Selection is going to be a key issue for future ground- and space-based observatories. Given lengthy observing times for targets of high interest, we have to know how to cull from our exoplanet catalog those specific worlds that can tell us the most about life in the universe. Recently, Ramses Ramirez (Earth-Life Science Institute, Tokyo Institute of Technology) went to work on the question of habitable zones for complex life, which are narrower than the classic habitable zone defined by the potential for water on the surface. In today’s essay, Alex Tolley looks at Ramirez’ recent paper, which examines the question in relation to the solubility of gases in lipid membranes. What emerges in this work is a constrained habitable zone suited to complex life, with limits Alex explores. The model has interesting ramifications right here in the Solar System, but it also points the way toward constraining the list of planets upon which we’ll apply our emerging tools for atmospheric characterization.
By Alex Tolley
Daggerwrist on Darwin IV. Artist Wayne Douglas Barlowe. Source: Expedition.
Life on Earth, until its last three quarter-billion years, was almost entirely represented by unicellular organisms. As we explored in Detecting Early Life on Exoplanets, biosignatures for microbial life are likely to be far more prevalent than for worlds with complex life. While rocky worlds in the classic habitable zone (HZ) are still relatively few, academic PR departments trumpet every find as “Earth-like”, and a selection of these worlds will be targeted for biosignatures. However, as the number of these worlds increases, scientists will want to distinguish worlds that have a biosphere that can be characterized as more Earth-like, with verdant landscapes and megafauna in the seas and on land.
When the term “Earth-like” is used, the public thinks of a world that looks like Earth, with oceans, continents variously clothed in verdant landscapes, and perhaps most importantly of all, “charismatic megafauna”, the animals that you went to see at the zoo, or watched on David Attenborough’s excellent nature programs. A blue sea lapping on a muddy beach, despite teeming with microbes and other unicellular life, looks dead to the unpracticed eye, which means most of the human population. It is those human-scale animals like the daggerwrist pictured above from Barlowe’s “Expedition: Being an Account in Words and Artwork of the 2358 A.D. Voyage to Darwin IV” that excites the public.
If life is rare, then the classic HZ will have the least constraints, although most of those worlds will still have biospheres populated only with microbes, and fewer probably with unicellular plants and animals. If life is not rare, then there will be a desire to discover true Earth-like worlds with complex life, which may mean limiting the range of the HZ that will allow for such life to flourish.
The classic HZ range is defined by the possibility of liquid water remaining continuously on the surface, warmed by the star’s radiation and an atmosphere of sufficient pressure and with some greenhouse gases. This is because all Earth’s life requires liquid water and this has led to the mantra “Follow the water” for missions in the search for life. Inside the inner HZ limit, there will be a runaway greenhouse that eventually desiccates the planet, like Venus. Towards the outer edge, the atmosphere needs to be increasingly composed of greenhouse gases, particularly carbon dioxide (CO2) until a limit is reached.
For the solar system, the classic HZ lies at about 0.95 AU, inside Earth’s orbit, but excludes Venus, and extends to about 1.67 AU, outside of Mars’ orbit. It is this that offers the possibility of a second genesis and possibility of finding extant life in refuges and in the lithosphere beneath the now inhospitable Martian surface.
Complex, or multicellular, life on Earth emerged less than 1 billion years ago as photosynthesis reduced the CO2 in the atmosphere and replaced it with oxygen (O2). Except for a few recently discovered species, all multicellular life is aerobic and requires a rich O2 atmosphere. It is the much greater energy released by aerobic respiration compared to anaerobic respiration that allows for the energetic lifestyles of multicellular animal life (metazoa). At least for our planet, we believe that the conditions for complex life to survive are constrained; Earth has its own habitable zone limits that are narrower than the classic HZ. The question is, “What might those HZ limits be for complex life, and how does that translate for exoplanets around different stellar types?”
CO2 is one of the main greenhouse gases that extend the outer boundary of the HZ. Nitrogen (N2) also helps extend the outer edge of the HZ although it is not a greenhouse gas but a main constituent of the atmosphere. Are there limits to the pressures of these gases due to effects on complex life that limit the range of the possible HZ for multicellular life living on the planet’s surface?
A new paper by Dr. Ramses Ramirez attempts to answer that question by applying the relationship between the solubility of gases in lipid membranes and their anesthetic potency (see figure 1 below). This theory, a partial explanation for the still imperfectly understood mechanism of anesthesia, is that the solubility of gases in lipid membranes is correlated with their anesthetic potency. Anesthetists must monitor the use of these gases to maintain unconsciousness. Too little and the patient remains conscious of the pain during surgery, too much anesthetic, and the patient stops breathing and dies.
The anesthetic gases are to the bottom right of the chart in figure 1. Nitrous oxide (N2O) is less potent and still used in dentistry (as well as at “nitrous parties”). Less well known is that CO2 also acts as such a gas with solubility similar to N2O. Although physiologically CO2 initially increases breathing rate to flush it out of the lungs, at higher concentrations it then invokes respiratory, and later metabolic, acidosis, which sets in as CO2 dissolved in the blood serum eventually causes cessation of respiration and death. As can be seen in figure 1 below, N2 has low solubility in lipid membranes, 2 – 3 orders of magnitude lower than CO2, and concomitantly similar orders of magnitude lower anesthetic potency.
However, we are probably also familiar with the effects of high-pressure N2 as nitrogen narcosis that is experienced by divers breathing compressed air at depth. The argument is that both CO2 and N2 dissolving in the lipid membranes of cells will cause death if those gas concentrations reach the anesthetic level for complex life.
Figure 1: The Meyer-Overton correlation of oil/gas solubility versus anesthetic potential of inhaled gases. Figure recreated from published data. Source Ramirez .
Figure 1 above shows the relationship between gases and their anesthetic potential. CO2 solubility is similar to nitrous oxide, while N2 is far less potent and therefore apparently less of a constraint. Note that helium is at the upper end of the range and has low solubility and low anesthetic potency. This is why helium is used to replace N2 when deep diving in soft suits.
While the Meyer-Overton correlation is primarily for humans, it has been shown to apply across several different phyla as it is a physical, rather than physiologic effect. Determining the tolerance limits for CO2 and N2 provides a constraint that limits the HZ to a “Complex Life Habitable Zone (CLHZ).” Dr. Ramirez supports the general applicability of the lipid gas solubility to metazoa from prior experimental work, primarily on mammals, but also with other animals, to suggest that 0.1 bar (1/10th of surface atmospheric pressure or 1.4 psi) of CO2 might be a reasonable, conservative limit for complex life to tolerate CO2. N2 limits are primarily set by experiments for human divers. 2 bar of N2 seems to be the safe limit at which divers do not get narcosis. This is just 10 meters below the surface, a depth even beginner scuba divers can safely operate for short durations. Using upper limits for 0.1 bar CO2 and 2 bar N2, Dr. Ramirez finds that his radiative-convective model (RC) gives an estimated HZ for complex life (CLHZ) of 0.95 – 1.21 AU. Using an advanced energy balance model (EBM) that allows for different temperatures on the Earth’s surface, thus allowing for liquid water at the equator, but not at the poles, this CLHZ is extended from 0.95 – 1.31 AU.
The new outer range for this 2 bar N2 and 0.1 bar CO2 is 1.36 AU using the Energy Balance Model (EBM). This range is shown in figure 2 below not just for Earth, but for a range of main sequence star types. The relative decrease in the CLHZ compared to the HZ is greatest for cooler stars, the type we have most exoplanet examples in the HZ currently.
Figure 2. The Complex Life Habitable Zone (CLHZ) for A – M stars (2,600 – 9,000 K) compared to other definitions.The CLHZ is for a 0.1 bar, 2 bar N2 atmosphere which is compared to the classic HZ. While the inner edge of the HZ and CLHZ are the same at 0.95 AU, the outer edge of the CLHZ is now well inside the orbit of Mars. Image source: Ramirez.
Dr. Ramirez compares his results to a similar paper by Dr, Edward Schwieterman that looks at the same problem but through the lens of CO2 chemistry, with the note that carbon monoxide (CO), while not limiting the CLHZ, is toxic and could be limiting to the evolution of complex life . (The CO is created by photolysis of CO2.) Schwieterman uses a 1D radiative-convective climate model for his calculations across a range of CO2 levels. Schwieterman does not investigate higher N2 pressures which results in his modeling having a narrower CLHZ than Dr. Ramirez’s most comparable modeling. However, the CO toxicity does not appear significant except for planets orbiting cool stars such as M dwarfs.
While both authors attempt to redefine the likely boundaries for the HZ of complex life based on Earth’s biological evolution, only Dr. Ramirez employs the possibility of increasing the N2 pressure to increase the outer limit.
To quote from the paper:
“The CLHZ is slightly wider at the higher N2 pressure because of increased N2-N2 collision induced absorption and a decrease in the outgoing infrared flux, which more than offset an increase in planetary albedo.”
Dr. Ramirez also states:
“I consider how our solar system’s HZ changes if we assume (for the moment) that complex life could evolve to breathe in a hypothetical 5-bar N2 atmosphere. For this sensitivity study, the RC model predicts that such worlds in our solar system can remain habitable at 1.24AU (SEFF = 0.65) whereas atmospheric collapse can be avoided as far as 1.36 AU (SEFF = 0.54) in the EBM (nearly 60% classical HZ width). I find that the additional N2 opacity is sufficient to counter the ice-albedo feedback, allowing for effective planetary heat transfer even at relatively far distances.”
Dr. Ramirez’s 0.1 bar constraint for CO2 should be put in context for life on Earth. CO2 is currently at about 0.04% (0.0056 psi) of the Earth’s atmosphere. Even during the Cambrian period when multicellular animals were rapidly diversifying into phyla, the atmospheric component of CO2 was never more than 1% and it fell fairly continuously during this period. The Great Permian Extinction which saw 90-95% of all complex life become extinct primarily by anoxia in the oceans, the CO2 levels were little more than 0.1% at their peak. [See “Climate Change and Mass Extinctions: Implications for Exoplanet Life”] and figure 3 below. For highly cognitive humans, NASA conservatively stipulated that the highest emergency level of CO2 in the Apollo Command and Lunar modules should be no more than 0.29 psi (0.02 bar) in an atmosphere of 5 psi O2 before cognitive skills become impaired . The Centers for Disease Control and Prevention (CDC) guidelines for CO2 is that 0.04 bar CO2 is immediately dangerous [i].
It should also be noted that the analysis is limited to surface living, air-breathing animals. Bathypelagic organisms, such as oceanic fish may be adapted to tolerate far higher N2 pressures.
Figure 3. O2 and CO2 levels in the Phanerozoic.  While the Permian extinction is associated with a rise in CO2 levels to about 0.1%, and a decline in O2 levels from the Carboniferous, the CO2 levels were far higher at 1% at the start of the Cambrian and still high in the Devonian (the age of fishes).
But what about multicellular organisms other than animals? While Dr. Ramirez acknowledges that complex life includes plants and fungi, not just metazoa (animals), he is unable to address the possible range of CO2 and N2 pressures these complex life forms might be adapted to because there is next to no data on the effect high pressures and concentrations these gases have on plants or fungi, beyond incremental increases in CO2 to experiment on plant photosynthesis limits and productivity. Where we do have data is Earth’s history of complex life that indicates that relatively low levels of CO2 in the atmosphere due to volcanic emissions, and reduced plant life to draw down CO2 and replenish the O2 due to sulfur acid rains and ash-darkened skies, are sufficient to force most species, including plants, to extinction. We do not know what factor or combination of factors is important, nor whether it is primary factors such as anoxia, or n-th order factors that resulted in their final extinction.
Now that the inventory of exoplanets is rapidly increasing, it is certainly time that we start thinking more critically about what sort of life we are looking for and what that might mean for the range of the habitable zone that supports these different life forms. Rather than allowing the widest possible HZ that allows any atmospheric composition and pressure allowing liquid water, we could also be looking for possible constraints that appear required for the sort of surface, air-breathing complex life that will give rise to the charismatic fauna that we have on Earth. Dr. Ramirez has posited one interesting idea for terrestrial complex life that is based on respiration across a range of metazoans which then constrains the atmospheric gas composition and hence the HZ.
As Ramirez’ CLHZ has an outer limit well inside the orbit of Mars, this invites speculation that if Mars ever had any life during its earlier, wetter, period, it did not have complex life. If this model proves correct, while we may find subterranean microbial life on Mars, we will not find metazoan fossils, such as mollusk shells or vertebrate skeletons.
It should be borne in mind that life as a whole maintains Earth’s low CO2 levels to keep the surface temperature equitable for itself, maximizing biodiversity and biomass. While hotter (e.g. the Eocene maximum) and cooler (ice ages) periods upset that equitable temperature, life in concert with much slower geological processes act as a thermostat. It is also the case that biomass and diversity are greatest in the tropical forests and the lowest at the poles. It must have been relatively sparse during the “snowball Earth” period but recovered once the global ice sheets melted. Life has evolved on the Earth as it is, and has biochemistry that matches that requirement.
Today, that requirement is for an atmosphere that has a low CO2 level. On exoplanets, where much higher CO2 levels are needed to keep the planet warm, different biochemistries might develop, and this is a caveat that Ramirez considers for his analysis. However, without examples of such life, we are forced to use Earth’s life as our only model. In a half-billion or so years in the future, as the sun increases its luminosity, the required CO2 level to keep Earth cool enough will be below that needed by plants. A technological species might utilize technology like orbital sunshades or perhaps genetic engineering to maintain life on Earth.
The more important point is that we may be able to provide more granular characterizations of exoplanets. Rather than the binary in or out of the classic HZ for exoplanets and therefore potentially living or not, we can add granularity, such as inside the CLHZ and therefore capable of hosting complex life too. This conclusion does depend on exo-life following our terrestrial biology. If it doesn’t then we have to fall back to the more generous HZ calculations alone.
1. Ramirez, Ramses M. “A Complex Life Habitable Zone Based On Lipid Solubility Theory.” Scientific Reports, vol. 10, no. 1, 2020, doi:10.1038/s41598-020-64436-z.
2. Schwieterman, Edward W., et al. “A Limited Habitable Zone for Complex Life.” The Astrophysical Journal, vol. 878, no. 1, 2019, p. 19., doi:10.3847/1538-4357/ab1d52.
3. CO2 and O2 levels in the phanerozoic. Web accessed May 11, 2020. https://notrickszone.com/2018/05/28/2-new-papers-permian-mass-extinction-coincided-with-global-cooling-falling-sea-levels-and-low-co2/
4. Michel, E. L., et al, SP-368 Biomedical Results of Apollo – Chap. 5 Environmental Factors. Accessed from web, May 11th, 2020. https://history.nasa.gov/SP-368/s2ch5.htm