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Is the Term ‘Habitable Zone’ Viable?

I’m not much for changing the meaning of words. True, languages always change, some at a faster clip than others (contrast Elizabethan English with today’s, though modern Icelandic is structurally very similar to the Old Norse of the sagas). But I love words and prefer to let linguistic variety evolve rather than be decreed. Even so, I get what Elizabeth Tasker is doing when she makes the case for exoplanet hunters to do away with the term ‘habitable zone.’

In a comment to Nature Astronomy, Tasker (JAXA) and quite a few colleagues point out just how misleading ‘habitable zone’ can be, given that when we find a new exoplanet, we usually only know the size of the planet (perhaps through radius, as in a transit study, or through minimum mass for radial velocity), and the amount of radiation the planet receives from its star. From such facts we can infer whether we’re dealing with a gas giant or a rocky world.

This is hardly enough on which to base a claim of habitability, but it gets worse. Among those planets where both radius and mass can be measured, we can work out an average density. Planets 40 percent larger than Earth are probably gaseous, but the cut-off is not definitive. And as to radiation from the star, this gets thorny indeed. Atmospheres come into play — consider that the equilibrium temperature on Earth is -18 degrees Celsius, at which temperature water is a solid. It takes our atmosphere to produce the 15 degree Celsius global average.

Image: An artist’s impression of Proxima Centauri b. But how much do we really know about the surface of this world? Credit: ESO/M. Kornmesser.

Equilibrium temperature is an interesting figure. It represents a surface temperature assuming a planet has no atmosphere. As Tasker and crew point out:

The heat-trapping properties of an atmosphere are highly variable, however, and depend on its thickness and gas composition. Our atmosphere raises Earth’s temperature to from below freezing to a life-friendly level. On Venus, by contrast, where the equilibrium temperature would be a comfortable 80 degrees F (about 27 degrees C), its thick atmosphere, made mostly of the greenhouse gas carbon dioxide, boosts the actual temperature to a hellish 860 degrees F (460 °degrees C), turning this Earth-size planet into an inferno hot enough to melt lead.

Arguing that ‘a quantitative measure of habitability is impossible,’ Tasker says in this Scientific American essay based on the Nature Astronomy piece that temperature and habitability don’t always go together. We can thank factors like our planet’s magnetic field for protection against the kind of solar flares that could impact the development of life. We can also thank plate tectonics, which sets up a carbon-silicate cycle, for its impact on our atmosphere. The list goes on.

Now if all this were simply a matter of lowering the expectations of headline writers, the case would be strong enough. Whenever I see a bold headline proclaiming the discovery of a ‘habitable second Earth’ or some such, I wonder how jaded the public will eventually become, especially since many such articles don’t explain how tenuous a call this really is. We want public participation in the exoplanet hunt, but we should also want the public to receive solid information. Trying to get news outlets to tone down the rhetoric, though, may not be possible.

But think ahead to the coming decade, in which we’re going to have at our disposal assets that may show us biosignatures (or not) in the atmospheres — if they are there — of planets of great interest, such as the seven worlds around TRAPPIST-1, or perhaps those interesting four planets we looked at yesterday around Tau Ceti. The reason why the astronomical community has worked so hard on the metrics for choosing which planets to look at is that these resources are going to be scarce, and we have to optimize our target list.

Thus we talk about ‘habitable zones’ where planets can have liquid water on their surface, and use such tools as the Earth Similarity Index, which lays out orbital parameters for smaller planets in a habitable zone. But again, the parameters in play come back to the two we’ve already discussed, the size of a planet and the radiation it receives from its star. A planet can be like the Earth in these two factors and still be a long way from habitability. Tasker again:

Over-interpreting these selection tools to claim they measure habitability is a dangerous game. It is absurd to suggest we can assess something as complex as life-supporting conditions based on just two properties, neither of which directly probe the relevant environment. It is equivalent to judging someone’s personality based on height and the distance between the eyes. The proliferation of such statements both in the popular media and even scientific literature risks planetary scientists being taken less seriously.

What to do? The authors of the Nature Astronomy paper want to remove the term ‘habitable zone’ altogether from the evaluation metrics astronomers use (Tasker suggests ‘temperate zone’ instead). Presumably astronomers would then stop saying they had found a planet in the ‘habitable zone,’ and the urge for misleading headlines would vanish.

I fully understand the concern these scientists are expressing and think all of us who write about these matters have an obligation to make no exaggerated claims. But as I said at the top, the pace of linguistic change over time is slow, and in this case, where hopes for finding a genuine Earth 2.0 are high, it will take more than a linguistic fiat to calm down fervid language.

By all means, go to ‘temperate zone’ within the scientific community if necessary — it makes no claims about ‘habitability’ – but I suspect our problems with foolish headlines are going to remain a part of the public experience. It takes patience to keep explaining the limitations of exoplanet science, and telling that story accurately isn’t always the way to maximize an audience. But fatigue with exaggerated headlines coupled with persistent education on these matters may eventually right the balance.

And from the standpoint of Internet commentaries, let me point out that Andrew Le Page, on his Drew ex Machina site, has proven the gold standard at analyzing each new claim. His ‘habitable planet reality checks’ are long on analysis and utterly devoid of hype. Slow and persistent, that’s the way to proceed, and at each step of the process, Drew gets it right.

The paper is Tasker et al., “The language of exoplanet ranking metrics needs to change,” Nature Astronomy Vol. 1, 0042 (2017). Abstract.


{ 50 comments… add one }
  • Rob Flores August 17, 2017, 13:01

    As a bit of lark maybe you could just show
    an index as to how quickly you would die if you
    spontaneously appeared unprotected on the surface of an exoplanet exomoon, with an oxygen supply.

    Example: from our solar system

    Jupiter at metallic hydrogen depth. 10^-42 secs. (plank time x 10)
    Venus surface .1 secs
    Mars surface 30 secs
    Titan Surface 60 secs.

    • Tulse August 17, 2017, 13:45

      Surely one would die faster on Titan’s −179 °C surface than on the surface of Mars.

      • Rob Flores August 17, 2017, 16:05

        If you curl up into ball and close your eyes (as much as possible), you can probably survive longer on titans cold and that on MARS space like “atmosphere” but I would not bet on it, you are right.

        But both are a waste of Oxygen masks, true.

        • J. Jason Wentworth August 19, 2017, 19:34

          A Titanian surface excursion suit would actually be–for a pleasant change–pretty easy to make. William K. Hartmann even depicted Titan explorers wearing them in one of his paintings. While it looked superficially like a spacesuit (which is a pressure suit), it wasn’t one, as it was un-pressurized and hung loosely on the wearer. Its main purpose was to protect the wearer from the cryogenic cold on the moon’s surface. Only the helmet need be fed air or oxygen (the X-15 rocket plane space suit was made this way; only its helmet was provided with oxygen, while the rest of the suit [below its rubber neck dam], which would have been pressurized if the cabin pressurization failed, was filled with nitrogen). On Titan, a small tank of liquid oxygen (or maybe liquid air–at least the oxygen/nitrogen mixture, without the minor constituents), with the evaporated gas warmed electrically (or maybe with a “pilot light” fueled with methane from the atmosphere) to comfortable temperatures for the wearer, could support a person’s breathing needs for hours or even days.

    • Alex Tolley August 17, 2017, 15:36

      Each of your examples doesn’t require an O2 supply. Even Titan is less time than one can hold his breath. So just hyperventilate and take a deep breath. ;-)

    • Steven W August 17, 2017, 22:44

      A couple of years ago , I seen a documentary on the Science Channel that was interviewing Planetary Scientist Chris McKay about Titan and he said that although Titan is extremely cold, there is no wind and one could wear a parka and a simple mask for O2 and be just fine.

      • J. Jason Wentworth August 19, 2017, 19:54

        Being an Alaskan who has worn a parka at -40 (no scale specification is needed here, as -40 degrees F. is also -40 degrees C.), I wouldn’t recommend that Chris McKay–or anyone else–try that. With Titan’s denser air (50% higher in pressure at its surface than our atmospheric pressure, and thus a more efficient conductor of heat than our air), the “dead-air space” that a parka creates in front of one’s face for local warmth wouldn’t keep one’s face–including the eyes, nose, and lips!–from freezing solid (and brittle…) at the -280 degrees F. temperature on Titan. The face’s heat would quickly be absorbed by the cryogenically-frigid air (walking would also create wind, speeding the heat loss), and the moisture in exhaled breath would immediately freeze, which wouldn’t assist the oxygen mask’s operation (and its rubber would crack very soon, too).

    • Antonio August 18, 2017, 2:24

      You would also die in Norway unprotected. So what? Does that mean that people don’t live in Norway?

      C’mon… a civilization that can travel to an Earth analog 50 light years away has surely colonized Mars, Europa and what not.

    • Joe H. August 18, 2017, 13:47

      Perhaps a more relevant question is how long could you survive on the surface of Venus if the planet had a nitrogen/oxygen atmosphere with similar pressure to Earth. I suspect such a planet would be hot, but bearable, especially near the poles.

  • Douglas Loss August 17, 2017, 13:14

    I’ve been wondering about a couple of things. If a planet is tidally locked, wouldn’t that extend the distance from the star at which liquid water could exist on the surface, at least for the illuminated hemisphere?

    Second, for the radial velocity method of discovering exoplanets, would a double planet like Earth/Luna appear to be one larger planet? Or alternatively, could some of the discovered exoplanets actually be double planet systems with two smaller components?

  • Alex Tolley August 17, 2017, 13:29

    Habitable also connotes human terrestrial multicellular life on the surface, where liquid water can exist. This is quite Earth-centric and parochial. If what we hope to find is life of some kind, the conditions may be quite different than the range we currently expect. Even liquid water on the surface may not be necessary.

    For that reason, perhaps temperate is a better word to use.

    • Antonio August 18, 2017, 2:15

      Totally agree. ‘Habitable zone’ was always a really stupid term. It should be called ‘water zone’ or whatever.

  • Brett August 17, 2017, 13:45

    I’ll second LePage’s “reality checks”. Those are good posts.

    Generally, I’m more optimistic about a planet’s habitability if it’s cooler than Earth (i.e. if it gets less sunlight) than I am if its solar insolation is greater than Earth’s. Cooler planets can make up the difference with thicker atmospheres and more greenhouse gases, whereas it’s harder for warmer planets to get rid of that extra heat.

    • Ronald August 18, 2017, 7:57

      I suspect that is also the reason why the inner limit of the HZ has been so strikingly sharp and constant over the years (from 0.95 to 0.99 AU in our solar system), whereas the outer limit of it has been rather fuzzy and subject to change.

  • Infinite123Lifer August 17, 2017, 14:44

    I am not sure what is best. On one hand I think it’s incredibly neat when family/friends, who do not share a deep love of space/science like I do, come to me and as a casual conversation starter mention to me “did you hear they might have discovered another planet in the habitable zone” or “did you hear there might be a planet made entirely of diamond” lol … I absolutely love this, every single time this happens it is accompanied with a smile and what seems some sort of actual genuine fascination on their behalf. Often it leads to imaginings and creative brainstorming that might never have taken place if it were not for those extravagant headlines. For me personally, I take the time to talk about Europa or Enceladus maybe Titan to ponder … what does it truly mean to be habitable. Most of the times we settle on what the headlines are referring to is looking for the planet that mostly resembles Earth, thus the headline. I do see these types of headlines making a difference in my Life by bringing those who are otherwise not interested, to me, to talk, to discuss and maybe most importantly to wonder about the possibilities that could be out there.

    On the other hand, scientists by their very definition are to be experts in their subject matter. That must surely conflict for some with … having no way to perhaps eliminate such massive ambiguity when trying to define/categorize the universe. Habitable zones and definitions of what a planet is to be two strong examples. The universe is just so brilliantly vast that it should come as no surprise that we are running into these types of situations and will continue to far into the future (with luck). Given our nature it should also come as no surprise that we are (thankfully) constantly and consistently updating and upgrading almost all facets of our information to improve accuracy, efficiency and precision.

    I can raise my glass to the concerned scientist who never wants to be misleading and thank them for their caution, concern and dedication, your tab is on me. To the attention getting reporters looking for the best science related headline they can muster … come on in fellow, sit down, let us discuss this in a little more detail, the first drink should be on us.

  • Robin Datta August 17, 2017, 16:27

    “Habitability zone” can be replaced by various acronyms for what “habitability” means in different contexts.
    Surface Liquid Aqueous Medium Possible: SLAMP
    Goldilocks Range Of Temperature: GROT

  • DJ Kaplan August 17, 2017, 16:49

    Excellent conversation!
    At this point, I think we still need to stick individual variables. The original “HZ” refers to an orbit about a star that assumes a certain amount of radiation, based on that which we can measure from terra firma.
    Other variables can be observed conservatively, such as ratio of earth mass, apparent composition of the exoplanet’s atmosphere, and the host star’s variability.
    I think it’s premature to toss out terms such as “class M planet”. ;)

  • Andrew LePage August 17, 2017, 17:21

    First of all, I’d like to thank Paul and other posters for the kind words they have for my “Habitable Planet Reality Check”. While I am always on the look out for a better way to quantify any world’s ability to support life, I still personally like the concept of a “habitable zone”. As I frequently bring up in my articles, the scientific definition of a “habitable planet” is simply a world which has surface conditions capable of supporting liquid water – a prerequisite for life as we know it. This is a handy definition at least in regards to looking for worlds with Earth-like habitability which are a reasonable places to start looking for life as we know it (with the understanding that these are only a subset of ALL types of worlds which could support biocompatible environments). While this term “habitable” has been abused by some to imply that such planets can support humans (the overwhelming majority of such worlds will not), that happens all the time when the media tries to hype some scientific discovery to generate traffic to support their ad rates. And changing the term “habitable zone” to “temperate zone” will do absolutely nothing to stop people from creating clickbait titles for their online content so I am personally against such a futile move. Just my two cent’s worth :-)

    • Ronald August 18, 2017, 7:44

      Could not agree more.

      • DJ Kaplan August 18, 2017, 12:20

        I agree as well. There’s a lot of shoddy science journalism out there; so relieved to find this place where that’s not the case.

    • Michael August 18, 2017, 17:11

      I find the quantification of a ‘habitat zone’ very difficult as well, verging on impossible to calculate. I mean there are so many variables not just temperature for water related to the distance from a stellar light source. Lets look at say organics, a hot world may have lost all of its organics through thermal degradation processes during a long contraction phase and still end up in a ‘ thermally habitable zone’, so is also time dependant. Its is the term ‘habitable zone’ that needs bracketing by a number of variables but there are many. Maybe it is time to look at what makes a ‘habitable zone’ in more detail and the many variables that can influence it.

  • Astronist August 17, 2017, 17:37

    I have commented before that, if a subsurface ecosphere is a possibility (as has been postulated for Europa and other large moons in the outer Solar System), then, unless one changes the meaning of words, the “habitable zone” is by definition coextensive with the entire astronomical universe.

    The term “temperate zone” would be meaningful, as would “surface liquid water zone”, but actually what people are trying to talk about is the “Earth analogue zone”, i.e. the zone around a star in which an Earth analogue planet would be a possibility, so I suggest that if this is what they mean, then this is what they should say!

    Oxford, UK

  • Dave Moore August 17, 2017, 20:52

    It’s definitely time to ditch the term “Habitable zone” when you consider that for most of Earth’s history our planet wasn’t habitable.

    • Ronald August 18, 2017, 7:42

      Disagree: the term HZ has never referred to human habitability, but to a (rather optimal) delimitation of the liquid surface water zone. And in that sense, our planet has been habitable for most of its history. Also see my comment below and Andrew LePage’s comment.

      • Dave Moore August 19, 2017, 15:51

        The more I think about the term Habitable zone the more I think it needs to go.

        If you are referring to it as the zone in which liquid water can exist on the surface without an ice-cap over it, then there is no theoretical outer limit to the habitat zone. A planet with a 13 bar atmosphere out at the orbit of Jupiter—or one orbiting a red dwarf at a distance that gives a similar level of insolation—would have a room temperature surface. And its been postulated that an Earth mass planet roaming between the stars with a 100 bar atmosphere could maintain liquid water on its surface through geothermal heat alone.

        Technically, a planet with a 13 bar Helium atmosphere with 1 1/2% oxygen and a couple of percent Nitrogen would have a breathable atmosphere.

        If we go back to the origin of the concept: Doyle’s, Habitable Planets for Man, we can examine the assumptions behind the concept of The Habitable Zone. First off, Doyle was using a snappy title. A more accurate title would have been Planets that could possibly be Habitable for Man. He was basically developing a series of exclusionary criteria that limited the type of planet that we could expect to habitable. (This was an important piece of work from a day and age when we still had pirates from Jupiter.)

        However, as we have gained knowledge of the field, some of his exclusionary criteria has been cast into doubt. For instance, he assumed tidally locked planets would be completely uninhabitable. Subsequent work indicate this may not be so.

        Another assumption that has been underlying our studies in this field is that our planet and our solar system are the natural arrangement. With respect to our solar system, this assumption was rudely shattered with the discovery of 51 Pegasi, but there still underlies the assumption that planets with Earth’s mass and insolation will converge on being like Earth. And if there is anything we should have learned from our planetary discoveries so far, it is that this is not likely to be so.

        We derive the habitable zone from the concept of taking Earth and increasing its insolation until we get a change of state, and decreasing its until the same thing happens, but this is a very Earth centric approach to one factor among many that will determine the surface composition of a planet. What we are really saying with the words Habitable Zone is a zone where planets like Earth may exist.

        And what we have noted so far is that the word Habitable does actually refer to livability by humans. I have show that Habitable zone doesn’t refer to the possibility of liquid water on the surface of a planet. If we think of habitability a place where life could form and exist, then look at our own solar system. We have four bodies that we consider are potentially life bearing: Earth, Mars, Europa and Enceladus. Two of these are clearly outside the habitable zone.

        So the term Habitable Zone has a fuzzy definition that varies with the person using it, and if used as a criterion to classify planets, its not a particularly useful one as insolation is only one of many factors in determining a planet’s type. This is not a good scientific term.

        If you want to refer to an Earthlike planet, then say Terran Planet zone and talk about that could it be a Terran type of planet.

        I have the following classification scheme for smaller planets (subject to modification and subdivision as our knowledge increases.)

        Mainly rocky planets like Mercury, Venus, Earth and Mars: These would be called the Lithic planets as opposed to gaseous.

        Planets with no atmosphere: Mercurian.

        Planets with an atmosphere too thin to for liquid water to be stable on the surface over anything other than a small temperature range: Martian

        Planets whose high surface temperature/pressure are such that liquid water cannot exist on the planets surface: Venusian
        (The term Venusian would describe planets of increasingly high atmospheric pressure until the pressure at the base of the atmospheric column was such that high temperature ices form. Then the planet would be referred to as Neptunian.)

        Planets whose surface is completely covered with water: Oceanic

        Planets that have a mix of bodies of water and rocky terrain on their surface where the carbonate-silicate cycle can operate: Terran (Mars would have been a Terran planet in its youth.)

        • Infinite123Lifer August 20, 2017, 2:58

          Wow, I am not qualified to attempt to poke holes in this but as a simplistic minded bystander I am definitely liking where you have gone with this … talk about saying what you mean and meaning what was said! I think this type of approach benefits both sides, the public and the scientific community … for now. It’s simplistic enough to be understood by the masses and really anybody who has a basic understanding of our solar system, not only that but it almost begs a person with the remotest interest in such things to advance the conversation to ask more questions. For example, “why is it Neptunian” might compel a person to do some research on Neptune, or any number of other things that in turn might cause that someone to speak to their children about such … and that child might grow to do extraordinary things, rinse and repeat and eventually we just might get to where we are wanting to go. (don’t get me wrong, I believe the work to get there is already being accomplished and sought after and I hope and trust it is for all the right reasons)

          As you can obviously see I miss the points quite often round these parts but I do realize that the direction or focus of conversation here at CD is more about the solving of problems rather than the why humanity is attempting to solve them but there in lies the dichotomy of the conversations which underly (by all of yours design) practically EVERY post on this blog that I adore so much.

          For topics/issues like this one, from the article:

          “Whenever I see a bold headline proclaiming the discovery of a ‘habitable second Earth’ or some such, I wonder how jaded the public will eventually become, especially since many such articles don’t explain how tenuous a call this really is. We want public participation in the exoplanet hunt, but we should also want the public to receive solid information. Trying to get news outlets to tone down the rhetoric, though, may not be possible. ”

          And you are probably not going to be able to perfect the definition of a planet, you are probably not going to be able to perfect the definition of a habitable zone … at least not perfect them in a manner that is parallel to the average person’s train of thought. So when it comes to these types of messy but publicly important messages, you are going to need some tact, some neatness and possibly something romantic. It can be done. A friend of a friend who passed away once told me “you can talk to people all day long, but if at the end of the day you have not touched the hearts or minds of anyone, you have wasted your breath” – Terry Smith. Change the terms, doing so in a way that improves the publics information slowly chipping away at the rhetoric employed by many without them even noticing it really. Use their downfall to your advantage. Wanna post nothing but click bait? Make them at least do it with some sustenance. Heck, even the term Habitable Zone has done wonders for the publics awareness. Perhaps that awareness is on the fringes of reality, but it’s a start.

          Which is why I really like this idea, it improves upon the definitions in a way that people can relate to, possibly even feel something for. I would say the average person knows more about one planet than all our planets as a whole, for instance people will know quite a bit about Saturn or Jupiter but nothing about Mercury, or maybe they have a love or interest in Mars or Venus but not Uranus. Which means that possible Lithic or Oceanic or Martian discoveries have more potential to seep into the hearts and minds of people. Those headlines, if changed to a system such as this, I believe, would do wonders for the publics interest, I mean of course above and beyond the current reaction to the term “habitable zone” it would add a new layer, one that I think we are ready for. And if I am not mistaken it provides a more realistic definition (which is obviously either the primary or secondary point to all of this depending on how/who you ask). Sure it still needs help, sure it’s still incomplete, sure it’s still messy but at last count there were practically an uncountable number of planets in the universe, couple that with a sometimes practically uncountable level of ignorance here on this one and that makes this task and jobs like it practically maddening. Explain it in further detail, but do it as simply and relatable as possible.

          I don’t think anybody here would argue against: more interest plus better understanding equaling greater possibilities.

          I would love it if when people came to me to discuss planetary discoveries (this coincides with discoveries but happens between my significant other, my kids and my family and close friends often enough about all sorts of topics) I would love it if they came and said “did you hear a Terran type planet was discovered”.

          I might be banking on my primitive imagination to carry me to worlds that might not be, but who can blame me, without it, I have nothing :)

        • Ramses Ramirez August 21, 2017, 7:08

          Just a couple of quick points (assuming the traditional CO2-H2o habitable zone).

          Dave Moore says.. “We derive the habitable zone from the concept of taking Earth and increasing its insolation until we get a change of state, and decreasing its until the same thing happens”

          This is close (for the inner edge), although water vapor concentrations would be much, MUCH higher than on the Earth. Things are also not quite “Earth-like” at the outer edge either. At farther distances (and stellar radiation levels) from the star, the assumption is that the volcanism from the carbonate-silicate cycle would produce the CO2 necessary to sustain warm surface temperatures. Earth’s atmosphere currently averages just above ~400 pm of CO2. In comparison, the models predict that a habitable planet right at the outer edge in our solar system would have 8 bars of CO2 in its atmosphere. So, I wouldn’t really say that the habitable zone refers to the region in which a planet receives a similar insolation as the Earth (nor such similar statements like it is “the region best fit for humans.”). If life could originate in an 8-bar CO2 atmosphere, there is no reason to think that it would be very “human.” That said, the narrow focus on CO2-H2O atmospheres is definitely something that has been criticized by myself and others. It is a valid point (as Dave mentions) that other greenhouse gas combinations can foster conditions warm enough to support liquid water on the surface.

          “We have four bodies that we consider are potentially life bearing: Earth, Mars, Europa and Enceladus. Two of these are clearly outside the habitable zone.”

          To clarify, the habitable zone is simply concerned with planets with liquid water on their *surfaces* so that any potential life can be possibly detected from space observations of the atmosphere. The habitable zone makes no assumptions regarding the possibility (or lack thereof) of life on planets with subsurface water (such as Enceladus or Europa). There could very well be life on Europa- or Enceladus- analogues in other stellar systems but (unfortunately) such life would be sealed away from having direct contact with the atmosphere and is not detectable with present technology…… well until such time that the habitable zone sweeps outward during the red giant phase of stellar evolution and these Europa- and Enceladus analogues could possibly foster habitable/warm surface conditions (at least for some time) as we have argued here:

    • andy August 18, 2017, 18:24

      Viewed from the perspective of the giant insects that evolved in the high-oxygen atmosphere of the Carboniferous, present-day Earth isn’t habitable.

  • Geoffrey Hillend August 17, 2017, 23:47

    Quote from Andrew La Pages Habitable Planet Reality Check: Kepler’s New Planet Candidates on Drew ex Machina “As a planet receives less energy from its sun, various processes such as the carbonate-silicate cycle allow more CO2 to build up in the atmosphere which helps to increase the greenhouse effect and maintain surface temperatures. The outer limit of the conservative HZ, as defined by Kopparapu et al. (2013, 2014), corresponds to the maximum greenhouse limit beyond which a CO2-dominated greenhouse is incapable of maintaining a planet’s surface temperature. Instead of helping to heat the atmosphere, the addition of more CO2 beyond this point makes the atmosphere more opaque causing the surface temperatures to drop instead of increase. The latest work suggests an Seff value of about 0.36 for the outer limit of the HZ of a Sun-like star with cooler stars having slightly lower values. There are some slightly more optimistic definitions of the outer edge of the HZ such as the early-Mars scenario or evoking some sort of super-greenhouse where gases other than just CO2 contribute to warming a planet. But these more optimistic definitions do not change the Seff for the outer limit of the HZ significantly”
    This idea is not supported by atmospheric science. The Co2 radiation absorption frequency does not change at lower temperatures at least not enough to weaken the green house effect of Co2. If we raised the amount of Co2 on Mars to 1000 millibars or one bar of Co2, the temperature would be raised significantly enough so there would no longer be any frozen Co2 on its surface; Co2 would not freeze anymore on Mars with an atmosphere of one bar. I don’t know what Kopparapu means by Co2 making an atmosphere opaque? Opaque to what? Co2 is opaque to infra red thermal radiation which means that Co2 absorbs infra red at all temperatures within a life belt with an atmosphere of one bar. At 3 degrees F, Co2 on Earth will still absorb at the peak of black body infra red radiation leaving the Earth. Co2 is transparent to visible light which is re radiated from the ground in the infra red which is absorbed by Co2 to give us a green house effect. Without any Co2, an atmosphere is transparent to infra red so that it passes right through it without any scattering by the atmosphere unless such an atmosphere has a high pressure such as many bars: Nitrogen and Oxygen are transparent to infra red photons at one bar. The heat escapes into space. We would freeze without any Co2. This is due to quantum theory: An atom or molecule will only absorb radiation if it is at the same wavelength or frequency as the ground state of the atom which is equal to a specific wavelength. For example: all EMR gamma rays, x rays, ultra violet, visible light and radio waves pass right through the Co2 molecule but not infra red which is absorbed and re emitted. Consequently, infra red is slowed down from escaping into space by Co2. Increase the Co2 and you increase the amount of thermal energy retained in atmosphere.
    The carbon cycle is what controls carbonate silicon through rain, and plate techtonics which recycle the Co2 and put it back the silicon carbonate in the form of Co2 back in the air via volcanoes and crust subduction.

    • Andrew LePage August 18, 2017, 9:48

      Your comment is filled with far to many misconceptions on of the how radiative transfer works in atmospheres to create the greenhouse effect and other aspects of Kopparapu’s work for me to address in the time I have available. I can only suggest that you get a copy of “How to Find a Habitable Planet” by James Kasting (the “father” of our modern understanding of Earth-like habitability and the mentor of Ravi Kopparapu who has continued the work). Published by the Princeton University Press, the book is quite readable and filled with peer-reviewed references which should clear up your misconceptions.

      • Ashley Baldwin August 19, 2017, 5:37

        I couldn’t agree more. Can’t recommend this book enough ! I’m lucky enough to know Jim Kasting and having sought his opinion on this yesterday ( he hadn’t yet read this article as result of being at a conference in Europe ) . He is considering responding formally to this article and can obviously do so far better than I . He obviously continues to supports the hab zone concept as intended . It’s is essentially a paradigm that provides a working framework within which planets may be scrutinised by future direct imaging telescopes and to aid with interpretation of any findings . He like you sees relative stellar insolation rates as the real driver rather than meaningless “effective temperatures ” . The term “habitable ” has been expanded too far by others and was never intended to imply that any planet lying within this zone would be “inhabited” or even “habitable” . It’s hard to see any planet lying outside this zone having a potentially habitable surface though .

        Further evidence comes from the fact that despite approaching its “Silver Jubilee ” , next year, the term still holds continued and near unchallenged currency in academia . Despite being posited before the first exoplanet was even discovered ! It is repeatedly cited ( pointedly without rebuttal ) in Rory Barnes’ “Tidal synchronisation ” article featured in the latest post.

        Interestingly Jim Kasting is not a fan on the Earth Similarity Index which many consider of limited utility and at its worse is openly misleading .

        Ravi Kopparapu has also previously written about his work in this area for Centauri Dreams

        • Ramses Ramirez August 20, 2017, 14:58

          Ashley.. In my view, Elizabeth does raise some good points regarding how habitability, especially in regards to supposed Earth-clones, is poorly communicated to the public by both scientists and the media alike. She is also right to question the utility of some of these metrics (although some of these, like the habitable zone, are likely better than others).

          That said, even though the habitable zone (HZ) concept may be one of the better metrics, it isn’t clear that the classic definition developed originally by my mentor and updated by us later in 2013 is the best way to think about it.

          For instance, planets located in the present day HZ orbiting M-stars (e.g. Proxima Centauri b and 3- 4 of the TRAPPIST-1 planets) may not be very habitable at all considering that their surfaces would have been completely irradiated and desiccated way before the star ever reaches the main-sequence. Basically, the theory predicts the HZ (in this sense) may work better for Sun-like stars but not so good for cooler ones.

          One other example.. It may also be narrow to think of a habitable zone that consists of only two gases (CO2 and H2O). Certainly other greenhouse gas combinations (perhaps in addition to CO2 and H2O) are possible. We have such additional gases on our own planet so there is no real reason to be too attached to just Co2 and H2o.

          In my view, the traditional CO2-H2O HZ is a good starting point but it can be significantly improved. My published work has been trying to show a more liberal view of the habitable zone. We should avoid the temptation of being too Earth- and solar system-centric regardless of what metrics are used (this is something that both the ESI and HZ concept have in common). I fear that by being too Earth- and solar system-centric, we may be really limiting our search prospects.

  • Geoffrey Hillend August 17, 2017, 23:54

    Mars has such a thin atmosphere that heat is not transported very well. It might be 50 degrees F on the surface near the equator during closest opposition but two feet above the ground, the temperature might be below zero F.

  • Geoffrey Hillend August 17, 2017, 23:57

    The silicon does not go back into the air just the Carbon

  • Antonio August 18, 2017, 2:19

    Also, I don’t understand the obsession with an Earth 2.0. How boring… I would not travel tens or hundreds of light-years to live in a place like the one I live now.

    • Matt August 18, 2017, 12:17

      I agree a second Earth would not be that interesting by itself. What could make it interesting is the possibility of some version of “earthlings.” Also the possibility of settling a remote planet.

    • andy August 19, 2017, 11:21

      I really don’t understand the whole “Earth 2.0 would not be interesting” attitude. Earth itself has had a wide variety of different conditions on its surface: supercontinents vs separate landmasses, snowball glaciations and hothouse climates, significant variations in atmospheric composition, oscillating axial tilt, flood basalt eruptions, oceanic anoxic events, etc. There is no reason to believe an Earth-twin would much resemble present-day Earth when Earth itself hasn’t much resembled present-day Earth for much of its history. As such, an “Earth 2.0” would be a fascinating place.

  • Ronald August 18, 2017, 7:37

    What a complete non-issue discussion in the Tasker paper! It looks like a pitiful attempt for ‘5 minutes of fame’.

    If this paper had substantively contributed to a better definition or re-analysis of the HZ (as Kopparapu et al. did with Kasting’s seminal work, and which resulted in a strikingly similar but somewhat more fine-tuned redefinition), then I would have appreciated the contribution. But no, just the laughable conclusion: that we should just change the term :-()

    By now we all know that ‘habitable’ in HZ does NOT mean habitable to unprotected humans, walking out of a spaceship.
    But that it is a (rather maximal) delimitation of the liquid surface water zone.
    Just as ‘earthlike’ ‘terrestrial’ does not equate to literally Earth-like, but to a small rocky planet capable of retaining an atmosphere but not a thick gaseous (H/He) envelope.

    These are, for the time being, very useful concepts, and everybody seriously using them is aware of their limitations.

    “It is absurd to suggest we can assess something as complex as life-supporting conditions based on just two properties, neither of which directly probe the relevant environment. It is equivalent to judging someone’s personality based on height and the distance between the eyes”.
    No, it is not, rather, this remark is absurd and ignorant, shame. A better comparison would be to judge a person’s well-being based on his temperature and heartbeat, grossly insufficient, but a useful beginning all the same.

    I agree with Paul’s comment and also Andrew LePage’s. I commend you both.

  • Andrew LePage August 18, 2017, 9:42

    For those readers who may be interested, here is my latest installment of “Habitable Planet Reality Check” where I address the potential habitability of the exoplanet candidates of Tau Ceti:


  • Rob Flores August 18, 2017, 11:06

    Thinking more seriously, here.
    A good measure of what type of planet we are dealing with is
    use the Support Level Rating with the weight or cost (in 1 oz gold value) equivalent of extra equipment/technology being used (the greater of the two , per 4 hour surface stay,
    In logarithmic scale, on the least hostile point on the planet

    Earth 0.

    Titan 4.0 (40 pounds of extra weight (cold protection) , small ox supply

    Callisto 8.0 80 pounds (Space suit and cosmic ray protection, low solar rad, very thin surface atmosphere may have high amount of oxygen which can gathered by space suit systems )

    Mars 15.0 (150 pound space suit, small ox supply

    Venus 90.0 (900 pound armored Space suit with heat sink, sm ox sup

  • Ramses Ramirez August 20, 2017, 13:53

    Hi everyone. Elizabeth’s new comment itself raised some interesting comments on this thread.

    On the topic of opacity of CO2 in dense atmospheres near the outer edge of the habitable zone…

    Although it is true that raising CO2 will increase the greenhouse effect, if too much CO2 is added to a planetary atmosphere in order to keep it warm at the outer edge (e.g. 8 bars for our solar system), two things can happen that will counter the warming:

    1) CO2 will condense out of the atmosphere and
    2) dense CO2 atmospheres reflect a considerable part of the incoming solar radiation back out to space.

    If these two effects become too strong, they can outweigh the greenhouse effect.

    On another point. We have recently shown that the addition of volcanically-outgassed hydrogen can significantly extend the outer edge of the traditional CO2-H2O habitable zone (from ~1.7 to 2.4 AU) well beyond the limits that Ravi and I had originally shown in our 2013 paper.

    This was covered earlier in Centauri Dreams:

    • Ashley Baldwin August 21, 2017, 13:18

      Thanks Ramses. Can I ask which effect is greater , condensation or reflection ? ( though they can’t be mutually exclusive ) I assume reflection will be dependent on incident wavelength too . If so should the outer limit of the HBZ as defined here vary from one stellar class to another and if so in what way ?

      • Ramses Ramirez August 21, 2017, 16:12

        Hi Ashley. Whether condensation or scattering is greater depends on things like the atmospheric composition, topographical distribution, pressure, temperature, stellar class, and incoming radiation. In the case of topography, higher (colder) elevations will favor more condensation that lower, warmer elevations (all else equal).

        An outer edge planet orbiting a G-star may have a planetary albedo of over 40%. However, around an M-star this may be only 10%, suggesting that condensation is more important in the latter. Around an F- or A-star, the very high reflectivity (~50% or so) becomes extremely important. So, this depends on the specific planetary/stellar circumstances.

        The definition of the outer edge itself, however, is the same whether CO2 condensation or scattering is dominant. The definition assumes both phenomena occur.

  • Geoffrey Hillend August 20, 2017, 17:51

    Quote by Richard Ramirez: “1) CO2 will condense out of the atmosphere and
    2) dense CO2 atmospheres reflect a considerable part of the incoming solar radiation back out to space.” If these two effects become too strong, they can outweigh the greenhouse effect. I don’t think there are many cases where the albedo or reflectivity can outweight the Co2. An atmosphere would have to be completely cloud covered without any Co2. Maybe a water world might be like that and the rain took all the Co2 out of the atmosphere. The clouds would have to be made of 100 percent water vapor. Otherwise we get a Venus like world or something between Venus and the Earth which would still have a big greenhouse effect.
    Co2 would only condense out of the atmosphere of an Earth sized body at Mars distance but not enough to remove most of the Co2 or stop the greenhouse effect. The problem is that colder air tends to hold less water, but I doubt the Earth would be completely cloud covered if we moved it 40 million miles further from the Sun. We would get a snowball Earth with all frozen oceans and not complete cloud cover. There would still be a greenhouse effect but not enough to stop mass extinctions. If we move the Earth a little closer to the Sun we’d have a warmer Earth moving towards a runaway greenhouse effect which would most likely give us more cloud cover with larger greenhouse effect. We’d have more evaporation of sea water to make more clouds. Warm air holds more water. The clouds albedo would not be enough to counter the greenhouse effect.
    Venus has an atmospheric pressure of 93 bars and a high albedo or reflectivity. It reflects 76 percent of the sunlight but that is due to its thick cloud cover and close proximity to the Sun. It has so much Co2 that the high abledo does not matter due to the large amount of Co2 which is 96 percent and huge greenhouse effect. In an atmosphere of one bar, this reflectivity would be considerably lower not offset any greenhouse effect. 8 bars would cause a greenhouse effect even without Co2.
    An exoplanet with a runaway greenhouse effect most likely does have a lot of Co2, regardless of albedo unless it is a huge world with a strong gravity to hold a large atmosphere. The Co2 depends on the volcanism which can replenish lost atmosphere from solar wind stripping and it’s rate of loss and a lot of other factors. Consquently in all cases an increase in Co2 will increase the greenhouse effect.

    • Ramses Ramirez August 20, 2017, 22:49

      Geoffrey. The way that the outer edge of the habitable zone (HZ) is defined (i.e. the maximum greenhouse effect limit for CO2) is that there is a distance beyond which the Co2 greenhouse effect is maximum.
      Beyond that distance the greenhouse effect becomes less efficient in large part because of the combined effects of Co2 condensation and increasing rayleigh scattering. Co2 is ~2.5 times more reflective than terrestrial air (which is why this is such a big effect), and near the outer edge of the habitable zone (in our solar system), planetary albedo easily exceeds 40%, cooling the planet off considerably. CO2 condensation for such outer edge planets can form clouds (which can be either reflective or absorbing) and/or form ice caps that could lead to atmospheric collapse if there is sufficient condensation near the poles. Remember also that for such distant outer edge planets, the water would likely freeze out of the atmosphere. So, the water-vapor feedback, which enhances CO2 absorption on the Earth, would be significantly weaker on such worlds.

      You are correct that there is enhanced absorption for high pressure CO2 atmospheres, but this will get countered by the above mentioned effects for distant outer edge planets. Venus is a different story because its close proximity to the Sun means it gets much more incident radiation (in addition to the pressure-broadening effects from its ultra-dense atmosphere as you mention).

      The combined effects of CO2 condensation and high reflectivity on distant planets is something that is well-understood and has been shown in many works, not just in studies of the habitable zone. In 1991, Jim Kasting had shown that no amount of Co2 can produce warm mean surface temperatures on early Mars. Subsequent models (like mines’) obtain mean surface temperatures no higher than ~230 K as CO2 pressures above ~2 or 3 bar lead to atmospheric collapse. Given today’s brighter Sun, warm conditions on present Mars are achievable if CO2 pressures were to exceed ~ 1 bar. However, the outer edge is far enough past the orbit of Mars that it needs higher pressures (8 bar) to sustain warm conditions.

  • Ashley Baldwin August 21, 2017, 13:35

    Which sadly raises the question , posited by JK in “How to find a habitable planet ” of just when ( and how ) this early Mars was temperate enough to have had liquid water on its surface , regardless of a ( much ) thicker atmosphere ? As is still commonly believed.

    • Ramses Ramirez August 21, 2017, 16:26

      Ashley.. If you were to ask me I would say that the early Martian atmosphere wasn’t composed of just CO2, but also of other potent secondary greenhouse gases (e.g. hydrogen, H2S, CH4..etc).

      In my 2014 and 2017 papers, I show that an early CO2 atmosphere with a few percent hydrogen could have conceivably warmed early Mars during the Noachian/Hesperian (~3.6 – 3.8 Ga), explaining the surface geology we see.

  • ljk August 25, 2017, 10:59


    Future Astrometric Space Missions for Exoplanet Science

    Press Release – Source: astro-ph.EP

    Posted August 21, 2017 11:27 AM

    High-precision astrometry at the sub-microarcsecond level opens up a window to study Earth-like planets in the habitable zones of Sun-like stars, and to determine their masses.

    It thus promises to play an important role in exoplanet science in the future. However, such precision can only be acquired from space, and requires dedicated instrumentation for a sufficient astrometric calibration.

    Here we present a series of concepts designed for handling this task. STARE is a small satellite concept dedicated to finding planets in the very nearest stellar systems, which offers a low-cost option toward the study of habitable planets. The NEAT concept is a set of two formation-flying satellites with the aim to survey the 200 nearest Sun-like stars for Earths in the habitable zone.

    Finally, THEIA is a proposal for an ESA M-class mission, with a single-unit telescope designed for both dark matter studies as well as a survey for habitable Earth-like planets among the 50 nearest stars. The concepts illustrate various possible paths and strategies for achieving exquisite astrometric performance, and thereby addressing key scientific questions regarding the distribution of habitability and life in the universe.

    Markus Janson, Alexis Brandeker, Celine Boehm, Alberto Krone Martins

    (Submitted on 18 Aug 2017)

    Comments: 12 pages, 3 figures. Invited review to appear in ‘Handbook of Exoplanets’, Springer Reference Works, edited by Hans J. Deeg and Juan Antonio Belmonte

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:1708.05560 [astro-ph.EP] (or arXiv:1708.05560v1 [astro-ph.EP] for this version)

    Submission history

    From: Markus Janson

    [v1] Fri, 18 Aug 2017 10:53:22 GMT (350kb,D)


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