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Two Takes on Habitability

Last week’s announcement about Kepler-186f presented a world that is evidently in the outer reaches of its star’s habitable zone, with the usual caveats that we know all too little about this place to draw any conclusions about what is actually on its surface. Is it rocky, and does it have liquid water? Perhaps, but as Greg Laughlin (UC-Santa Cruz) points out on his systemic site, the widely circulated image of Kepler-186f was all but photographic in its clarity. Listen to Laughlin as he looks at the image:

I stared at it for a long time, tracing the outlines of the oceans and the continents, surface detail vivid in the mind’s eye. Yes, ice sheets hold the northern regions of Kepler-186f in an iron, frigid grip, but in the sunny equatorial archipelago, concerns of global warming are far away. Waves lap halcyon shores drenched in light like liquid gold.

He goes on to look at how the press has handled earlier stories on habitable planets, dating back to the Gliese 581c frenzy of 2007. And you can see the evolution in artist’s renderings of the various worlds under discussion, culminating in a Kepler-186f image that could indeed be misconstrued as a photograph by someone who didn’t realize the limits of our capabilities. Here’s the image again (credit: NASA Ames/SETI Institute/JPL-Caltech):


It’s beautiful work, and I ran it last Thursday along with my story on the new planet. But we have to be careful not to get too far ahead of ourselves. Yes, researchers tracking the exoplanet hunt understand that this is entirely conjectural, but upon reflection, I think we’re sending a signal to the general public that we’re more confident about what these worlds are like than is justified. In the case of Kepler-186f, the fact that a planet is close to Earth-sized does not render it Earth’s twin in any other meaningful way, especially given that the planet orbits a red dwarf and pulls in only about a quarter of the insolation that Earth receives.

The Benefits of Axial Tilt

What we do surmise about habitable zones keeps getting tweaked around the edges, and on a more theoretical plane, the work on planets with ‘tilted’ orbits — this comes out of the University of Washington, Weber State and NASA — is intriguing because it pushes liquid water on the surface much further out than earlier habitable zone notions would allow. The paper, which appears in the April Astriobiology may, in fact, expand the habitable zone by ten to twenty percent, which would greatly increase the number of planets suitable for life.

We’re talking about planets whose axis has been tilted from their orbital plane thanks to gravitational interactions with other planets in the system. The contention here is that a fluctuating tilt in a planet’s orbit may enhance rather than diminish the chances for life, because glaciation becomes more difficult when polar regions melt thanks to the erratic spin. Says Rory Barnes (University of Washington), “…the rapid tilting of an exoplanet actually increases the likelihood that there might be liquid water on a planet’s surface.” From the paper:

We interpret our results to mean that planets with large and rapid obliquity oscillations are more likely to be habitable than those with negligible oscillations, such as the Earth. This perspective is at odds with the notion that the stability of the Earth’s obliquity is important to the development of life. While it still may be true that rapid oscillations can be detrimental, and certainly at some point obliquity cycles could be too large and rapid, our results clearly show that rapid obliquity evolution can be a boon for habitability. At the least, one should not rule out life on planets with rapid obliquity cycles.


Image: Tilted orbits such as those shown might make some planets wobble like a top that’s almost done spinning, an effect that could maintain liquid water on the surface, thus giving life a chance. Credit: NASA/GSFC.

One consequence is that future searches for living planets might be extended farther from the target star, given the deeper habitable zone available, a result with a bearing on how difficult it is to separate stellar and reflected planetary light. The researchers do point out in their conclusion that their simulations all began with planetary spin rates of 24 hours and obliquities of 23.5 degrees, clarifying the need for future work on a wider range of initial conditions. In particular, does a specific solar system ‘architecture’ always produce a particular obliquity cycle?

Interesting stuff, and bear in mind that it could have ramifications on another theory, that planets need a large, stabilizing moon to be suitable for life. The Earth’s axial tilt of 23.5 degrees would, in the absence of Luna, increase to the point where climate fluctuations became more extreme. This could be problematic for planets in orbits like ours, but at the outer edge of the habitable zone, in this view, those fluctuations could be precisely what is needed to prevent ice from becoming global, making the lack of a moon is a distinct plus. Large moons, then, may have a role to play in both directions, depending on the planet’s position in the habitable zone.

The paper is Armstrong et al., “Effects of Extreme Obliquity Variations on the Habitability of Exoplanets,” Astrobiology Vol. 14, Issue 4 (April 15, 2014), full text available. The University of Washington press release is here.


Comments on this entry are closed.

  • Paul W April 21, 2014, 13:01

    It’s a great “find” in the Kepler data, if it holds up.
    They trumpeted it as, finally, an earth sized planet in the habitable zone of it’s host star. Not mentioning that Kepler’s real goal was to find such a planet in the habitable zone of a “sunlike’ star.
    Still an important find.
    I give credit to the artist that produced such a pretty, and real looking image.
    Very beautiful. Probable very unlike whatever the real planet looks like.
    Perhaps the artist should have gone for the Mars look, but I know, it’s not trivial to keep the public interested, fascinated with the notion that there are other earths out there. This one probably ain’t it, but every nugget helps.

  • Andrew Palfreyman April 21, 2014, 16:07

    If the surface temperature is between 0 and 100 deg C, but there is almost no atmosphere, the phase diagram for pure water shows that only water vapour would exist around the surface. This is doubtless too simplistic, but I see no mention of the possibility.

  • Mike Jude April 21, 2014, 16:59

    Interesting point that depicting exo-planets in too realistic a way might give the general public the impression that we know more than we actually do. But one wonders if this might not be a good thing? If the public believes that we know where other habitable worlds are, the next logical question is, why aren’t we figuring out a way to get there?

    Also, it occurs to me that one way to get interstellar space flight going might be to just do it. You have discussed the notion of building very small (1 Kg) space probes. If we consider the Voyagers/Pioneers as interstellar probes V1, how about an X prize content for interstellar probe V2, with an order of magnitude higher velocity (but very small size). Give extra points for instrumentation and messaging, but make that optional. It wouldn’t matter if the probe took a thousand years to reach a star…any star, the idea would be that we are doing…not speculating. Over time, increase the velocity and instrumentation requirements…keep upping the bar. I am sure that by using existing boosters and gravity assists, some pretty impressive delta Vs could be achieved…even without exotic new forms of propulsion.

    Anyway…I think it is possible to talk something to death…perhaps it would generate more interest if we simply did what we could with what we have. :-)

  • ljk April 21, 2014, 17:21

    Why are we looking for another Earth? We already have one.

    Yes I know why, but bear with me.

    I thought exploring space was all about finding strange new worlds, seeking out new life and new civilizations. You know, to boldly go where no one has gone before.

    Instead the agencies searching for alien worlds and life are all seeking versions of ourselves. Maybe that is what is actually out there, but as we have seen even long before finding the first exoplanet, most stars in the Universe are not G2 Sol type suns. The same likely applies to alien worlds not being like Earth and alien life not being humanoid beings.

    This is why it is going to take a very long time for us to find real ETI at this rate, so long we just keep looking for copies of ourselves and our world. Also see here for the details on this:


    “We take off into the cosmos, ready for anything: for solitude, for hardship, for exhaustion, death. Modesty forbids us to say so, but there are times when we think pretty well of ourselves. And yet, if we examine it more closely, our enthusiasm turns out to be all a sham. We don’t want to conquer the cosmos, we simply want to extend the boundaries of Earth to the frontiers of the cosmos. For us, such and such a planet is as arid as the Sahara, another as frozen as the North Pole, yet another as lush as the Amazon basin. We are humanitarian and chivalrous; we don’t want to enslave other races, we simply want to bequeath them our values and take over their heritage in exchange. We think of ourselves as the Knights of the Holy Contact. This is another lie. We are only seeking Man. We have no need of other worlds. A single world, our own, suffices us; but we can’t accept it for what it is. We are searching for an ideal image of our own world: we go in quest of a planet, a civilization superior to our own but developed on the basis of a prototype of our primeval past. At the same time, there is something inside us which we don’t like to face up to, from which we try to protect ourselves, but which nevertheless remains, since we don’t leave Earth in a state of primal innocence. We arrive here as we are in reality, and when the page is turned and that reality is revealed to us – that part of our reality which we would prefer to pass over in silence – then we don’t like it anymore.” – Stanislaw Lem in Solaris (1960)

  • Ashley Baldwin April 21, 2014, 17:23

    Kepler -186 is not your typical M dwarf planet though. Although it orbits at .4 AU and is therefore on the edge of the “habitable” zone this is possibly an advantage. The biggest contributor to the synchrous orbit equation is semi major axis . The further out you are , the longer till you get tidally locked , which most people agree isn’t conducive to terrestrial type life. At this sort of distance, 186-f may not yet be tidally locked, increasing the chance of a dynamo induced protective magnetic field ( against all those nasty CMEs, EUV and X rays ) and a decent climate. The authors of the paper also report that 0.5-5 bar CO2/N2 atmosphere could get the temperature above freezing at this distance , helped by 186f’s greater mass and gravity . My feeling is that if we are to find life around M dwarfs, its examples like this that will have it. For reference, to avoid tidal locking for billions of years , you need to maintain a semi major axis of greater than .5 AU. That’s K dwarf territory.

  • Av April 21, 2014, 17:33

    Great news About nearby Brown Dwarf (free-floating Planet?) than have been found:



  • Henry Rouhivuori April 21, 2014, 18:01

    Kepler-186f has high propability to be tidally locked to It’s star. That leaves half of the planet in darkness and coldness, maybe very cold because of the orbit just within the outer boundary of HZ.

  • Alex Tolley April 21, 2014, 20:57

    Interesting point that depicting exo-planets in too realistic a way might give the general public the impression that we know more than we actually do. But one wonders if this might not be a good thing? If the public believes that we know where other habitable worlds are, the next logical question is, why aren’t we figuring out a way to get there?

    Also, it occurs to me that one way to get interstellar space flight going might be to just do it.

    “A new life awaits you in the Off-World colonies. The chance to begin again in a golden land of opportunity and adventure. Let’s go to the colonies! This announcement has been brought to you by the Shimago-Domínguez corporation, helping America into the new world.”

    Phil Dick would understand what you mean. (As would the Norsemen being enticed to “Greenland”). :-)

  • Alex Tolley April 21, 2014, 21:23

    While the HZ may be extended for microorganisms, such obliquity would be disastrous for terrestrial ecosystems. Look at Figure 9, which if I am interpreting correctly, shows major temperature swings of up to 170K over the year. Terrestrial biomes would not survive that. Rooted plants would need to evolve to survive these extreme temperature swings, as well as the animal life associated with it. We do not have terrestrial analogs that come even close to that. Life would have to adapt to extreme cold and extreme heat over the planet’s year. We wouldn’t see stable, lush forests on such a world.

  • Lionel April 22, 2014, 2:11


    Thanks! Very exciting news

  • ljk April 22, 2014, 9:27

    A review of a new book on how rare Earth might really be titled Lucky Planet:


    The author’s Web site here:


    Waltham is an academic geophysicist and so far as I can tell, there is not a hidden religious agenda to his Rare Earth stance. Unlike the godawful (pun intended) book The Privileged Planet, which was nothing but religious agenda, and a specific brand of religion at that.

  • ljk April 22, 2014, 9:43

    As Joan of Arc liked to say, at least in that movie about her, I am just the messenger here:


  • Rob Henry April 22, 2014, 20:30

    Careful Alex, high obliquity planets are very different from our own. Past a certain point (60 deg, I think annual tropical insolation is lower than polar insolation. I saw a simulation in which our current Earth given a 90 deg tilt had permanent ice-free poles (despite the six-month night), and tropics where the ice never completely melted. Nevertheless, tropics always have the more stable temperature. Finding the best combination of warmth and temperature stability may well entail finding the perfect latitude, and that would definitely migrate slowly and vary in width with changing obliquity. I think the whole point of this article is to argue that such a band will always exist somewhere for their model planet.

  • Alex Tolley April 22, 2014, 23:29

    @Rob – the high obliquity example shows that the temperature at any given latitude cycles from 217K to 320+K over the course of a year. There are no stable, low temperature range latitudes in the model the authors use. Only the Earthlike, low obliquity model has relatively stable temperature regimes that would allow the ecosystems we have. Perhaps their model is not very good, but this wide ranging temperature range is what I see in their simulations. Perhaps the temperatures are stable at particular longitudes, but that isn’t evident in their data to me.

  • Ashley Baldwin April 23, 2014, 14:40

    I don’t think anyone is in a position to talk about the Earth being unique, privileged, special, rare or anything or for any reason. We have barely scratched the surface of exoplanetology . We have just 20 years of knowledge limited to rough mass calculations based around Doppler spectroscopy supplemented with one significant transit mission looking at stars so far away that we can’t even use the the RV method to calculate more than a few bulk densities and then only in planets near their parent stars. Nothing like the Earth. We have only directly imagined a few new, big and hot planets that again are nothing like our own. Not because such planets don’t necessarily exist, but simply because we haven’t developed anything like the technology to see them . Our current habitable planet sample size is thus one. That’s not alot to go on. As a doctor I certainly wouldn’t be advocating the use of a drug based on an experiment in one person , however wonderful the results. I repeat, a sample size of one. You don’t have to be a statistician to appreciate that drawing any type of meaningful conclusion from that is meaningless. Even our pre exoplanet theories on our own solar system have been found to be wanting based on the meagre data we have gleaned so far from other star systems. If we don’t understand our own star system how can we comment on others or separate ourselves from them ? As to exobiology , the only living creatures to date have been discovered in one place and one place only. Earth. The entire science of exobiology is based on Earth based extrophile life and nothing else other than imagination and extrapolation.( nothing wrong with that provided you are aware of the many caveats) . Give me some decent sized space telescopes equipped with decent coronagraphs and spectographs and a decent mission time looking at nearby stars and maybe we will find a potential few bio signature molecules that justify a bigger Interferometer multi telescope TPF mission. Then , just maybe, we might be better positioned to comment just how special our dear old Earth is without resorting to 21st Century Ptolemy.

  • Rob Henry April 23, 2014, 16:42

    Those look like inland temperatures. Let’s take Earth at 90 again. TWICE a year, the tropics has a summer, where insolation is the same as the tropics today. The sea-ice begins to melt rapidly, but can never quite get to completion, then follow a three month winter where new ice begins to form. The sea is an enormous temperature buffer, that also effects the coast. Neither sea nor shore can get far from zero centigrade -but thousands of kilometers inland it is a far different story.

  • Michael April 24, 2014, 13:43

    At a 90 degrees tilt

    The way I am seeing it is that at the poles centres we would have half a year of light and half a year of darkness.

    At the equator we would have a small period of permanent daylight all the way around twice a year when it coincides with the terminator followed by a half year period of waxing light and waning darkness, then a half year period of waxing darkness and waning light. It looks like we would have one large moving ice pole over the entire planet in a year (anti sub-solar point).

    It does not sound like a good place to live as the whole surface would be scoured/affected by the ice pack in a year, hardly conductive to plant life.

    If we wanted to stay in the light always we would have to walk/swim/fly the circumference distance over the planets year following the sub-solar point, I said the equator in the other post, please ignore it.

  • Alex Tolley April 24, 2014, 20:19

    @Rob Henry
    Neither sea nor shore can get far from zero

    The sea itself won’t get far from zero, or at least a narrow range, but the surface will. Sea ice will allow extremely cold temperature above it once it insulates the sea from the atmosphere. Think of conditions above the Arctic ice cover from Canada to the North Pole. The constantly shifting winds might be the main ameliorating factor for temperature on the coasts and inland, and of course the pattern of land and sea will be important too on such a world.

    SF world building should be having a field day as authors can use studies such as these, and soon no doubt run their own simulations to see what results.It should make for much richer, more realistic venues.

  • David Waltham April 26, 2014, 10:08

    I’d just like to reassure IJK that there is no religious agenda to my book, Lucky Planet. I’m just a geologist who’s trying to work out whether or not the Earth is special. My view is that the, very limited, data we have points towards the Earth being a peculiar world but Ashley Baldwin is right to say that extrapolating from a single case is inherently risky. It’s my best guess, not definitive truth.

    I’d also like to correct the widely held belief (repeated in Paul’s blog) that a large Moon stabilizes our axis. If the Earth had had a larger Moon from the beginning, the Earth-Moon system would have evolved more quickly towards instability (it actually becomes unstable in about 1Gy). Hence, a larger Moon would actually have destabilized the Earth! Magically taking the Moon away today would also destabilize the axis but that’s asking the wrong question since removing the Moon is not the same thing as never having one in the first place. Hope that’s clear. If not, check out Lucky Planet:-)

  • Michael April 27, 2014, 1:08

    @David Waltham April 26, 2014 at 10:08

    ‘Lucky Planet. I’m just a geologist who’s trying to work out whether or not the Earth is special.’

    It must be special at least to us, other microbes that are now no longer dominant may think, if they could, otherwise.

    ‘I’d also like to correct the widely held belief (repeated in Paul’s blog) that a large Moon stabilizes our axis. If the Earth had had a larger Moon from the beginning, the Earth-Moon system would have evolved more quickly towards instability (it actually becomes unstable in about 1Gy). ‘

    If we did not have a liquid/continent interface or a rapid spin to form a bulge as well the moon would have to have a different mass to stabilise it. Another thing the continents are moving around, is that not the same as instability, I mean one eon you are in the sun then the next you are under a mile of ice!

    Life could have started out a billion times in the galaxy only to be become extinct or is developing slowly, we just don’t have enough of a sample.

    Yes we may need a goldilocks moon,
    yes we may need a very successful species, the dinosaurs to become extinct, yes we need to have had a lot of oil that we could use as a fuel source
    ….and so on.

    Are we lucky because we are here or lucky to be intelligent enough to philosophise on the subject? If intelligent life evolved on this planet it will occur somewhere else, but the probability is very, very small. In my opinion the great filter is an endless list of things that could go wrong and probably have.

  • ljk July 7, 2014, 11:43

    The Disappearing Habitable Planets of GJ 581

    By Andrew Lepage

    July 7, 2014

    During the past few days, word has started to spread about the latest study that has resulted in the “disappearance” of a pair of planets that some had claimed over the past few years as being potentially habitable: GJ 581d and g. These planets have not physically disappeared, of course. Instead, the subtle radial velocity variations that had originally been interpreted as being the result of as many as six planets orbiting the nearby red dwarf star, GJ 581, have instead been found to be the result of just three planets, the subtle effects of stellar magnetic activity and noise in the data.

    In the wake of these latest findings, there have been a chorus of nonscientists on the internet touting this as yet another example of the failure of science. To the contrary, this episode is a perfect example of how the scientific process is suppose to work.

    Full article here:


    To quote:

    While there are those who have pointed to this episode as further proof of the failure of science, nothing could be further from the truth. The past decade of experience with GJ 581 is a perfect example of how science is suppose to work. Science seeks to find natural explanations for observed phenomena. One of the key steps in this process is the formulation of scientific hypotheses that not only explain current observations but produce predictions of what future observations will find in order to prove or disprove the hypothesis. If a hypothesis fails these tests, it is either modified or discarded in favor of a new hypothesis that better explains the observations and the process begins again. It is this constant reassessing and self-correction that makes science such a powerful tool.

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