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Habitable, Not Earth-like

I’ve put off writing about Wesley Traub’s paper on the frequency of planets in the habitable zone because I knew Adam Crowl had reservations about Traub’s method. We talked about this at the 100 Year Starship Symposium, which led to Adam’s agreeing to writing this piece for Centauri Dreams. How you define a habitable zone is, of course, a critical matter, especially when you’re dealing with a topic as compelling as extrasolar planets that can support life. Adam places Traub’s work in the context of earlier attempts at defining the habitable zone and finds HZ estimates different from Traub’s, though one is surprisingly similar to a much earlier study.

by Adam Crowl

The recent paper by Wesley Traub [reference below] has estimated the frequency of terrestrial (“Earth-like”) planets in the Habitable Zone (HZ) of their stars based on statistical analysis of the recent Kepler data release, but the frequency computed, of ~34(+/-14)% around FGK stars, is dubious due to the assumption of wide HZ limits. Before I discuss the specifics, let’s look at the modern history of the “Habitable Zone”.

The modern discussion really began with Stephen Dole’s “Habitable Planets for Man”, a RAND commissioned study from the early 1960s, eventually updated in 1970, and popularized with Isaac Asimov. Dole based his HZ limits on the criterion that a significant fraction of a terrestrial planet would experience a “hospitable climate”. He didn’t examine the effect of atmosphere, and derived the HZ limits of 0.725 – 1.25 AU, from just outside the orbit of Venus and a bit closer to the Sun than Mars at its closest. Applying statistical analysis to various features of the known planets, then extrapolating to other stars, Dole found that potentially 645 million Earth-like planets might exist in the Galaxy.

In the mid 1970s Michael Hart developed the first evolutionary models of the atmosphere of an Earth-like planet, finding Earth to be poised on a virtual knife-edge, tipping towards a Runaway Greenhouse if closer than 0.95 AU and Runaway Glaciation if further out from the Sun than 1.01 AU. When this criterion was applied to other stars, the frequency of Earth-like planets was less than 1 in a quarter million stars, or less than 400,000 Earths in a Galaxy of 100 billion stars.

Hart’s limits seemed overly sensitive to climate perturbations, and further work on the evolution of Earth’s atmosphere in the 1980s led to the paper “Habitable Zones Around Main Sequence Stars”, (Kasting, Whitmire & Reynolds, 1993) , which redefined the debate. What James Kasting and colleagues discovered was a powerful feedback loop between the levels of carbon dioxide in the atmosphere, geological weathering and the heat input from the Sun.

This creates a self-regulated surface temperature which can keep water in its liquid range out to a significant distance from the Sun. The chief uncertainty came from the complication of dry-ice clouds. Past 1.37 AU clouds of dry-ice begin forming and by 1.67 AU the cloud cover becomes total, negating the effectiveness of the carbon dioxide greenhouse effect. Some preliminary work on water clouds also suggests the inner radius of the habitable zone, just 0.95 AU, might be extended to closer to Venus.

Traub’s paper has somewhat more generous HZ limits. Traub examined three cases, with the ranges from 0.72-2.0 AU in the best case, a nominal HZ of 0.8-1.8 AU, and a “conservative” 0.95-1.67 AU. Using the observed planetary radii distribution and the orbital radii, Traub was able to compute the frequency of terrestrial planets in these HZ as 34(+/-14)%, with the extremes providing the error bar limits.

Here’s where just what is computed and why is important. The ranges used by Traub for the HZ apply to specifically liquid water compatible planets with extensive greenhouse gas atmospheres. Such worlds, with up to several bars of carbon dioxide for atmosphere, are only distantly “Earth-like”, much like Mars or Venus can be called Earth-like. The Earth we know, with an oxygen rich, carbon dioxide poor, atmosphere is somewhat more sensitive to climatic instability. If more conservative HZ ranges are used a quite different result is obtained.

The HZ, inside of the CO2 cloud limit found by Kasting, et.al., is the more restrictive 0.95-1.37 AU. This gives a frequency of just 13.3%. If we use the Continuously Habitable Zone (0.95-1.15 AU), also from Kasting, et.al., then the frequency drops to a mere 6.3%. Using Hart’s even more restricted range drops the frequency to less than 2%. Another caveat is that the planet frequency estimated is limited to stars in the mass-range 1.13-1.01 solar masses and is yet to be extended into the wider population of stars which make up ~80% of the Galaxy.

The HZ limits derived by Kasting et.al. assumed ocean-dominated terrestrial planets. The broader range of land dominated “desert planets” (Abe et.al., 2011), with water bodies limited to circum-polar lakes/ice-caps, increases the HZ range to 0.75-1.3 AU, and a corresponding frequency of 17.3%. Incidentally this range is equivalent to that derived by Dole’s (1964) ground-breaking study.

So, in conclusion, the high frequency of “Earth-like” planets derived by Traub, is tempered somewhat when a more precise Earth-like Habitable Zone range is used. Planets warm enough for liquid water thanks to multi-bar atmospheres of carbon dioxide, methane or hydrogen, while probably conducive to extremophiles, aren’t “Earth-like” as usually understood, and this caveat should be more widely appreciated when making such estimates.

The paper is Traub, “Terrestrial, Habitable-Zone Exoplanet Frequency from Kepler,” available online as a preprint. Other references:

Y.Abe, A.Abe-Ouchi, N.H.Sleep, and K.J.Zahnle. “Habitable Zone Limits for Dry Planets”, Astrobiology, Volume 11, Issue 5, pp. 443-460 (2011).

S.H. Dole, Habitable Planets for Man, Blaisdell, New York (1964).

M.H. Hart, “Habitable zones about main sequence stars”, Icarus, 37: 351-357 (1978).

J.F. Kasting, D.P. Whitmire and R.T. Reynolds. “Habitable Zones Around Main Sequence Stars” Icarus 101: 108-128 (1993).


Comments on this entry are closed.

  • LarryD October 18, 2011, 8:59

    Perhaps what I’ve read is now obsolete, but the reconstructed history of Earth’s atmosphere had it originally around 90 bar, mostly carbon dioxide. With oceans of water, very early on. The water enable chemistry that formed carbonate minerals, sequestering enough of the carbon dioxide to thin the atmosphere considerably.

    We still have a lot to learn about Mars, but it seems to have had an early period that was warmer and wetter, with a thicker atmosphere and magnetic field.

    Venus is, insofar as we can tell, as dry as a bone in the desert. No internal dynamo, yet it retains a 92 bar atmosphere.

    We plainly still have a lot to learn about what make a terrestrial world habitable, insolation is only one of the variables, and we don’t understand if it has any connection to the presence of copious amounts of water.

  • Alex Tolley October 18, 2011, 9:37

    Isn’t even more complicated by the changing intensity of the star? The sun has increased in intensity during earth’s “habitable” period, and it is predicted that within just a billion year or so will be uninhabitable as life will no longer to able to regulate its temperature by removing CO2 from the atmosphere. In the past, we may even have had an ice covered snowball earth, just before the Cambrian explosion of life.

  • Eniac October 18, 2011, 11:31

    Following Alex Tolley, and going with a 30% change in solar output over the past gigayears, would that not mean that whatever we call the HZ should be at least wide enough to include the entire range over time? Given the square law, this would exclude quite a few of the narrower ranges proposed.

    To me it seems likely that the increase in solar luminosity was balanced by a decrease in greenhouse gases. This decrease is at least partly due to biological fixation of carbon via photosynthesis and calcification. In addition, the free atmospheric oxygen generated by photosynthesis prevents other greenhouse gases like methane or hydrogen from accumulating in the atmosphere. It seems that life has managed to modulate its own environment to remain temperate for a long time. This is a far cry from the “knife-edge” view of Hart. More like Kasting, although I wonder if Kasting did not underestimate the role of biology.

    It is also important to note that Dole assumes with little discussion that life will always arise under the right conditions. If this turned out not to be correct, there would be very few habitable planets under Dole’s criteria, which include a breathable atmosphere.

  • Ronald October 18, 2011, 12:03

    Good work, Adam, fascinating stuff!

    Since I was impatient, I wrote a small summary of Traub’s paper (my comment of 12 October) under the post ‘A Wary Look at Habitable Worlds’,

    In this I mention that another reason for the very high estimate of the frequency of “Earth-like” planets in the HZ of their stars is the fact that in Traub’s paper (quote):
    “Terrestrial planets are taken to be those with 0.5 ≤ r ≤ 2.0, corresponding to roughly 0.1-10 Earth masses”

    I find that a rather wide mass range, it also includes the super-earths, possible holding on to a very dense not-so-earth-like atmosphere and the smallest terrestrial planets down to about Mars size, too small to hold on long enough to a significant atmosphere.
    I would find a planetary mass range of about 0.3 – 3 Earth masses more realistic.
    The size distribution of planets in this range is not even (the larger terrestrial planets, super-earths, seem to be more common), but even if we assume a more or less equal distribution in this range, my mass range suggestion would reduce the frequency of earthlike planets in the HZ by another factor of at least 3.
    Combining that with Kasting’s HZ of 0.95-1.37 AU then gives a frequency of about 4%, with Kasting’s CHZ of 0.95-1.15 AU gives a frequency of about 2%.
    Still a bit higher than previous recent estimates of about 1%.

  • Martin J Sallberg October 18, 2011, 12:31

    While a multi-bar CO2 atmosphere would not be human-breathable, it would still, with liquid water and livable temperatures, be less human-hostile than Mars and MUCH less so than Venus. If a “outer edge” world has lots of oceans, the original CO2 atmosphere evens the temperature, creating high evaporation worldwide, and the water vapor may replace CO2 as greenhouse gas, assisted by plant life producing condensation-inhibiting chemicals to avoid shadow. Even without such a change, there could still be an ecosystem with complex native life, and human explorers would only need a mask, not a full pressure suit. Of course there can be habitable worlds orbiting dimmer stars. Modern research shows tidelocked worlds would NOT lose their air to freeze-out, winds would redistribute heat. Antifreeze worlds with mixed water-ammonia oceans would be hostile to humans, but native complex life would be possible.

  • Athena Andreadis October 18, 2011, 13:11

    The burning (or freezing) question, of course, is: habitable to whom?

    As long as we don’t have a second life sample, all these calculations are strictly mental exercises. Also, it’s almost certain that Hart’s personal biases colored his work (as did Guillermo Gonzalez’). It’s discouraging, though not surprising, that the habitable zone research is crowded with hierarchists of various stripes.

  • Leon October 18, 2011, 13:29

    I think we now know 2 habitable worlds, mars being the second. It meets the requisite of water in the surface , we can´t dismiss it just because we are living in another era of the universe

  • Dave Moore October 18, 2011, 14:56

    I think this is an area that is suffering from a lack of hard definitions. It is very hard to get the frequency of something that is only fuzzily defined.

    I would suggest that those involved in the field think carefully about the matter and come up with a set physical parameters for the definitions then get them recognized by the IAU. We now have enough of an idea about planets to start coming up with useful definitions.

    I’ll throw my initial suggestion into the ring with the following definitions (This are off the top of my head):

    Human Habitable:
    Temp range: 0-40 deg. C somewhere on planet.
    Partial pressure of O2: 150-400 millibar
    Liquid water. Some land surface?
    Max atm. pressure: sufficiently low to avoid narcosis.
    Max surface gravity: 2g?
    No gases at toxic levels.

    Liquid water. Some land surface?
    CO2, N2 in atm.
    Temp range: 0-40 deg. C somewhere on planet.

    This definition would probably be broad enough to cover Earth in all its iterations during its history from when life first developed. It would not cover such things a carbon planets with a dry, CO2 atm and liquid CO2 oceans, Hydrocarbon planets or their ilk.

    There would then also be the broader definition Terrestrial planets, which again needs careful definition. For instance, would a Earthlike, Earthsized planet covered with a deep ocean be considered a terrestrial planet? or would there be a separate category of Oceanic?

    The first step in understanding something in science is classification.


  • Bob October 18, 2011, 17:14

    “The burning (or freezing) question, of course, is: habitable to whom?”

    I would consider the Moon and Mars both habitable to our growing technological civilization as well as many other places including free space itself as long as one has access to energy and resources (which we do). A much more advanced civilization might find almost any stellar system habitable. Of course, any civilization capable of even a very slow migration to other stellar systems would likely have the capability of using a wide variety of situations to make a way to live around that system if it has basic resources such as water, metals and minerals.

  • andy October 18, 2011, 17:53

    Hart’s estimate of the outer boundary of the habitable zone is implausible in the light of the luminosity evolution of the Sun over the past 4.6 billion years.

    And now we see that low-mass planets can have fairly substantial atmospheres, it is worth considering how hydrogen/helium can affect habitability: certainly it would seem that a mini-Neptune could maintain liquid water substantially further from the star than a traditional carbon dioxide greenhouse atmosphere. (Neptune itself is too hot and dry to maintain liquid water at the current time.)

    Another thing to bear in mind is that all of us, save those who have had visual disabilities from early life, have seen with our own eyes a full-disc view of a terrestrial planet located in the habitable zone of a G2-type star. It looks like this. The HZ is not set by the star alone!

  • Enzo October 18, 2011, 18:02


    Mars looks deceptively habitable because it does look a bit like Earth with its polar caps, its 24 hour something day and its dunes. The fact is that the atmosphere is almost vacuum and there’s no substantial body of liquid water on and below the surface. If there was, SHARAD and/or MARSIS would have found it. Even under the insulating blanket of the polar caps, where residual heat from underneath could have created lakes, nothing has been found. Nothing of any substance. From the surface, Mars looks like it has been bone dry for billions of years.
    Some liquid water must exist somewhere under the surface of Mars since there is a lot of ice and, eventually, deep enough some heat is still there.
    No sign of current volcanic activity has been spotted, not even hydrothermal springs. If it had, Mars Odissey would have spotted them with THEMIS from space, shining like a beacon in the freezing martian night.
    No organic compounds have been found by two missions. Hopefully MSL will be more lucky.

    Europa, ad possibly Enceladus, are much more habitable with plenty of liquid water, abundant energy source in the form of hydrothermal vents and, for the former, the possibility of an oxygenated ocean:

  • Adam October 18, 2011, 19:46

    Hi All
    Great comments.

    Alex & Eniac, to clarify these ranges do take into account the Faint Young Sun.

    Athena, perfect aim, as always. Our sample size is excessively small and we extrapolate warily.

    Dave, almost exactly the same as Dole’s criteria. My friend Martyn Fogg years ago distinguished quite usefully two grades of biologically interesting: Biocompatible, and (Human) Habitable. One is naturally a much narrower range than the other. Would be useful if the dichotomy was more widely used.

    Martin, a very good point. Such worlds can be inhabited, and not just by us, but they aren’t strictly “Earth-like”.

    Glad to provoke intelligent discussion.

  • Rob Henry October 18, 2011, 20:07

    We like to believe that no planet could possibly be more conducive to the development of higher life than an Earth-like one. We keenly write off the fact that complex animal life only appeared in our fossil record immediately after we had experienced a rare phase of Europa-like Snowball Earth conditions as a coincidence… but was it?

    If such conditions are the real key to triggering the development of higher life then I realise that a Water World covered by a crust of ice is not sufficient. We would additionally need sufficient energy flow for the biosphere to become interesting and probably need sufficient gravity to hold an (oxygen) atmosphere outside this shell. Since (to my mind) only photosynthesis or photodissociation can supply enough energy to provide this interest, we would need large areas where the surface ice is melted through, and the easiest way to supply this high level of geothermal activity per surface area would be if that planet was a super-Earth with a high initial inventory of radionuclides.

    Thus even drawing from our anthropocentric example, it would not seem too much of a stretch to conclude that almost all other higher life in our galaxy hailed from planets in the 2-10 Earth mass range at the equivalent distance of 2 – 5 AU.

    You may point out that I have drawn too heavily from a single case, but I note that all others calculations are similarly ill-founded. However, every journey must begin with a single step.

  • Abelard Lindsey October 18, 2011, 20:30

    “The burning (or freezing) question, of course, is: habitable to whom?”

    This is what many of us have been saying from the beginning. Especially considering that planets with multi-bar CO2 atmospheres probably outnumber “Earth” planets ten or a hundred to one. At least the heavy CO2 atmosphere ought to be good for the plants.

    “I would consider the Moon and Mars both habitable….”

    Are you kidding? I want to stand outside without having to wear a pressure suit or even having to use breathing apparatus (like in climbing Mr. Everest).

  • amphiox October 18, 2011, 23:28


    I think @Leon’s point re: Mars’ habitability is that now we have very good evidence that at some point in the distant past Mars was fully habitable, by our current definition of habitable, with a thick atmosphere and large quantities of running water on its surface (at a time when the sun was less bright than today, no less), which means that if we were to one day detect a Mars-sized exoplanet in a Mars-sized orbit around a sun-like star, we should not dismiss the possibility that this exoplanet is habitable, just because it is apparently too small (like Mars), or too far away from its parent star (like Mars).

  • jkittle October 18, 2011, 23:38

    Everyone has come up with lots of factors
    I propose a ” main sequence” approach to organizing these parameters:
    By analogy, the main sequence of stars is platted on a two D plot absolute maganitude vs color ( or temperature)
    We could producea three dimensional plot
    Absolute energy radiance of the star , Size of the planet, and average distance between planet and star. ( highly elongated orbits need not apply)
    The trace of (predicted) habitable planets would thread there way through the plot as a distinct elongated “cloud’ shape and meander according to composition of the planet. (this composition – related meandering is consistent with the stellar diagram.)
    Habitability is not just a funtion of the star energy output and orbital parameters, the planet mass is also important!

    What do you think?

  • coolstar October 19, 2011, 1:17

    I’m afraid that this analysis just pretty much totally misses the point of the Traub paper and the earlier paper of Catanzarite and Shao. That being to conservatively estimate the value of eta-earth from the existing Kepler data.
    To do that in a meaningful way one has to adapt SOME standards as to what HZs to assume, along with the radii of “habitable” planets. Traub and C&S simply used values that are common in the literature (none of these authors are atmospheric modelers, and defining the HZ is not what their papers are about). This allows others to play whatever games they want with defining THEIR favorite HZ and planetary radius ranges. These games have some value, but the mere fact that the different HZs cover such a wide range really illustrates the present state of our ignorance. Given that ignorance, setting conservative standards seems imminently sensible. The important and hard work in these papers lies in their careful analysis of the Kepler data. First one has to FIND the planets, THEN one can quibble over whether you’ve find the RIGHT planet.
    As I’ve stated before, the really important thing in these papers is the different way in which they extrapolate the Kepler data to longer orbital periods. This accounts for a difference in eta-earth of a factor of 30x between the two studies. Once this is nailed down, then one can resume the sure to be decades long discussion of how to best define the Goldilocks Zone,
    with everyone free to choose and defend their favorite values, based on their unique definitions of “habitable”.

  • spaceman October 19, 2011, 1:48

    Even if a planet roughly the size of Earth orbits within its star’s habitable zone, this does not necessarily mean that it must be habitable to humans. This may seem obvious to some, but I think the media in particular tends jump, when they hear the term “habitable zone” along with the word “planet”, to unwarranted conclusions. This is not too say that the discovery of smaller planets in the habitable zone is not truly exciting, it just means that we need to be careful before making the leap from “being in the habitable zone” to being “habitable.”

    There are reasons why roughly earth-sized planets smack in the middle of their stars’ habitable zones may not be habitable. For example, what if the orb had insufficient water? Water is crucial for life on our planet and probably crucial for alien life on extrasolar planets as well. What if many of the earth-sized planets in the HZ of their stars happen to be bone dry?

    Another issue at work usually not considered in these discussions is this: in the case of earth-sized worlds in the HZ that have enough water and even have a biosphere, what if that biochemistry is toxic to humans? Imagine a world with some type of pollinating planet life that filled a temperate oxygen rich atmosphere with molecules that would make hay-fever allergies seem like a walk around the park? Is there really any way to determine whether or not a planet of this type has a toxic biochemistry without traveling there? Sending a well-designed probe there first would help us glean more info on the local biochemistry, but once it comes time for humans to travel there we might have to live in space domes, or, maybe even terraform the planet. What would be the ethical implications of terraforming another planet that had preexisting life? And if terraforming was found to be necessary, then why travel there in the first place other than for adventure…why not just take a stab at terraforming planets and/or moons within our own solar system? It seems to me that combining interstellar travel with terraforming would be an extreme rigorous task to put it mildly. Indeed, could there be a toxic local biochemistry solution to the Fermi paradox?

    The toxic local biochemistry solution to the Fermi paradox would work as follows: an alien civilization uses very sensitive space telescopes to find a planet with atmospheres similar to their planet’s atmosphere. Next they send out an interstellar probe which takes breathtaking pictures and gathers invaluable scientific data. Unfortunately, the probe determines that the planet has pathogens and/or toxic molecules emanating from the local planet life. This would very much dampen the rationale for interstellar travel and refocus a spacefaring civilization back towards maximizing the use of its own solar system. Not to mention that even “toxic local biochemistry planets” would probably be very apart maybe ~tens of light years.

  • Ronald October 19, 2011, 7:41

    @Alex, Eniac, with regard to increasing solar output: that is precisely why the Continuously Habitable Zone (CHZ) has been defined, the range which remains HZ over a sufficiently long time period to allow for liquid water and water dependent complex life (at least 3 gy, if I am not mistaken).
    That is also why the CHZ of our Sun, ref. Kasting, is narrower (0.95-1.15 AU) than the present HZ (0.95-1.37).
    With regard to the narrower HZ and CHZ of dimmer stars: while it is true that dimmer (later spectrum) stars have a narrower HZ, their long-term stability and life-span are also much longer, so that for those stars the CHZ practically coincides with the HZ.
    And vice versa: bright early spectrum stars (F) may have a wider HZ, but it moves outward so fast that the resulting CHZ becomes narrow. In fact, beyond a certain brightness and spectrum (about F5-F7) the CHZ probably becomes zero, i.e. there will not be any place within the HZ during a few gy.

    What is remarkable and important here is that, while the outer edge of the HZ (or CHZ) is quite far away and rather fuzzy, also depending on the planet’s size and atmospheric conditions, the inner edge is, however, quite sharply delimited. Most authors (after Dole) put it at about (0.93-) 0.95 AU. This is, because any closer there will always be a runaway greenhouse effect due to high insolation and resulting early photo-dissociation of water. The root problem on Venus is not the excessive CO2 but rather its chronical lack of water, even inhibiting necessary ‘lubrication’ of the mantle and plate tectonics.
    For the same reasons (early high insolation induced photodissociation of water plus fast-moving HZ) I think that we will find that ‘habitable stars’ are sharply delimited on the hot/bright side of the stellar spectrum (probably somewhere around latest F), whereas they may continue far into the later (dimmer) spectral types (early/mid K or even further?).

    With regard to Mars and habitability: the real problem with Mars is probably not foremost its location, at about 1.5 AU, but its too small size: with only 0.11 Earth mass, it is simply too small to hold on to a significant atmosphere long enough plus the fact that due to this small size Mars cooled down too quickly for plate tectonics to continue. Mars is basically a geologically (near-) dead planet. It is regrettable that Venus is not in Mars’ location. It would also have been a very good test to establish the outer boundary of the HZ.

    So habitability does not only depend on the stars’s HZ but at least as much on the planet’s characteristics, foremost its mass and composition, and particularly on the outskirts of the HZ.
    0.1 Me, as used by Traub as a lower limit for earthlike planetary mass, is probably just too low. 0.3 Me may be closer to it.

  • Alex Tolley October 19, 2011, 9:51

    I do find the idea of “habitable zone” almost colonial in it’s expression, as if a world should be able to support a human colony without resort to much technology.

    As we have discussed on other threads, star flight difficulties not withstanding, it is probably far more likely that human expansion will be in space colonies, rather than planetary surfaces. The “planetary chauvinism” idea that Asimov[?] noted after the space colony idea was proposed.

    My interest is rather which worlds may support life, micro or macro. What I find interesting is that whilst we still have only a single sample, speculation has widened from HZ limits to a much broader category. Finding definitive bio signatures, or direct imaging at some point, is more interesting, especially if it turns out that HZs turn out to be not the main abode of life, just one category.

  • Ronald October 19, 2011, 10:45

    Several people here suggest that the definition of ‘habitable’ and ‘habitable zone’ would not be clear and needs further specification.
    I do not agree with that and think that the definition of the HZ, as most commonly used (partic. by Kasting et al.) is both the most unambiguous and the least ambitious: it is simply defined as the maximum zone within which liquid surface water can exist on a terrestrial planet.
    So, this is not necessarily habitable to humans, but rather biocompatible to any liquid water dependent life.
    Particularly on the outside that also depends on the planet’s atmospheric conditions, which is also why that outer limit varies so much among researchers.

  • Ronald October 19, 2011, 10:55

    One issue that can really puzzle, or even worry me, when it comes to the atmospheric aspects of habitable planets and the prospect of terraforming (Mars!) is the presence, or absence, of atmospheric nitrogen (N2).

    I find it surprising that this very important atmosphic component is rarely mentioned with regard to planetary habitability, even among the terraforming adepts (Fogg), or I have missed something.

    E.g. when terraforming of Mars is discussed, it is usually mentioned that the abundantly present CO2 could be evaporated out of the soil and polar caps and subsequently converted to O2 (plus C in plant biomass, of course), however, where will all the required N2 come from, as a largely inert atmospheric component? Both Venus and Mars seem to have a paucity of (atmospheric) N.
    And with regard to habitable planets: how likely and common will an N2 atmosphere be, also relative to CO2/O2?

  • Athena Andreadis October 19, 2011, 11:27

    To clarify, my question (habitable to whom?) intended to point out that a planet can be teeming with life but be hostile to humans. Hence, any extrapolations of HZ based on our own single sample are based on incomplete data. Carbon and water are privileged; some source of energy and some kind of atmosphere is required; but that’s the extent of generalization we can make.

    Ironically, the more Earth-like and “human-friendly” a planet is, the likelier it will be to have developed native life — and any attempt to settle humans and their impedimenta on it will result in a process that ends in -cide, unless the humans change themselves to fit into the new home without leaving a wake.

    Some have suggested that this is the back story of Avatar’s Pandora — which would explain why the Na’vi look humanoid while the rest of the planetary fauna is six-limbed and four-eyed. But Cameron vitiated this intriguing possibility by stating the Na’vi are not mammals, despite the promiment mammary glands he gave Whatsername in an explicit bid to ensure the attention of adolescent boys of all ages.

  • Martin J Sallberg October 19, 2011, 12:55

    @spaceman: Biochemistry is shaped by the environment. The cliche about a globally shared arbitrary biochemistry is nonsense, since if there was no strong selection pressure to keep biochemistry the same, life would evolve fundamentally incompatible biochemistries to protect themselves from being eaten. As the example of anaerobic Lorificerans evolving gradually from aerobic ancestors shows, Stephen Jay Gould´s distinction between diversity and “disparity” is just as wrong as the creationist distinction between “microevolution” and “macroevolution”. Therefore an otherwise habitable world would not have an incompatible biochemistry. If you want to find a biosphere where you could NOT live of eating any of the local life you should search for life in non-water solvents.

    @Ronald: It does not necessarily take billions of years to evolve complex life. Modern research on evolution shows that the limiting factor on evolution is selection pressure, not mutation rate. Suitable environmental change, the sort that creates new life opportunities at the same time as it destroys other life opportunities and wipes out specialists while body size hardly matters at all for the risk of dying out during them, is the most important factor.

    @Alex Tolley: It is absurd to think all space colonists would use the same strategy. Some will prefer artificial habitats, others will prefer planetary surfaces. There just is no single method for everyone.

  • Winchell Chung October 19, 2011, 13:54

    Adam, what is your opinion on the 2002 paper by Jill Tarter and Margaret Turnbull: Target Selection for SETI, a Catalog of Nearby Habitable Stellar Systems?


  • Athena Andreadis October 19, 2011, 14:28

    Martin Sallberg, where exactly do you get your biology? Gould is (was) right: diversity is totally different from disparity. All terrestrial biochemistry is a single system and an independently evolved biochemistry could/would be different in a myriad ways. Spaceman mentioned one — different chirality is another (that would not make it toxic, just completely inedible in terms of metabolism and energy production); so is reliance on different heavy metals.

  • Bob October 19, 2011, 14:45

    “I would consider the Moon and Mars both habitable….”

    “Are you kidding? I want to stand outside without having to wear a pressure suit or even having to use breathing apparatus (like in climbing Mr. Everest).”

    No, serious. I stated “I would consider the Moon and Mars both habitable to our growing technological civilization as well as many other places including free space itself as long as one has access to energy and resources (which we do).”

    I assumed technology which is a natural assumption for a technological civilization. You cannot live in most places on Earth without some technology some of the time.

  • jkittle October 19, 2011, 16:18

    We will live with the the technology we take with us and the technology we evolve in Situ. Terraforming is for vacation worlds perhaps, or to make a nice park to play in… but here is a key point: Terraforming is TOO SLOW to keep ahead of an aggressively expanding technological civilization. By the time you have an oxygen atmosphere and advanced ecosystem in place, the Human population would already be fully invested in more practical technologies to use resources as is- making pressurized habitats of climate-enclosed spaces, using technology to regulate temperature, atmosphere, food production, purify water, reproduction of symbiotic ( pets) species etc. we can live on these worlds with machines/technology and no Terraforming but we cannot survive there with Terraforming and no machines. Terraforming is expensive and risky if it is not needed.

  • LarryD October 19, 2011, 17:54

    @Ronald: Venus actually has about as much nitrogen in its atmosphere as does Earth, it’s just that all that CO2 makes it relatively negligible.

    What Mars mainly lacks to today is atmosphere, only in the lowest lying areas does the pressure get up high enough (barely) that water could exist in liquid state, elsewhere on Mars it just sublimates and freezes. I think s step one of terraforming Mars would be “just” to add several million tonnes of nitrogen to its atmosphere and then analyze the results.

    Venus needs water, and it’s too close to the sun. But we don’t have the tech to even begin to contemplate moving Venus out. It also needs to be spun up, to get a dynamo effect going. Or maybe not, it’s still got a massive atmosphere, after all.

    Chirality may not be a random factor.

  • Dave Moore October 19, 2011, 18:52

    Just remember, that for most of Earth’s history. Earth has not been “habitable” and that if we colonized it, say via time machine, we would have to live in a sealed off environment not much different to a colony on Mars.

  • Jim Baerg October 19, 2011, 20:55

    Ronald: “Both Venus and Mars seem to have a paucity of (atmospheric) N.”

    Actually Venus has about 3x as much N2 as earth. It’s just diluted in excessive amounts of CO2

  • Adam October 19, 2011, 21:29

    Excellent discussion everyone.
    Coolstar, I appreciate the effort in both papers and I do get the point of their analyses. What I am high-lighting is an issue which needs to be made clear to the interested public – biocompatible doesn’t mean “Human habitable”.

    Win, I read that paper years ago. I will need to reread it to give you an updated view on it. A lot of climate modeling with more sophisticated 2-D and 3-D models has occurred since then.

  • Ronald October 20, 2011, 7:31

    @Jim Baerg and LarryD: you are (luckily) right aboiut Venus having plenty of atmospheric N (it just being too diluted by the excessive overdose of CO2).
    This could indicate that abundant atmospheric N2 is rather normal for an earth-sized terrestrial planet, which is not too surprising, because N is one of the more common elements in our galaxy (and the universe), besides O, C, Si, Fe and a few others (and of course way behind H and He).

    Venus’s problem is indeed its proximity to the sun and its resulting lack of remaining water, which would make any terraforming a heck of a job, as Zubrin has also pointed out (one would have to import enormous numbers of comets from the Kuiper Belt or Oort Cloud.

    Compared to this terraforming of Mars seems te be a walk in the park, because of its relatively abundant water.
    Question remains, though whether there is enough N on Mars, either atmospheric (there isn’t), in the soil (probably not), or in the polar ice caps (not enough).
    It could be that Mars lost most of its N2 because of its lower gravity.
    The need for an atmospheric buffer gas (most likely N2 or Argon) is one of the greatest challenges of terraforming Mars.
    In fact this may set a ‘minimal mass requirement’ to planetary mass (= mass great enough to hold on long-term to a significant N2/O2 atmosphere), which in turn may set another limit to planetary habitability.

  • Ronald October 20, 2011, 7:42

    @jkittle: “Terraforming is expensive and risky if it is not needed”.

    I disagree: planets, because of their large size (in comparison with any space habitat constructed by humans in the foreseeable future) and their natural origin, possess a great deal of intrinsic stability that a space habitat would not have. One could perhaps counter that a terraformed planet would not be in an equilibrium condition either, however, even if so, the natural adverse changes (reverting back to original state), once terraformed, would only take place over very long time periods.
    Besides, as Dave Moore also points out, even many potentially habitable terrestrial planets may not yet be in that state (i.e. still in a more primordial one) and terraforming could then speed up certain processes and rapidly push such a planet toward habitability.

  • Martin J Sallberg October 20, 2011, 11:55

    @Athena Andreadis: Anaerobic Lorificerans have gradually evolved from aerobic ancestors, which debunks any distinction between diversity and “disparity”. It proves that fundamental biochemical traits can change through gradual adaptation. The high omega 3 content in the fat make-up of aquatic mammals is similar to that of fish, which shows that biochemistry is environmentally adaptive and non-arbitrary. Over billions of years any common biochemistry would fade into unrecognizability if there was the evolutionary pressure, and predation provides evolutionary pressure to evolve biochemical incompatibility (as the widespreadness of natural poison proves). Deeming a trait arbitrary is just ignorance of what its selective usefulness is. How do you think “disparity” could be different in kind from diversity when evolution is modification of what already exists? It contradicts itself.

  • Athena Andreadis October 20, 2011, 11:58

    To LarryD: It is worth pointing out that 1) handedness is a property of all biomolecules, not just amino acids, 2) isovaline, the topic of the chirality article you linked to, is not found in terrestrial proteins — so their conclusions only apply to amino acids found in meteorites, which may not have contributed to the rise of proteins on earth and 3) the authors themselves say at the end of their article that other parts of the galaxy or universe may well produce radiation or polarized light that encourages right-handed amino acids.

    In other words: planets with native life may well have a different chirality from ours.

    To Ronald: space habitats are stable compared to planets? Think this through. And if you think we have the faintest idea how to “push such a planet toward habitability” think again.

  • Athena Andreadis October 20, 2011, 13:31

    Martin Sallberg: a little knowledge is a dangerous thing, and so is fuzzy logic. Of course, scientific gobbledygook is in a class by itself.

  • andy October 20, 2011, 15:42

    Talking of Venus, one of my favourite things about the climate of that planet is how energy-efficient it is. The Venusian surface is not hotter than the Earth’s because the planet is closer to the Sun: the highly reflective global cloud layer more than compensates for the increased energy flux at 0.72 AU, so in fact the absorbed flux is less than that of the Earth.

    (The other point is that it doesn’t have an especially high amount of carbon dioxide relative to Earth, provided you take into account the various stored forms of carbon in the crust, e.g. carbonates.)

  • Adam October 20, 2011, 19:02

    Hi andy
    Nice reminder about Venus. Something very odd happened to Venus to perturb it into its current end-state IMO. Pierrehumbert has looked at the stability of all that CO2 at lower insolation or higher IR levels. It’d collapse into an ocean of liquid CO2 if Venus was much further from the Sun – only 8% of the incident flux reaches the ground to drive the greenhouse effect, so a different light-balance would make it unstable. If a Venus was around a red-dwarf, the atmosphere would begin condensing and would probably end in up in the rocks.

    There’s about ~45 bar equivalent of CO2 in Earth’s crust, so Venus is only x2 more than Earth. Perhaps the transition to the current atmosphere occurred same time as the global resurfacing event?

  • Ronald October 21, 2011, 9:16

    @Athena: you must have read my comment in a hurry, I actually said the opposite: “planets, (…), possess a great deal of intrinsic stability that a space habitat would not have”.
    So: planets are much (MUCH) more stable than any artificial space habitat.

    How to push a (potentially suitable, primordial terrestrial) planet toward habitability: anything that converts a CO2 dominated primordial atmosphere into an O2 rich atmosphere. This could for instance involve the use of (genetically engineered) prokaryotes (cyanobacteria).

  • Ronald October 21, 2011, 9:25

    @andy, Venus: yes, confirms how truly a sister planet Venus is, (originally) similar in composition.
    But indirectly it is indeed Venus’ proximity to the sun which most probably made it so much hotter: the high insolation caused water evaporation and dissociation, H2 leaving the planet and O2 being bound to C as CO2, in turn causing the present runaway greenhouse. And inhibiting plate tectonics, which also seems to be facilitated (lubricated) by water in the mantle.

    If, as Adam points out, the sun would have been a dimmer star, or Venus farther away from our sun, this most likely would not have happened. This also sets an absolute lower limit to the inner edge of the HZ: Venus is way too close.

  • Eniac October 21, 2011, 11:15

    @Andy: This is interesting about the CO2 in the Earth’s crust. Is it plausible that, had it not been fixated through photosynthesis and biological calcification, all that CO2 would still be in the atmosphere? That Earth would be much more like Venus today? That perhaps it was more like Venus early on? That would mean it is Earth that is odd, not Venus. This would also fit with the CO2 atmosphere on Mars.

    @Ronald: From what I understand nitrogen (or any filler gas) is not at all necessary for a breathable atmosphere. Pure oxygen at the right pressure (20% atmospheric, I think) would do. I think that was laid out in the Dole study, and was not the Apollo 1 fire caused by a pure oxygen “atmosphere” that they gave the astronauts to breathe?

  • Eniac October 21, 2011, 11:38


    … H2 leaving the planet and O2 being bound to C as CO2, in turn causing the present runaway greenhouse.

    I am not sure this part makes sense to me. Where was that carbon before it got bound by the oxygen? Methane, perhaps? That would be a greenhouse gas too, or not?

  • Athena Andreadis October 21, 2011, 14:55

    Ronald: you’re right, I misread your first sentence! So we agree on that, although my point about terraforming still stands. We’ve made a lousy job of “terraforming” even Earth and the scenarios I hear for stabilizing the climate are frankly hair-raising.

  • Rob Henry October 21, 2011, 18:39

    Eniac and Ronald seem to be skirting around a factor in the above article that I (nor apparently they) have really understood. Every study done on habitable zones seems to claim any Earth-like planet that is just a little closer than the insolation equivalent of 1 AU will become Venus-like in under a billion years.

    We all know that this involves moving the cold trap higher where uv light can photodissociate water, and then the planets gravity is too low to retain hydrogen from thermal loss, and geological processes easily absorb the oxygen due to its high reactivity. But such an overview is grossly inadequate.

    1) I would love to know how high the oxygen levels are during the transition period, and thence the ozone protection. Why is hydrogen loss expected to be so much greater than a pre-biotic Earth with a lower cold trap but an ozone free reducing atmosphere? and do oxygen levels during this process get so high that their chemical potential would destroy most “interesting” carbon based molecules – is there any possibility that such a process once destroyed a Cytherean biosphere?

    2) how quickly can all the oxygen sinks remove this oxygen, and once saturated how quickly can geological processes bring fresh reduced material from that planets depths, and would all planets be expected to be similarly efficient at this process.

    3) Could oceanic life absorb the hydrogen so quickly as to reabsorb the majority of this hydrogen before it escapes, and could this change the analysis significantly? Something like this has been implied for Titan.

    4) Finally I would love to know what would happen if we suddenly added Earths inventory of water to Venus and came back in just a million years. Would the surface be only tens of degrees hotter than the Earth today or would it be hotter than it is now due to the greenhouse effect of steam. Even then there would still be a zone for Earth-type life there, but it would be high in the atmosphere!

  • Ole Burde October 22, 2011, 11:58

    On earth it seems that microbial life started almost as soon as the conditions made it possible . Therefore we have no empirical reason to suspect it wouldn’t do the same on any earthlike water-rich planet.
    In a few years it should become possible to verify the existence or non- existence of such planets , incuding the presence of fhotosyntesis .
    Only a fotosyntesis- carrying planet can truely be called “habitable ” in a realisic scenario, where a small group of humans arrieves after having done the almost impossible feat of surviving and maintaining their starship for hundreds of years .
    There wil be no returnticket , nobody to call for help , and they wil probably be running out of time in a thousand ways . Only if they can land on a hospitable life-carrying planet do they have a chance of longterm survival at all . Only such a planet would make them start the journey at all .
    All these considerations leads me to the conclution that a solid common sense benchmark plan must be one involving atleast a 50 LY distance to the target planet , and a travel time for atleast 500 years . Anything better than that would be unrealisticly optimistic ,until prooved otherwise .
    With a 500 years traveltime , the propulsion system becomes just one of many heavy problems that have to be overcome ….

  • Eniac October 22, 2011, 14:03


    On earth it seems that microbial life started almost as soon as the conditions made it possible .

    From what I can tell, it “seems” like life started at most 1 billion years after conditions made it possible. The evidence is scant, though, and it may have been as much as 2 billion. Not “soon”, exactly, in either case.

    Therefore we have no empirical reason to suspect it wouldn’t do the same on any earthlike water-rich planet.

    We can suspect just that. We have a few, weak, empirical reasons: We have seen no signs of ET life anywhere, and we have not detected any ETI. You are right that the current exoplanet boom may substantiate this as yet very scanty evidence, but as long as it remains negative you will always get: “We did not look hard enough, yet”.

  • Rob Henry October 22, 2011, 15:25

    Ronald, I can explain the lack of discussion on the apparent need for a high N2 inventory. At 3kg per square metre there is ample N for all biological needs, but we want an Ecological balance, and achieving that in terraforming currently means much handwaving.

    Unless biological nitrogen is continuously cycled from atmospheric N2, it will nearly all end up in the seas and waterways, and land based life would be very limited. Fixing nitrogen is hard, but so useful that bacteria that do it have clear adaptive advantage. Biological denitrification suffers the much worse problem that it is against the chemical equilibrium, and gives no killer advantage to its practicing bacteria. They are thus susceptible to displacement, and may be required to exist in huge inventories of nitrates (as measured by their total presence within the biosphere, not their microhabitats) before they will perform their tasks adequately. Without them all land based life is doomed!

  • Rob Henry October 22, 2011, 15:29

    Ronald, I can explain the lack of discussion on the apparent need for a high N2 inventory. At 3kg per square metre there is ample N for all biological needs, but we want an Ecological balance, and achieving that in terraforming currently means much handwaving.

    Unless biological nitrogen is continuously cycled from atmospheric N2, it will nearly all end up in the seas and waterways, and land based life would be very limited. Fixing nitrogen is hard, but so useful that bacteria that do it have clear adaptive advantage. Biological denitrification suffers the much worse problem that it is against the chemical equilibrium, and gives no killer advantage to its practicing bacteria. They are thus susceptible to displacement, and may be required to exist in huge inventories of nitrates (as measured by nitrogens total presence within the biosphere, not just in their microhabitats) before they will perform their tasks adequately. Without them all land based life is doomed!