, such compounds are not likely to be emitted in ant quantity due to the damage to the ozone layer.

      On a related note, if life is succeeded by machines, what might such machines emit as signatures?

      • Eniac March 5, 2016, 13:11

        One of the strongest bio signatures might be the absence of CO2. All biomass on Earth is derived from CO2 by the action of RuBisCo, leaving only minute traces free in the atmosphere. No other atmospheres in this solar system are as thoroughly depleted of CO2 as Earth’s.

        • Alex Tolley March 5, 2016, 17:21

          Depends on what you mean by minute. CO2 levels were much higher in the past:



          CO2 really started to decrease one photosynthesis started, as you suggest. However it was much higher in the past, and needed to be so as the sun was generating less energy.

          Currently (as in the last 100 my), CO2 levels have been decreasing in response to increased solar radiation, to the extent that possibly within less than a billion years or so, CO2 levels will need to be so low that they cannot sustain plant life.

          My takeaway is that we need to consider where the planet is in the HZ, and how long has life been established. For planets towards the outer edge of the HZ, CO2 levels would need to be high to warm the planet enough to keep liquid water on the surface.

          • Eniac March 13, 2016, 19:23

            It may be, as I detect you suggest, that in the past, CO2 levels were limited by the greenhouse effect (i.e. too little CO2, too cold, plants freeze, less photosynthesis, more CO2). They are also limited, though, by the ability of RuBisCo to selectively take up CO2 vs. O2. This ability is bound to have improved over time, as competition for the scarce resource favored evolutionary improvements.

            Today, the RuBisCo limit is in effect: O2 vs. Co2 uptake is around 20-30%, meaning plants are barely scraping by. Any less CO2, and they’d start dying. It is possible that for some periods in the past, RuBisCo was better than it needed to be, and plants died from frostbite caused by lack of greenhouse gases before they ran out of CO2. Perhaps this would explain the “Snowball Earth” episodes. But, it is also possible that RuBisCo was the limit all along, and it just evolved to get progressively better over time.

  • Ivan Vuletich March 6, 2016, 20:25

    I wonder how habitable (by Earth life) such an O2 rich but otherwise sterile planet would be. It could be a favorable terraforming target, nice thick breathable atmosphere ready for colonisation by modern Earth life without having to wait millennia (+) for the atmosphere to be transformed.

    • Michael March 9, 2016, 14:43

      The problem with terraformation is that it takes so so long. We could build thousands of rotating habitats in the time it takes to terraform the atmosphere.

  • Stan Erickson March 11, 2016, 2:01

    I think what the article said is that O2 is a necessary but not sufficient indicator of photosynthetic life. By the way, did the modeling look at any feedback effects of higher O2 levels on the ocean, increasing carbonate sinks or anything else?

    • Eniac March 13, 2016, 19:30

      But that would be wrong. Photosynthesis predates the O2 atmosphere by a billion years, at least. O2 is therefore not a necessary indicator.

  • Eniac March 13, 2016, 19:27

    I guess what I am trying to say that it is not as simple as “CO2 levels have been decreasing in response to increased solar radiation”. There is no such direct relationship. CO2 level cannot decrease beyond the RuBisCo uptake limit, no matter the solar radiation, and that is where they are and likely have been for a while.