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The Seasons of Exoplanetary Life

Looking for biological products in planetary atmospheres is how we’ll first study exoplanetary life, assuming it exists. The tools for characterizing atmospheres have already developed to the point that we are examining the gases surrounding some ‘hot Jupiters,’ and even talking about the movement of clouds — exoplanet meteorology — on giant worlds. The hope is that TESS will find targets that we can then investigate with new space telescopes.

The way forward is exciting, but my guess is that as we start looking into the atmospheres of transiting planets around nearby red dwarfs, the most accessible targets in the near future, we’re going to find ourselves awash in controversy. Did we just find oxygen? Maybe we’re on the way to a biosignature detection, but then again, ultraviolet radiation can break down atmospheric water to produce oxygen. For that matter, UV can split carbon dioxide molecules.

What about methane? Abiotic methane from geothermal activity on the surface could account for that, and there are other production mechanisms as well. Ideally, we’ll find gases that are many orders of magnitude out of thermochemical equilibrium with the planet’s atmosphere. A combination, say, of an oxidizing gas like oxygen and a reducing gas like methane — these shouldn’t co-exist without some replenishing source — is suggestive of metabolism. But these are going to be tough detections, and we’re likely to see huge controversy over just how significant the early findings are.

We have to start somewhere, but bear in mind that life on Earth produces thousands of different biosignature gases, most of which are in such small quantities that they accumulate little in the atmosphere. Most are produced for reasons specific to the organism. Sara Seager is doing brilliant work compiling all the molecules that exist in a planetary atmosphere under conditions of temperature and pressure similar to the Earth. This is by way of compiling a comprehensive list of biosignature gases and the false positive possibilities that accompany them.

Image: Sara Seager’s volume Exoplanet Atmospheres (Princeton University Press, 2010) is a core contribution to this developing work.

All of which gets us to an interesting new take on biosignature detection out of the University of California at Riverside. What lead author Stephanie Olson and team have produced is what is being described as the first quantitative framework for dynamic biosignatures based on seasonal changes. The idea here is straightforward: Axial tilt creates weather changes and also changes in atmospheric composition. Thus a northern hemisphere summer shows increased plant growth, hence lower levels of carbon dioxide and higher levels of oxygen.

“Atmospheric seasonality is a promising biosignature because it is biologically modulated on Earth and is likely to occur on other inhabited worlds,” Olson says. “Inferring life based on seasonality wouldn’t require a detailed understanding of alien biochemistry because it arises as a biological response to seasonal changes in the environment, rather than as a consequence of a specific biological activity that might be unique to the Earth.”

We can also keep in mind that extremely elliptical orbits could produce seasonality on extrasolar planets in the same way that axial tilt does, which could widen the range of planets under future investigation. Homing in on the seasonal formation and loss of oxygen, carbon dioxide and methane and their detection through spectroscopy, the authors also extended the work to include planets with low oxygen content, where fluctuations on a world like the early Earth might be detectable. Here, ozone (O3) turns out to be an easier marker to identify through seasonal variability than oxygen (O2).

That’s a useful finding, because after all, a planet like the early Earth would have shown a markedly different atmospheric composition than the blue and green world of today. Start with hydrogen and helium in an early atmosphere and add in water vapor, carbon dioxide and sulphur from volcanic eruptions. The process is stochastic (i.e., randomly determined); when does oxygen appear? Ozone? Ideally, we’d like to see a world with water vapor, oxygen, ozone and carbon dioxide because these are strong absorbers that show up well in planetary spectra.

I notice that the atmosphere of ‘early Earths’ is something that co-author Timothy Lyons, also at UC-Riverside and director of the Alternative Earths Astrobiology Center, emphasizes:

“We are particularly excited about the prospect of characterizing oxygen fluctuations at the low levels we would expect to find on an early version of Earth,” Lyons says. “Seasonal variations as revealed by ozone would be most readily detectable on a planet like Earth was billions of years ago, when most life was still microscopic and ocean dwelling.”

Image: Satellites monitor how ‘greenness’ changes with Earth’s seasons. UC-Riverside scientists are studying the accompanying changes in atmospheric composition as a marker for life on distant planets. Credit: NASA.

In any case, I fall back on my prediction. We are going to start getting exoplanet atmospheric data around rocky worlds orbiting red dwarfs within the next decade or so, and our results will be ambiguous at best. Erwin Schrödinger said that living matter ‘avoids the decay into equilibrium’ by fighting entropy, which is why finding out of balance gases co-existing would be highly significant. But while oxygen is comparatively easy to detect, trace methane is not.

I expect a battle royal to break out over the significance of such gases in the amounts we find them, and the quest to filter out all the false positives may take us many years.

The paper is Olson et al., “Atmospheric Seasonality as an Exoplanet Biosignature,” Astrophysical Journal Letters Vol. 858, No. 2 (9 May 2018). Abstract. On Sara Seager’s work on potential biosignature gases, see “Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry,” Astrobiology. June 2016, 16(6): 465-485 (abstract).

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{ 24 comments… add one }
  • Ron S. May 10, 2018, 14:24

    Something that doesn’t quite come through here is that while we may first be able to detect bio-signatures in the atmospheres of red dwarf planets these will not exhibit seasonal variation. The reasons are that the “year” of planets within the habitable zone is very short, and these planets are also likely to be tidally locked. Seasons and seasonal variation require a habitable zone that is further out, as it is for a star such as our own.

    • Michael C. Fidler May 10, 2018, 20:14

      You are correct in the sense of the axial tilt of planets like earth but what will create seasons on tidally locked planets around red dwarfs will be libration (oscillating motion) of the planet. Take the example of the Trappist 1 system, tidal influence and librations will be large as each planet passes by another, this will cause seasons similar to earth’s but on a much shorter time period. There will longer and larger librations the further out in the seven-planet resonant chain in TRAPPIST-1.

      • Ron S. May 10, 2018, 20:54

        Then you missed one of my key points: variation on short time scales cannot result in seasons. Also, libration does not have the scale of effect you seem to assume. Further, the effect of conjunctions cannot possibly affect angular momentum with the violence you suggest.

        • Michael C. Fidler May 11, 2018, 10:06

          Ok, maybe in a few years we will see.

  • Alex Tolley May 10, 2018, 14:51

    Seasonality effects on a global level depend on the distribution of continents and their coastlines.

    For our contemporary Earth, the north distribution of temperate and boreal forests causes the CO2 fluctuations. For the early Earth, photosynthesis seasonality will be reflected by aquatic photosynthesizers in the rich coastal waters and upwelling of nutrient-rich waters. The distribution of these will impact the seasonality of O3 production averaged globally by atmospheric distribution.

    The sensitivity of the instruments is going to have to be exquisite to detect such changes. Along with eccentric orbits, I have no doubt a host of abiotic explanations will emerge to suggest any data could be a false positive.

    I wonder whether observation of photosynthetic molecules either spectroscopically or by imaging of forests on continents might be the clincher for life detection.

  • Bert Molloy May 10, 2018, 15:07

    Thank you Mr. Gilster for a thorough yet brilliantly simplified explanation of a very complicated topic !
    I have been meaning to express this thought to you for the past year+ but haven’t done so out of sheer laziness and inertia !!
    My apologies for this, along with my sincere thanks !

    • Paul Gilster May 10, 2018, 16:42

      Why thank you, Bert! Your thoughts are much appreciated, as is your continued participation. Glad to have you with us.

  • Gary Wilson May 10, 2018, 15:44

    Very interesting. Does anyone know at what level we will be able to detect gases such as CO2 or methane with a clean transit of a red dwarf? And what about ozone? Would we need 1% or more to detect a gas even if it is a strong absorber in a spectrum of gases? I realize each planetary system would offer different detection challenges but I’m just trying to get a feel for detection levels. Sorry if this is a stupid question

  • Andrew Palfreyman May 10, 2018, 15:46

    It is humbling to realise that we are living through, at this moment, a golden age of astronomy.

  • Robert L May 10, 2018, 16:02

    How many planets, on average, would we expect to have axial tilt? The word “tilt” implies to me a deviation from a non-tilted norm, but I assume that that is a misinterpretation and that the large majority will exhibit tilt to some degree.

  • Geoffrey Hillend May 10, 2018, 16:45

    With an Earth twin, it might depend on how early it has a strong, magnetic field. The false positive from the breaking down of H20 into hydrogen and oxygen would be found only on a world with a magnetic field? Without one, the solar wind atmospheric stripping might cause the escape of oxygen. What is the rate of oxygen production from H2O divided by ultra violet compared to oxygen produced by life? The former might be much less and the oxidation of Iron might remove that signature since the rate that oxygen is replenished is crucial. Life tends to replenish a lot of oxygen. If there is a false oxygen positive, it’s spectroscopic lines might be more faint than today’s Earth would produce. The same might be true about Earth over three billion years ago.

  • Hamilton1 May 10, 2018, 17:30

    This all runs up against the Fermi paradox. We expect to find only biosignatures but why should we not find aliens looking back at us with their bigger telescopes? We are like the natives building reed boats, but with an assumption that Columbus will not arrive tomorrow – which logically contradicts the justification for seeking biosignatures in the first place.

  • Ramses Ramirez May 10, 2018, 23:01

    I agree with Paul. In the upcoming decades, even should a paper come out finding “evidence” of atmospheric biosignatures on a particular exoplanet, 10 other papers will immediately follow arguing how it is a false positive.

    I believe that the major impediment to progress in this field is the lack of detailed observations. Our methods and techniques are just not good enough to unambiguous detect and characterize biosignatures in terrestrial planetary atmospheres . Our crude methods also make computer modeling predictions difficult to make. Solving this dilemma will require major technological advances to our imaging techniques, including possibly direct imaging.

    We discuss this in our recent white paper to the NAS “Exoplanet Science Strategy” Call:
    https://arxiv.org/abs/1803.00215?

  • andy May 11, 2018, 14:25

    Unfortunately I can’t access the paper, but is the case of Mars addressed? Mars shows seasonal variations, including dust storms that tend to occur more around perihelion. Could Mars-like seasonal effects cause false positives for this proposed biosignature?

    • Paul Gilster May 11, 2018, 16:50

      andy, this is what the paper says about Mars:

      “Although seasonal CO2 cycles may be vulnerable to nondetection (a false negative), large-magnitude CO2 easonality is unlikely to arise from abiotic processes on habitable planets with liquid water, potentially making this biosignature robust against false positives. An exception to biological controls involves seasonal CO2 ice sublimation and deposition, which produces a seasonal CO2 cycle on Mars today (Wood & Paige 1992). However, significant levels of water vapor, which would suggest temperatures incompatible with the seasonal CO2 ice cycle, could preclude a Mars-like scenario. Seasonality in CO2 in the presence of an ocean (Robinson et al. 2014) would be a powerful but difficult to detect indicator of life on habitable planets.”

      and this:

      “These abiogenic signals complicate but do not preclude CH4 seasonality as an independent biosignature. Indeed, Earth’s seasonal recovery of CH4 following elevated photochemical destruction requires substantial flux from the planet’s surface that, depending on broader planetary redox (Krissansen-Totton et al. 2015), would strongly imply a biological source of CH4 to the atmosphere because it precludes stochastic delivery of exogenous CH4 (Court & Sephton 2012) or episodic geologic inputs, such as those that occur on Mars today (Mumma et al. 2009).”

  • Geoffrey Hillend May 16, 2018, 0:56

    The problem with the methane on Mars is that is only one biogsignature gas. Oxygen is only a trace gas on Mars and not easy to detect spectroscopically. We couldn’t detect O2 with a telescope from Earth in before the space age in the 1950s. It might just be abiogenic CH4 on Mars. I don’t think that our early exoplanet spectroscopy is going to detect the minute quantity of O2 on a Mars twin or even an Earth twin exoplanet. Consequently, it should be pretty easy to rule out trace gases which don’t count as false positives. If we see all the biosignature gases, it must be life.

    I am confident that we won’t see such worlds around exoplanet M dwarfs within the next ten years. You can see why I hope we find a real Earth twin with a Moon not too far away. If not, we will have to look further or improve the technology. Also how many stars are nearby that have been not been on the main sequence hydrogen burning already for several billion years, so what are the odds of finding an Earth twin in a period of time which was like early Earth 3.5 billion years ago nearby? They might not be good.

  • Geoffrey Hillend May 16, 2018, 1:11

    Composition of the atmosphere of Mars. Co2: 95.97 percent, Argon 1.93%, nitrogen 1.89%, oxygen, 0.146% Wiki.

  • Jesus Olmo May 16, 2018, 1:32

    Distant Planet Terrified It Might Be Able To Someday Support Human Life:
    http://www.theonion.com/article/distant-planet-terrified-it-might-be-able-to-somed-35179

  • Geoffrey Hillend May 16, 2018, 1:38

    Our telescopic spectroscopy of Mars before the space age was still inside Earth’s atmosphere so it is not a good example, but due to the redox and photodissociation processes already mentioned, I think CH4 and O2 will only be at most trace gases in any Earth sized exoplanets around M dawf stars and not false positives.

  • stephen May 16, 2018, 22:08

    Ron S. thinks there can’t be seasonal variation on a planet with a very short year. I don’t understand why. We have temperature and weather changes during a 12-hour period from day to night and day again, so why not during a 12-day year?

    Regarding the book, Exoplanet Atmospheres, does the author consider some of the unusual compositions of stars? Some stars have unusual levels of some particular element like bismuth, which implies their planets might have unusual levels as well. Or a non-solar binary companion star might have unusual levels of an element which gets transferred via flares and solar winds, to the planet of a solar-type star in the same system. Is that too unlikely to be worth considering?

    Somewhere–I wish I’d noted the name and date of the publication and authors–somebody was suggesting two chemicals as potential solvents for life. One was Mercuric Bromide; I think the other was Potassium Pentasulfide; I have it written down *somewhere*. I realize the first one would be unlikely, and I don’t know if they’re likely to be detected in an atmosphere.

    I wonder how far afield into speculation, Sara Seager goes? I look forward to reading her book.

    • Ron S. May 17, 2018, 11:01

      A day/night cycle is not a season. But let me be more expansive since your not the first to make this criticism.

      It is not any dictionary definition of season that is at issue. The issue is whether there are seasonal variations amenable to detection by stellar light transiting the exoplanet atmosphere. Any variation must be of sufficient duration to fully suffuse throughout the atmosphere and particularly at altitudes well above the surface for detection to occur. Any so-called season measured in hours day or weeks is insufficient. Even several months may not be enough. But that’s about all you can get with an exoplanet around an M dwarf and still remain in any sort of habitable zone.

      Even for detecting seasonal variation on a planet such as Earth has serious challenges. Assume for the moment that our 12 month cycle of seasons is detectable by the described method from a large distance (this is not at all certain). We have north and south hemispheres that are 180 degrees out of phase with regard to the seasonal cycle. Any detectable signature, even if global atmospheric mixing doesn’t mask it, is undetectable at a distance since this observation technique does not have the resolution to separate north and south atmospheric hemispheres. In a sense the detection is just one pixel, mixing everything together.

      I suspect the only reliable signal would have to come from an exoplanet in the HZ of a star with a similar size to our own so that the seasons are reasonably long, and that exoplanet would have to experience global seasons by virtue of a highly eccentric orbit (which Paul’s article mentions).

  • Geoffrey Hillend May 17, 2018, 15:45

    The methane in Mars atmosphere is only 10 to 35 parts per billion. Much less than it’s oxygen. We can’t yet prove it’s not from Martian life. We need to do a test to see if it has the same isotope as a waste product of life. Both Earth and Mars don’t have a lot methane, a percentage which is not easily detected as a biosignature gas on exoplanets.

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