We’re in the midst of a significant period defining the biosignatures life can produce and determining how we might identify them. Centauri Dreams regular Alex Tolley today looks at a paper offering a unique contribution to this effort. The work of Sarah Rugheimer and Lisa Kaltenegger, the paper looks at how exoplanet spectra change for different types of host star and different epochs of planetary evolution. As Alex points out, the effects are profound, especially given the fact that red dwarfs will be our testbed for biosignature detection as we probe planetary atmospheres during transits around nearby stars. How stellar class affects our analysis will affect our strategies especially as we probe early Earth atmosphere equivalents. What will we find, for example, at TRAPPIST-1?
By Alex Tolley
As the search for life on exoplanets ramps up, the question arises as to which types of stars represent the best targets. Based on distribution, M-Dwarfs are very attractive as they represent 3/4 of all stars in our galaxy. Their long lifetimes offer abundant opportunities for life to evolve, and to resist extinction as their stars increase in luminosity. On Earth, terrestrial life might last another one billion years before the level of CO2 in the atmosphere is forced to be reduced below photosynthetic requirements for plants to survive. All but lithophilic life might be extinct within 1 ½ billion years. An additional advantage for astronomers is that spectra of exoplanet atmospheres will be easier to distinguish around low luminosity stars. [6, 7]
From a purely numbers game, M-Dwarfs are most attractive targets:
“Temperate terrestrial planets transiting M-dwarf stars are often touted as the poor-astronomer’s Earth analog, since they are easier to detect and characterize than a true Earth twin. Based on what we currently know, however, M-Dwarf planets are the most common habitable worlds“ 
Image: Gliese 581 from a planet in its HZ. Credit: David Hardy.
That M-Dwarf rocky worlds may be the most common habitable world is due to:
“1. rocky planets are much more common in the temperate zones of M-Dwarfs (…) than in the temperate zones of Sun-like Stars (…)
2. small stars are more common than big stars (…)
3. the tidally-locked nature of these planets is not a challenge to climate and may double the width of the habitable zone (…)
4. the red stellar radiation results in a weaker ice-albedo feedback
and hence stabler climate (…), and (…)
5. the slow main sequence evolution of M-Dwarfs means that a geological thermostat is not strictly necessary to maintain habitable conditions for billions of years (…). Studying temperate terrestrial planets around M-Dwarfs is our best shot at understanding habitability writ large.” 
There are negatives for life around M-Dwarfs too. The closeness of the habitable zone (HZ) to the star results in tidal locking that may impact the stability of the atmosphere, as well as the intense flares that may strip the atmospheres from these worlds. However, these negatives for habitability and hence life may be compensated by the ubiquity of such worlds and the relative ease of studying them remotely. For lithophilic life, surface conditions largely can be ignored.
After the lifeless Hadean, the Archean and Proterozoic eons had life that was purely prokaryotic. During this time photosynthesis evolved that eventually resulted in an atmosphere with O2 and very little CO2 and CH4. This phase of life’s history covers the long period when Earth’s atmosphere changed from a largely reducing one of N2, CO2, and some CH4, to one that becomes oxidizing. The Phanerozoic, starting around 500 mya encompasses the period when O2 pressures increased to the level they are today and terrestrial, multicellular life blossomed in diversity.
If Earth’s history is any guide, life in our galaxy will be mostly unicellular bacteria, living in a reducing atmosphere. If that is a correct hypothesis, then most life in the galaxy will be non-photosynthetic, perhaps with biologies similar to the Archaea. A biosignature of such microbial life will still require looking for a disequilibrium in gases, mainly CO2 and CH4, rather than O2 and CH4 [2, 3]. Archaea include the extremophiles living in a diverse array of environments, including the lithosphere. Such organisms may well survive the harsher conditions of a tidally locked world, especially regarding the impact of flares.
The question then arises, if we look for a biosignature around stars of different spectral types, will the star’s type have an impact on the planet’s atmosphere, detectable spectral markers, and any potential biosignatures?
This question is examined in a paper by Rugheimer and Kaltenegger . The authors modeled the spectra of atmospheres to simulate Earth-like worlds – rocky worlds large enough to hold an atmosphere and presumably with a mix of ocean and continents, rather than water worlds – orbiting in the HZ of different star types F, G, K and M. Their simulations cover the state of evolution of those worlds as if they were an Earth relocated to other stars, so that the spectra for different gas mixtures could be modeled.
The light of an M-Dwarf is shifted so that the UV component is much diminished. This affects the reactions of the gases in the atmosphere. Photolysis is reduced, reducing the loss of H20, which in turn, as a greenhouse gas, warms the surface more than with a hotter star. CH4 in particular is not lost and may even result in a runaway accumulation in some cases. The increase in H2O increases the cloud cover in the troposphere, which in turn increases the planet’s albedo. The increased IR component of the M-Dwarf’s output increases the surface temperature as well and may well further increase cloud formation.
The photolysis of water and the oxidation of CH4 is shown below. UV is required which results in the reduced loss of H2O and CH4 on exoplanets around M-Dwarfs.
H2O + hv (? < 200 nm) -> H + OH
CH4 + OH -> CH3 + H2O
Similarly, UV is required to split O2 allowing O3 formation.
O2 + hv (? < 240 nm) -> O + O
O + O2 -> O3
Previously, Kaltenegger  had modeled the atmospheres of Earth-like worlds around different stars and constructed synthetic spectra to determine the visibility of different biosignature gases in the visible and near-infrared.
Following on, Rugenheimer et al modeled gases for 4 different periods – 3.9, 2.0, 0.8 and 0 Ga for the 4 star types. The initial gas mixes are shown in Table 1.
Table1. Gas mixing ratios for 4 eons. N2 not shown.
Because the stars age at different rates, the periods are standardized to Earth. As M-Dwarfs age far more slowly than our sun, the different luminosities are modeled as if their planets are further out from their star earlier in its history to simulate the lower luminosity.
The result of the simulations shows that some markers will be difficult to observe under different spectral types of stars.
The impact of the star type is shown in Figure 1. Temperature and 5 gases are profiled with altitude, The M-Dwarfs show clear differences from the hotter star types. Of particular note are the higher H2O and CH4 atmosphere ratios, particularly at higher altitudes.
Figure 1. Planetary temperature vs. altitude profiles and mixing ratio profiles for H2O, O3, CH4, OH, and N2O (left to right) for a planet orbiting the grid of FGKM stellar models with a prebiotic atmosphere corresponding to 3.9 Ga (first row), the early rise of oxygen at 2.0 Ga (second row), the start of multicellular life on Earth at 0.8 Ga (third row), and the modern atmosphere (fourth row). Source: Rugheimer & Kaltenegger 2017 
Figure 2 shows the simulated spectra for the star types. Because of the loss of shorter wavelengths with M-Dwarf stars, the O2 signatures are largely lost. This means that even should a planet around an M-Dwarf evolve photosynthesis and create an oxidizing atmosphere, this may not be detectable around such a world.
Figure 2. Disk-integrated VIS/NIR spectra at a resolution of 800 at the TOA for an Earth-like planet for the grid of stellar and geological epoch models assuming 60% Earth-analogue cloud coverage. For individual features highlighting the O2, O3, and H2O/CH4 bands in the VIS spectrum. Source: Rugheimer & Kaltenegger 2017  [TOA = Top of the atmosphere – AMT]
In contrast, the strong markers for CO2 and CH4 are well represented in the spectrum for M type stars. This creates a complication for a biosignature for early life comparable to the Archean and early Proterozoic periods on Earth. An atmosphere of CO2 and CH4 assumes that the CH4 is due to methanogens being the dominant source of CH4, far outstripping geologic sources. On the Hadean Earth, CH4 outgassing should be rapidly eliminated by UV. During the Archean, the biogenic production of CH4 maintains the CH4 and therefore the disequilibrium biosignature. But on an M-Dwarf world, this CH4 photolysis is largely absent, resulting in a CO2/CH4 biosignature that is a false positive.
If photosynthesis evolves, the O2 signal can be detected at the longer wavelength of 760 nm, but only if there is no cloud cover, as shown in figure 3. For an M-Dwarf planet, clouds mask the O2 signal, and we expect more cloud cover due to the increased H2O on such worlds.
Figure 3. Disk-integrated spectra (R = 800) of the O2 feature at 0.76 m for clear sky in relative reflectivity (left) and the detectable reflected emergent flux for clear sky (middle) and 60% cloud cover (right). Source: Rugheimer & Kaltenegger 2017 . Note the loss of detectable O2 feature for M-type stars – AMT
Fortunately, ozone (O3) can be detected strongly in the IR around 9500 nm, so we can hope to detect photosynthetic life when the O2 partial pressure increases. Figure 4 shows that the O3 signature can be detected in the Archean in the Phanerozoic, but not the Proterozoic.
Figure 4. Smoothed, disk-integrated IR spectra at the TOA for an Earth-like planet for the grid of stellar and geological epoch models assuming 60% Earth-analogue cloud coverage. For individual features highlighting the O3, H2O/CH4, and CO2 bands in the IR spectrum see Figs. 9, 10, and 11, respectively. Source: Rugheimer & Kaltenegger 2017 
While current instruments cannot resolve spectra in sufficient detail to detect the needed signatures of gases, the authors conclude
“These spectra can be a useful input to design instruments and to optimize the observation strategy for direct imaging or secondary eclipse observations with EELT or JWST as well as other future mission design concepts such as LUVOIR/HDST.”
To conclude, the type of star complicates biosignature detection, especially the co-presence of CO2 and CH4 in Archean and early Proterozoic eons that dominate the history of life on Earth. Not only is the star’s light shifted, hiding shorter wavelength signals, but the light itself impacts the equilibrium composition of atmospheric gases which can lead to biosignature ambiguity.
While the ubiquity of M-Dwarf stars and the longevity of low O2 atmospheres due to the time to evolve photosynthesis on Earth and the delay before the atmosphere builds up its O2 partial pressure, favors M-Dwarf stars as targets for looking for early life, the potential of false positives for the Archaean and early Proterozoic equivalent eons complicates the search for life on these worlds using expected biosignatures for worlds around sol-like stars. There is still work to be done to resolve these issues.
1. N. B. Cowan et al “Characterizing Transiting Planet Atmospheres through 2025”
2015 PASP 127 311. DOI: https://doi.org/10.1086/680855
2. Tolley, A, “Detecting Early Life on Exoplanets”, 02/23/2018. https://www.centauri-dreams.org/2018/02/23/detecting-early-life-on-exoplanets/
3. Krissansen-Totton et al “Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life” 2018 Science Advances Vol. 4, no. 1. DOI: 10.1126/sciadv.aao5747
4. Kaltenegger et al “Spectral Evolution of an Earth-like Planet”, The Astrophysical Journal, 658:598Y616, 2007 March 20 (abstract).
5. Rugheimer, Kaltenegger “Spectra of Earth-like Planets Through Geological Evolution Around FGKM Stars”, The Astrophysical Journal 854(1). DOI: 10.3847/1538-4357/aaa47a
6. Burrows, A. S ”Spectra as windows into exoplanet atmospheres,” 2014, PNAS, 111, 12601 (abstract)
7. Ehrenreich D “Transmission spectra of exoplanet atmospheres” 2011 http://www-astro.physik.tu-berlin.de/plato-2011/talks/PLATO_SC2011_S03T06_Ehrenreich.pdf
Photosynthesis originated a billion years earlier than we thought, study shows.
“The earliest oxygen-producing microbes may not have been cyanobacteria. Ancient microbes may have been producing oxygen through photosynthesis a billion years earlier than we thought, which means oxygen was available for living organisms very close to the origin of life on earth. Researchers studied the molecular machines responsible for photosynthesis and found the process may have evolved as long as 3.6 billion years ago.”
That’s true, but the reductants in the oceans meant O2 didn’t build up for three aeons after.
The author of the photosystem paper has to make some fudges as his molecular clock approach puts the development of photosynthesis as older than the age of the Earth. [Perhaps this is a clue that life originated elsewhere? ] So he constrains the data to get his early evolution of photosynthesis.
However, this isn’t really relevant because, as Adam states, we have geologic data on oxygen levels that constrain its partial pressure over Earth’s history. IMO, that indirect observational data is the relevant point for types of biosignature.
RARE METALS ON MARS AND EARTH IMPLICATE COLOSSAL IMPACTS.
“The amount of siderophiles accumulated during late accretion should be proportional to the ‘gravitational cross section’ of the planet. This cross section is effectively the cross hairs that an impactor ‘sees’ as it approaches a target planet. The gravitational cross section extends beyond the planet itself, as the world’s gravity will direct an object towards it even when the object was not on a direct collision course. This process is called gravitational focusing.”
“The earlier paper showed that Earth has more siderophiles in the mantle than it should, even according to the gravitational cross section theory. The scientists explained this by showing that an impact of a lunar-sized body on the Earth (in addition to the event that formed the Moon) would have enriched the mantle with enough siderophiles to explain the current value.”
I think what we are seeing in the M-Dwarf systems is a more volatile rich environment that may be continuing long after the star settles down. The original M-Dwarf nebula was high in volatilizes elements because of the low radiation levels at the larger distances that the comets would form. This in turn would create a large reservoir of high volatile comets that would still be feeding these low density planets.
The example in our Sun’s case is a much higher density and drier environment for the inner solar system with the Earth having the highest density of any planet or moon in our system. Therefore M dwarf planets should have a fascinating tale to tell and could be the most lively place in the universe.
The Alien Planets of TRAPPIST-1 May Be Too Wet for Life.
“The seven rocky planets circling the nearby star TRAPPIST-1 have lots of water, a new study suggests — perhaps too much to make them good bets for life.”
Now we have either burned out hulk’s or to much water!
The question I have is if a deep large ocean would be salty or like a fresh water lake since there is no run off from the continents. How would this effect the atmosphere? Could there be several layers of different mixing between atmosphere and ocean? What about floating life and again are we seeing a bias because we do not live in the ocean???
Inward Migration of the TRAPPIST-1 Planets as Inferred From
Their Water-Rich Compositions.
Cayman T. Unterborn, Steven J. Desch, Natalie R. Hinkel, Alejandro Lorenzo.
February 7, 2018
“Multiple planet systems provide an ideal laboratory for probing exoplanet composition, formation history and potential habitability. For the TRAPPIST-1 planets, the planetary radii are well established from transits [1, 2], with reasonable mass estimates
coming from transit timing variations [2, 3] and dynamical modeling . The low bulk densities of the TRAPPIST-1 planets demand significant volatile content. Here we show using mass-radius-composition models, that TRAPPIST-1f and g likely contain
substantial (? 50 wt%) water/ice, with b and c being significantly drier (? 15 wt%).
We propose this gradient of water mass fractions implies planets f and g formed outside the primordial snow line whereas b and c formed inside. We find that compared to planets in our solar system that also formed within the snow line, TRAPPIST-1b and c contain hundreds more oceans worth of water. We demonstrate the extent and timescale of migration in the TRAPPIST-1 system depends on how rapidly the planets formed and the relative location of the primordial snow line. This work provides a framework for understanding the differences between the protoplanetary disks of our solar system versus M dwarfs. Our results provide key insights into the volatile budgets, timescales of planet formation, and migration history of likely the most common planetary host in the Galaxy.
Implicit assumption: the star has the same metallicity as Sol. However, a first-generation M dwarf would be composed entirely of H and He, and could not have any planets that even vaguely resembled “earthlike”. Life (as we know it!) requires “metals”. It is based on C H N O P S, and requires (among many others) Fe, Mn, Zn, Cu … up to Iodine. I doubt that even a second or third generation star would have these in sufficient abundance, so their lifetime of trillions of years is not equivalent to inevitability that life would arise. The sheer abundance of M dwarfs is not a good statistic. I believe that metallicity is the key parameter.
There aren’t that many “first generation” M-dwarfs around. They’re kinematically distinct from the majority, as they’re not denizens of the Galactic Disk and pass through it at high speed.
I’d be interested to read a paper that supports that statement. Also, my argument also pertains to 2nd- and 3rd- (maybe 4th-) generation M-dwarfs. Metals are a Must.
The scope of the authors’ analysis assumes an Earth-like world. Not all aspects of the planets’ origins are looked at as the variables are too numerous. We only have a sample of one so far. Let’s not be too constrained by Earth’s state when evaluating other stars for life.
Earth’s atmosphere becomes like Venus in only 300 million years. The Sun increases in brightness 7 to ten percent every billion years.
Quote by Alex Tolley: ” During the Archean, the biogenic production of CH4 maintains the CH4 and therefore the disequilibrium biosignature. But on an M-Dwarf world, this CH4 photolysis is largely absent, resulting in a CO2/CH4 biosignature that is a false positive.” I don’t understand this. Photolysis is the splitting or separation of molecules by the action of light. There’s plenty of EUV and x-ray radiation from a M red dwarf star. Why wouldn’t there by the photolysis of CH4? There would be and that is why any CH4 not produced by life would be lost or wouldn’t hang around for long unless replenished by life so there would be no false positive.
Also, the James Webb space telescope can detect O2, and O3 since these are well above the cloud tops on Earth. Also all molecules and gases have a thermal, infrared frequency emission especially when you have the photolysis of O2 into two O1 molecules which combine to make O3 or ozone. Without a significant oxygen source there will be little or no ozone.
The energy spacings are smaller in the larger, heavier atoms and also in molecules consequently, these will absorb light more towards the infra red as opposed to lighter or small atoms like H, and He which have large spacings and higher energy absorption more towards the ultra violet. Remember, it’s not the spectral light of the star but the the absorption spectrum of the gas that we detect.
Kasting puts it more like <1.5 billion years (How to Find a Habitable Planet). However, surely the point is that life will be mostly prokaryotic during its history. If so, we will most likely need a biosignature for such life.
Re: CO2/CH4 bisognatigure being a false positive for red dwarf worlds:
The authors’ point is that the reduced UV in the spectrum reduced the CH4 breakdown via the reactions shown. Their models show that abiogenic CH4 will not be nearly as transient as that on Earth, and that in some cases, can even accumulate. Thir model clearly shows that in Figure 1.
The point here is that the detection of O2 directly depends on the shorter wavelengths that are much reduced with RD sars. [O3 has a strong signal in the IR and is therefore a useful proxy]. Figure 3 is the one I think you are contesting, that shows that the 760nm O2 signal is washed out for RD planets.
I don’t have expertise in spectroscopy beyond the papers that discuss this. (I did dig up a number of papers and reviews after you raised the question previously). I accept the authors’ expertise here.
“Earth’s atmosphere becomes like Venus in only 300 million years.”
Some governments want it to happen a lot sooner, by burning even more coal and oil.
I think it has been pretty much debunked that Earth will have a runaway greenhouse effect if we burn all our fossil fuels. We will get our “Venus” state because the sun will increase its luminosity. Which is not to say the Earth will not get much hotter than it is now, something like the Eocene Thermal Maximum.
“The benefit of making infrared observations is that it is the infrared wavelengths that molecules in atmospheres of exoplanets have the greatest number of spectral features.” P. 293, Space Telescopes, English.
pardon me, it the light from the star, and the absorption frequency of the gas. A particular gas can still absorb at different frequencies of EMR, but are easier to detect at certain wavelengths where they absorb the most at one frequency.
I think M dwarf habitability should be addressed in a more fundamental way . We’ve seen first hand the deadly flare activity of Proxima and can only imagine it’s effect on the proximal “hab zone” planet Proxima b.
I think that even before atmospheric searches for biosignatures , the real breakthrough will be in simply demonstrating the existence of any ” terrestrial like” atmosphere on a hab zone planet around an M dwarf . Further out, larger, older- more quiescent star – whatever. Any or all. It’s the observation that counts . Some relatively low pressure mix of CO2, H2O and CH4 in the first instance .
That’s enough . Demonstrating that such atmospheres can and do exist -and don’t get stripped away – will be a huge , indeed the biggest step . These will surely have potential to demonstrate or evolve biosignatures or at worse act a proof of concept that they will be discovered somewhere , sooner or later , as technology and technique improve . We’ve heard about following the water , but how about going back a pace and following the terrestrial atmosphere ?
There may be as couple of ways that detection of atmospheres may be easier – Auroras – Ozone – Radio frequencies – Air-glow. All these relate to some type ionization of atmospheric components and may be easier to detect when flares take place. As I have suggested before, a watch for flares, especially when on the far side of M dwarf stars, could lead to a snapshot of the planets ionized atmosphere. The planets with a magnetic field would also take longer for the flares or CME (coronal mass ejection) to cause the magnetic re-connection and lights up the powerful auroras in the exoplanets atmosphere. This would lead to a longer time period to detect the the auroras signal then from the direct reflection of the UV from a flare. So in this situation timing and a large group of larger telescope that could be brought to bear on the exoplanet as the flare or CME impacts the magnetic field of the planet. On Trappist 1 the time scale could be a few hours but on earth the time is two to three days before the magnetic re-connection sends the particles down the magnetic field lines into the earths atmosphere.
Proxima b May Have An Aurora That We Can Detect.
There are two more important reason this is the way to go for detecting the atmosphere around M dwarfs. The planet does not have to be transiting the star and in multiple planetary systems, the individual planets should light up in sequence.
Magnetic Fields of Extrasolar Planets:
Planetary Interiors and Habitability.
A white paper.
“Jupiter’s radio emission has been linked to its planetary-scale magnetic field, and spacecraft investigations have revealed that most planets, and some moons, have or had a global magnetic field. Generated by internal dynamos, magnetic fields are one of the few remote sensing means of constraining the properties of planetary interiors. For the Earth, its magnetic field has been speculated to be partially responsible for its habitability, and
knowledge of an extrasolar planet’s magnetic field may be necessary to assess its habitability. The radio emission from Jupiter and other solar system planets is produced by an electron cyclotron maser, and detections of extrasolar planetary electron cyclotron masers will enable measurements of extrasolar planetary magnetic fields.
Would the magnetic poles on tidally locked planets around M dwarfs be facing the star? Could the M dwarf magnetic field flipping also cause there nearby planets to also flip? It would be interesting to see what the very intense flares and CME would cause on a planet with a magnetic pole facing the star. The magnetic re-connection would cause interesting effects on planet’s surface, atmosphere and aurora with a Uranus like magnetic field that close to the star. Could a magnetic field be generated from a correctly doped liquid H2O ocean, especially traveling thru the stars magnetic field? Could there be gradients caused on these planets by this unusual arrangement that would be more conducive to life, i.e., less radiation, more ionization even geologic structures frozen in place by the fields?
I agree. Models are useful for suggesting what to look for, but observations are most important. Most likely we will be surprised by what we actually find.
Getting any kind of constraint on atmospheric composition may be quite challenging. The recent crop of arXiv papers includes a detailed look at GJ 1214, the poster child for atmospheric studies of M-dwarf planets, and concludes that several of the inferred properties for the planet’s atmosphere are less certain than thought, because the observed transit spectroscopy is likely influenced by spots on the star.
Mallonn et al. (arXiv:1803.05677 [astro-ph.SR]) “GJ1214: Rotation period, starspots, and uncertainty on the optical slope of the transmission spectrum“
After the lifeless Hadean, the Archean and Proterozoic eons had life that was purely prokaryotic.
I thought Proterozoic had Protozoans which are eukaryotic single cells.
Purely prokaryotes at first, nucleated later, and the great fusion of a host anaerobe and a mitochondrion bearing aerobe – all before multicellularity.
Short of panspermia, if the evolution of life elsewhere in the accessible universe repeats the early events, it would suggest some as yet unrecognized constraints shortly after abiogenesis.
One reason to read blogs like CD is to become more educated. I hope that Robin Datta’s comment and link has helped in this regard. I’ve certainly had some misconceptions corrected over the years, and I expect to have more corrected in the future.
I was and am always fascinated by biology, although I last studied it formally 44 years ago (not counting medical school).
There is much more near infra red and mid infra red coming of a M class RD star than our Sun. I don’t know where they got the idea that there is not any such light coming from RD which makes no sense.
The idea that O2 spectra will be washed out makes no sense when you consider that 760nm is .76 microns or the near infra red, .7 to 5 microns. O2 or molecular oxygen absorption is in the near infra red. The James Web space telescope is designed to detect the near and mid infrared. Also light polarization techniques make it possible to easily distinguish between star light and the planet.
There is actually a better way to detect an exoplanets magnetic field; the broadening of spectral lines from the solar wind interaction with the magnetic field of the exoplanet. https://www.sciencedaily.com/releases/2014/11/141120141800.htm
Alex: Since well over 50% of our atmosphere is composed of nitrogen, any “Earth-like” atmosphere should have an abundance of it as well. This may not be the case for temperate planets orbiting M-dwarf stars. Although initially this was probably the case in their infancy, stellar wind and flares from the host star break down BOTH O2 AND N2 molecules which would re-form into molecules such as nitrogen oxide, nitrogen dioxide, nitrous oxide(laughing gas), and, in the UPPER troposphere and lower stratosphere(where TRACES of hydrogen may still be available)nitric acid. Case in point: TRAPPIST-1d. Even six-plus billion years of time is not enough for stellar wind and flares to totally remove its mega-ocean, but its original N2 primary atmosphere is probably totally gone. The above mentioned nitrates would make the atmosphere very noxious to Earth-like multi-cellular life-forms(as would be the fictional atmosphere of the fictional moon, Pandora, whose atmospheric components should be very similar due to its jovian host-world’s intense radiation belts). I envision cirrus clouds of nitric acid forming at even higher altitudes than the ones of ice on this planet. Other nitrate clouds may or nay not be present at lower altitudes. I have two questions for you. ONE: Are nitrates like the ones I mentioned above more or less detectable with current telescopes? TWO: Is there a class of extremophilic single-celled organism present on Earth that can REVERSE the above-mentioned transformation and RETURN N2 to such an atmosphere? RSPV.
As you can see from the charts, N2O is shown as far higher a gas ratio with M-Dwarfs.
The paper shows that N2O has a strong marker in the IR so I would assume it can be detected by future telescopes.
Soil bacteria are the main denitrification agents converting NO3, NO2 back to N2. However, N2O is also produced in this process and can escape to the atmosphere. H2O is soluble in water, so I assume it would be slowly washed out of the troposphere by rain and then reduced too N2 by bacteria. Whether some extremophiles can also reduce nitrogen oxides I don’t know.
I don’t think the EUV hydrolysis can make NOX compounds like NO, N2O and NO2, or nitric oxide, nitrous oxide, or nitrogen dioxide. High temperatures are needed as those found in automobile engines, the furnaces of coal power plants and forest fires to make NOX which combines with water to make nitric acid or acid rain. P. 60, Atmospheres, Barbato and Ayer.
Also without the life there is no nitrogen cycle to put the nitrogen back into the air so without life, we could expect to not see much nitrogen in the atmosphere of an exoplanet.
Quite so. If you recall from the “disequilibrium paper”, N2 is converted to oxides and dissolved in the oceans relatively quickly without life. The N2/O2 disequilibrium was a major part of the atmospheric disequilibrium energy. The problem is that N2 is very difficult to detect.
Because N2O is also produced by organisms, it is sometimes posited as a biomarker. However, this paper shows that this would likely be a false positive for RD stars.
We also need oxygen to combine with the nitrogen. The high temperature of lightning combines these and you get Nox. Rain carries it to the ground, but if you don’t have much O2 or N2, then you don’t get any NO2. NO to NO2 to HNO3 (nitric acid).
Don’t forget that photolysis of water will produce the O2 needed to oxidize nitrogen, therefore we don’t need to assume life is the only source of O2.
Venus’ atmosphere still contains some fraction N2 (actually far more than Earth in total mass), so N2 oxidation may not be possible in a CO2 dense atmosphere, or the N2 is still being outgassed in some way. My guess is that the oxygen is combined with other gases preferentially to nitrogen on Venus.
A relevant paper by Hu, Seager and Bains may be of interest here, as it models sulphur chemistry of different atmospheres.
Most of the oxygen on Venus in not free or bound to carbon in Co2. An atmosphere which has mostly Co2 and S gases atmosphere is likely to have a big greenhouse effect and little water. Not much Co2 but a lot of SO2 and S gases and we have the snowball Earth since the particulates or dust of S gases would reflect sunlight and cool the planet. Interesting paper on the photochemistry of sulfur dioxide and other S gases. It doesn’t mention tidal locking and tidal forces might produce more inner heat and volcanism and more S gases?
UV hydrolysis can split up H2O to make more oxygen. The problem is that it can also combine with rocks to be removed from the air like on Mars and also atmospheric stripping from the solar wind on a planet without a magnetic field or close to an M star.
If we don’t see any spectral biosignatures like CH4, O2, and O3, it does not completely rule out life which might exist in some sea on a super Earth, but it has not been able to adapt to land or affect the atmosphere and we can’t go there to check under the water yet. The James Web space telescope should be able to get some spectra from nearby exoplanets. We should be able know this in the next couple of years based on whether or not there are any biosignature spectra. I would be surprised to see one from an M dwarf exoplanet just based on the high EUV and solar wind stripping to reduce an atmosphere over time.
Oxygen combines with Iron to make rust like on Mars.
The source of Martian O2 was the photolysis of water. M stars have relatively low UV to split water so according to the authors of the paper, M-Dwarf planets have more H2O and CH4 than hotter stars. I would expect relatively low iron oxide formation as a result.
It is a pity that the most we can hope for in the foreseeable future are spectrographic analysis and later direct imaging. A physical mission to a star would be awesome, although the results would take a [very] long time to arrive.
I hope that various approaches to detecting life might find a living world, but I suspect ambiguity will remain with us for some time.
You know, the red Hematite.
If there was low iron oxide formation as a consequence of less splitting of H2O, then there would have to be low oxygen so we could rule out any false positive for oxygen. Maybe Methane can be more of a false positive for life than oxygen.
If you are talking about Mars, I am skeptical that it has any life today, although there is a possibility of life in its lithosphere where the depth ensures warmth and liquid water. I am hopeful that we may eventually find fossil signs of microbial life. Normally we would need to drill for a sample, but an impactor might do the work for us, throwing up a deep sample onto the surface for examination. Either way will need a substantial human presence on Mars I think.
If you are talking about exoplanets, without a sample return how could we determine the presence of haematite, or declare it absent?
Ramses Ramirez: I hope that you are reading the comments on this post. Here’s why. A new paper has come out in “Nature Astronomy” taking the chi squared goodness fit discrepancy you mentioned in one of your comments on this website, but taking it in the OTHER DIRECTION that you and your collegues have! “Inward migration of the TRAPPIST-1 planets as inferred from their water-rich compositions”. Authors(from Arizona State and Vanderbilt Universities)unknown. DOI: 10.1038/s41550-018-0411-6. The authors claim that TRAPPIST-1b and c are composed of 15% water, and that TRAPPIST-1f and g are composed of a WHOPPING 50% water! This is far greater than the values given by Grimm et al in their paper. What gives? RSVP.
Harry R Ray: Please read the article and learn the situation. Nature Astronomy received the paper by Unterborn et al in June 2017, which was long before the publication of Grimm et al. The planet interior model used in Unterborn et al was based on the masses derived from Wang et al (arxiv.org/abs/1704.04290), which is still under peer-review after a year. In fact, the masses in Wang et al deviate by a substantial amount from the discovery paper by Gillon et al. As the TTV date and analysis methods were updated after Grimm et al, masses used in Unterborn et al are not longer considered valid, so as their planetary composition study. If Unterborn et al submit their paper a year later and use the date in Grimm et al, they would get the same water mass fractions as Grimm et al.
Inward Migration of the TRAPPIST-1 Planets as Inferred From
Their Water-Rich Compositions.
Cayman T. Unterborn, Steven J. Desch, Natalie R. Hinkel, Alejandro Lorenzo.
February 7, 2018
Hello Nicky, this is the updated paper for Unterborn for Feb. 7, 2018, I have not had the time to compare the two, but we be interested to hear your comparison. The scorched exoplanets that is in the current vogue right now is based on the migration concept, but in these small M dwarf systems there is another way in which these planets can still be high volatiles. See my above comment in the “RARE METALS ON MARS AND EARTH IMPLICATE COLOSSAL IMPACTS” from March 16, 2018, 12:07. These systems are like a clocks that are ticking at a speed that is 100 times faster then our solar system. This process will cause the large reservoir of volatiles comets to refresh the M dwarf planetary systems of their atmosphere and water. The way I look at this is the the rain of cometary impactor flux onto these fast moving tightly pack planets would be like the rainstorms on planet earth – a miniature planetary system with a hydrologic cycle that spans from the water reservoir of cometary aquifers to the large oceans on these planets.
Hello Michael C. Fidler, arxiv.org/pdf/1706.02689.pdf is not the updated paper from Unterburn. It was submitted a year ago, and it just got published like two days ago. Currently, the most reliable interior models and planetary masses are from the paper by Grimm et al (0.5% water) is extremely difficult (impossible!) to detect, and can such planet be habitable at all is debatable. I put more prospect on dry and high density planets, like TRAPPIST-1e and LHS 1140b (Earth has <0.2% water).
Also, gravitational perturbation is needed to scatter water-rich bodies (comets) to closer orbits, and such perturbation is weaker in M-dwarf system.
It is V2.
(Submitted on 8 Jun 2017 (v1), last revised 6 Feb 2018 (this version, v2))
As Alex Tolley says “The first such ambiguous discovery will make headlines and reputations, even though it will ignore the possible spectrum of worlds that may be living but are not Earthlike.” I’ afraid this is going to be a long turf war that will take a least a decade to settle.
It is revised, but the main content and conclusion are still the same. The masses Unterborn et al in v2 used for their interior model are still from the outdated Wang et al.
“We adopt the Mobs and ?M data of  for this study.
 Wang, S., Wu, D.-H., Barclay, T. & Laughlin, G. P. Updated Masses for the TRAPPIST-1 Planets. ArXiv e-prints (2017). ArXiv:1704.04290”.
First, Wang et al is under peer-review after even a year. Second, the masses presented in Wang et al are systamally lower than the masses presented in the discovery paper. Third, problems in Wang et al TTV-method were already pointed out in Grimm et al, which was published in A&A.
The rocky nature (<5% water) of Trappist-1 planets are rather determined, and a long turf war seems unlikely in the face of advancing methods and date. Additionally, the upcoming SPIRou should also be able to detect the masses with RV method.
UPDATE: Cayman Unterborn is the LEAD author. And Correction: TRAPPIST-1b and c are composed of 10% water, NOT 15%. ALSO: The ESTIMATED percentages for TRAPPIST-1d and e lie somewhere between 10% and 50%>
To be clearer, the masses in Unterborn et al are derived from TTV-date collected from Late 2016 Spitzer observation, ground-based observation and Campaign 12 of K2 mission, and the masses are systematically low resulting high volatile mass fractions (Wang et al).
After the submission of Unterborn et al, a series of intense ground-base photometric monitoring campaign and Early 2017 Spitzer observation finished and new TTV-date was gathered. The result is now presented in Grimm et al showing a rather Earth-like composition with low volatile mass fractions (<5%).
Grimm et al also criticized the TTV method in Wang et al for not showing "how the correlations between parameters are taken into account and how comprehensive the search of the parameter space is".
CHARACTERIZING EARTH ANALOGS IN REFLECTED LIGHT: ATMOSPHERIC RETRIEVAL STUDIES FOR FUTURE SPACE TELESCOPES.
“Space-based high contrast imaging mission concepts for studying rocky exoplanets in reflected light are currently under community study. We develop an inverse modeling framework to estimate the
science return of such missions given different instrument design considerations. By combining an exoplanet albedo model, an instrument noise model, and an ensemble Markov chain Monte Carlo
sampler, we explore retrievals of atmospheric and planetary properties for Earth twins as a function of signal-to-noise ratio (SNR) and resolution (R). Our forward model includes Rayleigh scattering,
single-layer water clouds with patchy coverage, and pressure-dependent absorption due to water vapor, oxygen, and ozone. We simulate data at R = 70 and R = 140 from 0.4–1.0 µm with SNR = 5, 10, 15, 20 at 550 nm (i.e., for HabEx/LUVOIR-type instruments). At these same SNR, we simulate data for WFIRST paired with a starshade, which includes two photometric points between 0.48–0.6 µm and
R = 50 spectroscopy from 0.6–0.97 µm. Given our noise model for WFIRST-type detectors, we find that weak detections of water vapor, ozone, and oxygen can be achieved with observations with at
least R = 70 / SNR = 15, or R = 140 / SNR = 10 for improved detections. Meaningful constraints are only achieved with R = 140 / SNR = 20 data. The WFIRST data offer limited diagnostic information,
needing at least SNR = 20 to weakly detect gases. Most scenarios place limits on planetary radius, but cannot constrain surface gravity and, thus, planetary mass.”
Skimming the paper indicates that they can only weakly detect O2/O3 and H2O for earthlike worlds, which rules out biosignatures worlds where life is bacterial and photosynthesis has not yet resulted in the “great oxidation event”.
The paper does not mention which star type they use, but as the paper repeatedly mentions an Earth-twin world, and this is mentioned:
I think it is clear that the work only applies to a sun-like, G type star.
Clearly, the authors want to show that detecting biosignatures for an Earthlike world in its current evolutionary state is possible using the upcoming telescopes. The first such ambiguous discovery will make headlines and reputations, even though it will ignore the possible spectrum of worlds that may be living but are not Earthlike.
Geoscience and the Search for Life Beyond the Solar System.
“How can scientists conclude with high confidence that an exoplanet hosts life? As telescopes come on line over the next 20 years that can directly observe photons from terrestrial exoplanets, this question will dictate the activities of many scientists across many fields. The
expected data will be sparse and with low signal-to-noise, which will make disentangling biosignatures from abiotic features challenging. Our Earth is not just unique in that it hosts life, it is also the only terrestrial planet with direct observations of its interior through seismic waves, and compositional evolution through field and laboratory measurements. This extensive research reveals a planet born from collisions between worlds (Canup & Asphaug, 2001), followed by a
complicated biogeochemical evolution (Lyons et al., 2014). Exoplanet interiors, on the other hand, can only be constrained by the following observations: 1) photometric and spectroscopic analysis of the planet’s atmosphere, 2) spectroscopic and photometric analysis of the host star, and 3) companion planet properties. From these (future) data, astrobiologists must generate plausible compositional and evolutionary models that constrain a potentially habitable exoplanet’s internal
properties and history, provide environmental context, and rule out geochemical explanations for any putative biosignatures. The goal of this white paper is to frame the role of geophysical and geochemical processes relevant to the search for life beyond the Solar System and to identify critical, but understudied, areas of future research.
The emerging field of “exogeoscience” is the study of how galactic, stellar system, atmospheric, and internal processes of terrestrial exoplanets affect the properties, evolution, and
observable features of their surfaces and interiors. These phenomena and their couplings are central to the concept of planetary habitability, an environmental state that permits the origination and sustainment of life, because all plausible theories require a solid surface under a liquid water layer. As biospheres sit atop a tectonically active solid planet that can generate a magnetic field above the atmosphere, solid body processes are fundamental to both theoretical models (Foley &
Driscoll 2016) and retrieval algorithms (Meadows et al. 2018). Yet the challenge of measuring internal properties remotely, e.g. with photometric and/or spectroscopic data from future spaceand
ground-based facilities, is profound: Exoplanets are too distant for robotic exploration, and solid surfaces are opaque. Without significant investment of resources in theoretical and laboratory research to understand the full range of interior processes on exoplanets, interpreting spectral features as biosignatures will be purely speculative. Below we describe the current state of exogeoscience, and then suggest research initiatives that could dramatically improve the chances of unambiguously identifying active biology on an exoplanet.”
HABITABILITY FROM TIDALLY-INDUCED TECTONICS.
“The stability of Earth’s climate on geological timescales is enabled by the carbon-silicate cycle that acts as a negative feedback mechanism stabilizing surface temperatures via the intake and outgas of
atmospheric carbon. On Earth, this thermostat is enabled by plate tectonics that sequesters outgassed CO2 back into the mantle via weathering and subduction at convergent margins. Here we propose a
separate tectonic mechanism — vertical recycling — that can serve as the vehicle for CO2 outgassing and sequestration over long timescales. The mechanism requires continuous tidal heating, which makes
it particularly relevant to planets in the habitable zone of M stars. Dynamical models of this vertical recycling scenario and stability analysis show that temperate climates stable over Gy timescales are
realized for a variety of initial conditions, even as the M star dims over time. The magnitude of equilibrium surface temperatures depends on the interplay of sea weathering and outgassing, which in turn depends on planetary carbon content, so that planets with lower carbon budgets are favoured for temperate conditions. Habitability of planets such as found in the Trappist-1 may be rooted in tidally-driven tectonics.
Spin-orbital tidal dynamics and tidal heating in the TRAPPIST-1 multi-planet system.
“We perform numerical simulations of the TRAPPIST-1 system of seven exoplanets orbiting a nearby M dwarf, starting with a previously suggested stable configuration. The long-term stability of this configuration is confirmed, but the motion of planets is found to be chaotic. The eccentricity values are found to vary within finite ranges. The rates of tidal dissipation and tidal evolution of orbits are
estimated, assuming an Earth-like rheology for the planets. We find that under this assumption the planets b , d , e were captured in the 3:2 or higher spin-orbit resonances during the initial spin-down,but slipped further down into the 1:1 resonance.
Dependent on its rheology, the innermost planet b may be captured in a
stable pseudosynchronous rotation. Non-synchronous rotation ensures higher levels of tidal dissipation and internal heating. The positive feedback between the viscosity and the dissipation rate — and the ensuing runaway heating — are terminated by a few self-regulation processes. When the temperature is high and the viscosity
is low enough, the planet spontaneously leaves the 3:2 resonance. Further heating is stopped either by passing the peak dissipation or by the emergence of partial melt in the mantle. In the post-solidus state, the tidal dissipation is limited to the levels supported by the heat transfer efficiency. The tides on the host star are unlikely to
have had a significant dynamical impact. The tides on the synchronized inner planets tend to reduce these planets’ orbital eccentricity, possibly contributing thereby to the system’s stability.’
If these are low density, large ocean worlds the continual mixing of the lithosphere and upper mantle with H2O would create a completely different tectonic structure then in high density planets like earth. The mid-ocean ridge system would be a better analogy in this situation, with a much deeper and faster turnover then in earth’s case.
The riddle of TRAPPIST-1b has most likely been solved. Its orbit is CHAOTIC(i.e. the eccentricity VARIES considerably over time)and therefore it is NOT tidally locked! The tidal heating of this planet is SO GREAT that its surface temperature is MOST LIKELY 1400K! Obviously the ONLY WATER on this planet would consist of a very small amount of water VAPOR in its atmosphere,if it still has one. To PROVE this, the Spitzer Space Telescope should try to detect a SECONDARY ECLIPSE of this planet ASAP!!! ArXiv: 1803.07453. “Spin-orbital tidal dynamics and tidal heating in the TRAPPIST-1 multi-planet system.” by Valeri V. Makarov, Ciprian T Berghea, Michael Efromisky.
The ambiguouty of biosignatures only applies to far away exoplanets. The James Web space telescope and the Extremely Large space telescope should not have a problems with nearby M dwarf stars since they both can image their exoplanets directly as well as transit spectroscopy. Light polarization techniques will remove any ambiguity of conventional spectroscopy.
I was only using the hematite as an example of the loss of non biogenic oxygen. Some oxygen can be also produced by the splitting of CO2 molecules which leave atomic oxygen to recombine into O2 and O3, but that oxygen is lost the same way through oxidation with iron to make hematite. P. 137, Atmospheres, Barbato and Ayer.
It’s clear we can’t use Mars as a universal example in every cases since it has a weaker gravity and it’s easier to strip gases molecules from it’s atmosphere with the solar wind. There is not a lot of methane produced volcanically, Biogenic methane sources make a lot more methane. The hydrolysis of H20 creates OH hydroxyl radicals which remove methane. Methane volcanically produced on explanets by volcanoes won’t amount to much and removal process might make it less or very difficult to detect spectroscopically.
ESA’S NEXT SCIENCE MISSION TO FOCUS ON NATURE OF EXOPLANETS.
The nature of planets orbiting stars in other systems will be the focus for ESA’s fourth medium-class science mission, to be launched in mid 2028.
ARIEL, the Atmospheric Remote?sensing Infrared Exoplanet Large?survey mission, was selected by ESA today as part of its Cosmic Vision plan.
Very well done Pdf’s on Ariel with lots of eye candy!
Additional Exoplanet Science Enabled by FINESSE.
Submitted on 19 Mar 2018
“An Introduction to FINESSE
The Fast Infrared Exoplanet Spectroscopy
Survey Explorer (FINESSE) mission, recently
selected by NASA’s Explorer program to proceed
to Step 2 study, would conduct a large-scale,
uniform survey of exoplanet atmospheres and
create a statistically significant sample for
comparative planetology studies. The technical
capabilities of the FINESSE mission are driven
by a focused scientific program to study planetary
formation and climate mechanisms. In addition,
its mission capability enables a broad range of
exoplanet science topics including exploring the
role of non-equilibrium chemistry, atmospheric
evolution, and the star-planet connection (Fig. 1).
FINESSE’s survey capability would enable
follow-up exploration of TESS discoveries and
provide a broader context for interpreting
detailed JWST observations.”
FINESSE mission to investigate atmospheres of hundreds of alien worlds.
If selected for development, FINESSE is targeted for the launch around 2023.
The Ariel spacemission is very nice. There targeting gas giants and super earths with temperatures over 600 Kelvin thought. It’s something definitely needed to learn more about planetary atmospheres, but not biosignatures for life though.
Planets COMPLETELY COVERED BY WATER may be more habiotable than previously thought. ArXiv: 1803.07717. “The Ice Cap Zone: A Unique Habitable Zone for Ocean Worlds”. by Ramses M. Ramirez, Amit Levi. The only down side is this MOST LIKELY does NOT apply to water worlds orbiting M4, M5, M6, M7, M8, M9, M10, and main sequence L stars.