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What Makes a Planet ‘Superhabitable’?

Friday’s look at habitable zones, and the possibilities of life below the surface or in the atmosphere of an exoplanet, segues naturally into the fascinating notion of ‘superhabitable’ worlds. René Heller (McMaster University) and John Armstrong (Weber State University) ponder the possibilities in a recent paper for Astrobiology. What if, the scientists ask, our notions of habitability are too closely crafted to our own anthropocentric viewpoint? Could there be planets that are actually more habitable than the Earth? Should the Earth itself be considered, with respect to a broader view of biology, only marginally habitable?

The question has important ramifications for how we approach the search for other habitable worlds. We study extremophilic life forms on Earth and question whether conditions even more bizarre than these could still produce life. But Heller and Armstrong reframe the issue:

The word ‘bizarre’ is here to be understood from an anthropocentric point of view. From a potpourri of habitable worlds that may exist, Earth might well turn out as one that is marginally habitable, eventually bizarre from a biocentric standpoint. In other words, it is not clear why Earth should offer the most suitable regions in the physicochemical parameter space that can be tolerated by living organisms. Such an anthropocentric assumption could mislead research for extrasolar habitable planets because planets could be non-Earth-like but yet offer more suitable conditions for the emergence and evolution of life than Earth did or does; that is, they could be superhabitable.

SuperEarth_3

Image: A super-Earth, as depicted in this artist’s impression of HD 215497 b, could actually be more ‘habitable’ than the Earth, according to Heller and Armstrong’s new work, which re-examines our ideas about the conditions needed for life. Credit: Wikimedia Commons.

Heller and Armstrong stick with liquid water as a prerequisite for life, so their extrapolations are incremental, but the proposal to enter a new word into the lexicon is bold enough. So let’s consider what ‘superhabitability’ might mean. One possibility is that a larger world (though not large enough to cause plate tectonics to cease) could offer more spacious conditions for life’s development. Planets with what the duo call ‘fractionate continents and archipelagos’ should produce a wider diversity of habitats, their spacious shallow waters offering higher biodiversity than Earth’s deep oceans. The paper leans toward dry planets with a lower fractional surface coverage of water, where liquid water is abundant but distributed into many reservoirs.

The elements of superhabitability multiply. Plate tectonics allow us to speculate on super-Earths with masses up to about two Earth masses. Beyond this, high pressures in the mantle reduce the likelihood of tectonic activity (the two Earth masses used here is a deliberately conservative figure). Also needed: An intrinsic magnetic field produced by a liquid, rotating, convecting core to protect the planet from high-energy cosmic radiation. And the biodiversity of Earth seems to multiply in periods of warmer conditions, suggesting that a warmer version of Earth might have extended tropical zones and, over the course of aeons, a greater variance in biological forms.

The factors at play in Heller and Armstrong’s speculations make for interesting scenarios. Imagine a solar system with more than a single terrestrial planet or moon in the habitable zone. More even distribution between the Earth and the Moon would have resulted in a double-planet, with both objects habitable. Or imagine switching Mars and Venus, to potentially produce three habitable planets. Some systems, indeed, might be ‘multihabitable,’ with massive moons around gas giants and the possibility of panspermia within the system to spread local biology.

Or ponder moving the Earth, which some recent work describes as being at the very inner edge of the Sun’s habitable zone. A terrestrial world located closer to the center of the HZ could be considered superhabitable because it would be more resistant to runaway greenhouse states than the Earth is. Or perhaps age is itself a condition for superhabitability. Heller and Armstrong make the case that the Earth experienced greater biodiversity as it aged — thus the introduction of oxygen about 2.5 billion years ago (from oceanic algae) led to greater habitability on the surface, allowing life to move onto the continents, an increase in planetary habitability.

And what of the star the planet orbits? The authors see K stars as offering a compromise between initial and long-term high-energy radiation, making them favorable hosts for superhabitable worlds. From the paper:

Higher biodiversity made Earth more habitable in the long term. If this is a general feature of inhabited planets, that is to say, that planets tend to become more habitable once they are inhabited, a host star slightly less massive than the Sun should be favorable for superhabitability. These so-called K-dwarf stars have lifetimes that are longer than the age of the Universe. Consequently, if they are much older than the Sun, then life has had more time to emerge on their potentially habitable planets and moons, and — once occurred — it would have had more time to ‘tune’ its ecosystem to make it even more habitable.

That, of course, gets us to the K-class star we have so often discussed in these pages, Alpha Centauri B. We already have one planetary candidate around it, an Earth-mass planet in a 3.235-day orbit. In terms of age, recent work based on asteroseismology, chromospheric activity and other means shows it to be a bit older than the Sun, with estimates ranging from 4.85 billion years up to about 6.5 billion. The authors note that a planet in the habitable zone around this star, collecting water from objects beyond the snowline, could have had primitive life forms when the Earth had just collided with the Mars-sized object that would be responsible for the Moon.

centauriB_mann

Image: Artist’s impression of Alpha Centauri B, with Centauri A in the distance and the planet candidate Centauri B b also marked. Credit: Adrian Mann.

Heller and Armstrong note that our exoplanet studies have shown us that the Solar System is anything but typical when it comes to planetary systems, so it may well be that the Earth is itself atypical when it comes to inhabited worlds. In that sense — and they make this point specifically — their work refutes the Ward and Brownlee ‘rare Earth hypothesis’ that saw life as an extremely unlikely phenomenon emerging from a wide range of precise conditions. Perhaps what Ward and Brownlee are seeing, says this new work, is the emergence of an only marginally habitable world. If so, our search for inhabited planets should take in worlds slightly older and slightly more massive than our own, preferably those orbiting K-class stars like the close-by Centauri B.

Can we create a biocentric model to use in our search for habitable worlds that replaces our anthropocentric expectations? This paper makes a spirited run at the idea. The paper is Heller and Armstrong, “Superhabitable Worlds,” Astrobiology Vol. 14, No. 1 (2014), available online. The work is thick with ideas, and I didn’t have time today to go into the ‘terrestrial menagerie’ discussed in its first part, which shows not only how planets in the habitable zone can be rendered uninhabitable but how exomoons beyond the HZ may be rendered life-bearing. I suspect, though, that we’ll be coming back to these arguments again soon. Thanks to the numerous readers who forwarded links to this paper.

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Comments on this entry are closed.

  • ljk January 27, 2014, 11:06

    Paul Gilster wrote in the main article:

    “Heller and Armstrong note that our exoplanet studies have shown us that the Solar System is anything but typical when it comes to planetary systems, so it may well be that the Earth is itself atypical when it comes to inhabited worlds.”

    Is the Sol system really atypical, or even truly unique? Our current exoplanet detection methods are still best at finding exogiants circling very close to their suns. We have found a few systems that parallel our own to varying degrees. The fact that we have found them out of a few thousand detections says to me they are not uncommon after all.

    Beware anyone saying the Sol system and Earth are unique for any reason other than scientific ones. Many talking primates with car keys on this planet still want us to be unique and special, even though in the literal grand scheme of existence we are comparatively microscopic.

  • wajai January 27, 2014, 11:32

    While the whole concept is interesting, and the authors have good points, their view on red dwarfs may only be the full half of the cup :
    not a word on tidal locking? not a word on magnetic field squashing?
    If a superhabitability factor such as the star life duration comes at an even greater cost, i m not sure it can help to select candidates for extraterrestrial life.
    A usefull answer to the rare earth hypothesis anyway.

  • RobFlores January 27, 2014, 12:08

    What is the frequency of catastrophic magma outflows from deep mantle
    plumes on these larger worlds? An event that lowers the avg temperature
    of a planet by -10C for decades, I would call destabilizing because it’s effect
    is enough to kill a large amount of life forms, but it’s not long enough to
    create an evolutionary pulse. I don’t think you can evolve multi-celled
    higher animals if the volcanism is a constant presence.
    If your world is larger than our Earth, is it not the case that the internal heat in the mantle and core will loose heat much slower. While this favors life in the long run, since nutrients will be recycled for a longer period of a planets existence, will most of that existence be with a surface scarred by
    heating and cooling events.

  • Robert Steinke January 27, 2014, 12:14

    “Also needed: An intrinsic magnetic field produced by a liquid, rotating, convecting core to protect the planet from high-energy cosmic radiation.”

    I’m not sure if this is a valid assumption. Radiation and mutation is, on average, bad for any particular individual, but as long as some individuals of a population survive to reproduce higher mutation rates can be better for the population because it speeds up evolution. We may find that life can adapt just fine to high radiation environments even if the life expectancy of each individual is shortened.

  • Jim Baerg January 27, 2014, 12:48

    For some speculation on the variety of worlds that could have multicellular & even intelligent life see:
    http://www.worlddreambank.org/B/BIOSPHER.HTM

  • Michael January 27, 2014, 13:24

    I am thinking a very dense atmosphere, high oxygen levels and lower surface gravity would be good for promoting super habitability.

  • Leo Serrano January 27, 2014, 13:30

    is the plates tectonic a solid theory or just one example, the earth? How is plate tectonic on Venus? Too much speculation and no examples …. i think there are many variable we probably are unaware of in terms of habitability and how bigger worlds might work

  • Bill January 27, 2014, 13:51

    “In that sense — and they make this point specifically — their work refutes the Ward and Brownlee ‘rare Earth hypothesis’ that saw life as an extremely unlikely phenomenon emerging from a wide range of precise conditions. ”

    My understanding is that Ward and Brownlee suggest that microbial life is actually very common, but that advanced multicellular life is what is rare.

  • don wilkins January 27, 2014, 14:07

    I am not sure Heller and Armstrong refute Ward and Brownlee so much as disagree.

    Do not Ward and Brownlee still have the upper hand? If older K sequence stars are homes to complex life than where is the intelligent life that the mediocrity principle demands?

  • David Cummings January 27, 2014, 16:08

    ljk, The categories “microscopic” and “unique” are not mutually exclusive.

  • Paul Gilster January 27, 2014, 17:18

    Bill writes:

    My understanding is that Ward and Brownlee suggest that microbial life is actually very common, but that advanced multicellular life is what is rare.

    Good point — I think I over-stated the point. It’s been some time since I’ve read Ward and Brownlee, but as I recall, you’re right.

  • ljk January 27, 2014, 18:22

    My primary intellectual frustration is that even though we are now aware of how vast the Universe is (and the Cosmos may be ever vaster than even we can currently imagine), many if not most humans still look upon our species as the focal point of existence for a variety of reasons. In a physical, literal cosmic sense if nothing else, we are anything but prominent.

    And even if there is no other intelligent life in the Milky Way galaxy, which I contest but will argue about elsewhere, there is probably plenty of life in general throughout our stellar island and beyond.

    If our SETI and interstellar probe programs were not so grossly underfunded (and in certain cases may be widely off the mark), we might have a better idea by now if we have any celestial neighbors or not. Instead we are left to a combination of a few clues from one data point (Earth), lots of guesses, and often wild and very questionable anecdotes from the general public.

    I am grateful that we have some scientific ideas about the Universe and any life in it, but now that we know Earth orbits the sun and the stars are other suns, it is time to take the next steps seriously.

  • Alex Tolley January 27, 2014, 19:48

    Some of their conclusions about teh distribution of land and water don’t seem correct to me.

    For example, we know that biodiversity is highest in the tropics, therefore one might want a land distribution weighted in this zone. The fractal distribution of land probably has limits in terms of island isolation, as large islands (and continents) have far higher biodiversity than small islands. The land area meeds different types of environment to be contiguous and allow relatively easy plant and animal migration as climatic conditions change, but not too easy to prevent separation of populations.

    From Earth’s history, we know that species diversity has been very much tied to environmental disasters, due to excess volcanism (Permian extinction) and asteroid impact (KT extinction). Recovery in each of these cases involved a “rolling of the dice” in terms of which forms dominated the succeeding epochs. Would a larger world be inherently more resistant to mass extinction events?

    My guess is that the most important feature of a world that is superhabitable, based on actual biological diversity, is stability (i.e. fewest major disruptions), because this allows forms to co-evolve complex communities and have the time to co-evolve into narrow niches. A world that is relatively unstable will be in a state where pioneer forms are forever filling vacated ecosystems.

    So I would plump for a warm, wet world, with a tropical climate covering the planet from pole to pole. The world would be protected from major asteroid impacts, and would have a low level of vulcanism to prevent major climatic instability. A longer lived star might be good too, so that the increasing radiance level would be slower, allowing forms to have longer to evolve between the “snowball” early period and the drying out in the late period. This last may be optional if evolution is always fast enough to cope with the changing star’s output.

  • P January 27, 2014, 21:36

    If Venus ever had plate tectonics, then the ‘continents’ we see on its surface today might be remnants of that era, curtailed by the loss of water perhaps as recently as a billion years ago.

    That haunts me, the idea that an alien visitor to the early earth, surrounded by only microscopic life, could have trained a telescope on Venus and seen not impenetrable cloudtops but something much more earthlike.

    P

  • Lionel January 28, 2014, 5:26

    Yes, discussing ‘habitability’ is typically a discussion of anthro-centric and earthling-centric habitability, discussions about omni-habitability, such as we are having here, are of great value and should be .

    Also agree with comments above about the factors that are likely for super-habitability, especially long-term stability. Further, I think there will be a ‘goldilocks’ optimal mix of disruptions. This would likely mean many frequent minor disruptions providing the impetus for evolution, adaptation and specialisation, and little or no major catastrophic disruptions that permanently erase progress

    Thinking what might become of a super-habitable planet is interesting, I see no reason why bio-spheres couldn’t exist that are so dense and layered with life that they make our rainforests look like bleak arctic tundra by comparison

  • Lionel January 28, 2014, 5:27

    Yes, discussing ‘habitability’ is typically a discussion of anthro-centric and earthling-centric habitability, discussions about omni-habitability, such as we are having here, are of great value and should be on the menu more often as new exo-planet findings pour in

    Also agree with comments above about the factors that are likely for super-habitability, especially long-term stability. Further, I think there will be a ‘goldilocks’ optimal mix of disruptions. This would likely mean many frequent minor disruptions providing the impetus for evolution, adaptation and specialisation, and little or no major catastrophic disruptions that permanently erase progress

    Thinking what might become of a super-habitable planet is interesting, I see no reason why bio-spheres couldn’t exist that are so dense and layered with life that they make our rainforests look like bleak arctic tundra by comparison

  • Astronist January 28, 2014, 6:49

    Sorry, I find this unconvincing. I think it’s just attention-seeking.

    The idea of Earth as “marginally habitable” seems bizarre, considering the quantity, diversity and durability of life that has inhabited it for the past several billion years.

    The idea that our “anthropocentric assumption” (the authors mean: “terrestrocentric”) “could mislead research for extrasolar habitable planets” is spurious, because all exosolar planets that come within range of detailed close-up examination will be interesting in their own right, and have their own surprises. No doubt it could mislead speculation, but it’s not speculation that matters in the end, it’s actual observations.

    Stephen A.

  • kzb January 28, 2014, 8:54

    I think some people on here are confusing superhabitability with what would be nice (or not nice) for us (as a human civilisation).

    Biodiversity and productivity are highest around fault lines. Human populations are concentrated in areas of highest geological activity (Tokyo, LA, etc). There is something good about high geological activity.

    Also, a high rate of catastrophic events would drive evolution faster. It might not be good for the individuals concerned, it’s possibly not good for an advanced civilisation, but it’s good for evolution.

  • Tarmen January 28, 2014, 12:16

    There must be some riotous worlds out there. Planets that make our ecology seem a paltry endowment. Imagine a bi-planet of super sized oceanic worlds to drive monster tides on each other. And if one takes a comet strike, the other is missed, but gathers material from the ejecta. A fast rotation and steep tilt to drive the seasons. Imagine them both with a low surface gravity. They might have numerous volcanic archipelagos, nuclear decay and orange sunlight reaching to the floor of shallow, wind-roiled seas. Probably make some great wild-life TV productions there, if you don’t get eaten !

  • Lionel January 28, 2014, 13:12

    @Astronist

    Thinking about what you and others wrote, I now basically agree that Earth shouldn’t be labelled “marginally habitable” since it seems that conditions made it very likely that life would emerge and develop here, and that it would, when Earth’s conditions are replicated elsewhere

    That being said, I don’t think the authors are just attention seeking, I think they are making legitimate points but using a poor choice of label. I think what they are *trying* to say is that conditions far more favourable for the emergence, sustenance and development of life than those on earth are likely and even inevitable to be found elsewhere. I see it as logical to describe such highly supportive conditions “super-habitable”, and also believe that we’ll find Earth to be a relatively good place for life but by no means the best. It’s good they point out that we’re just somewhere on a spectrum of habitability, and that we’ll probably discovery some amazingly life-favoring places if we search long enough.

    The label “Marginal habitability” however, conjures a sense of a place where life can only get a tenuous hold, restricted to a few hospitable niches in an otherwise inhospitable biosphere. Sounds like Mars rather than Earth.

  • Heath Rezabek January 28, 2014, 13:19

    @ljk – I think one of the data points now being grappled with is the finding that while we haven’t found quite so many worlds our own size, mini-Neptunes seem to be quite common indeed (3/4 of recent survey sample) http://www.nasa.gov/ames/kepler/nasa-kepler-provides-insight-about-enigmatic-but-ubiquitous-planets-five-new-rocky-planets/#.UswFcvRDtHU

    In that sense, unless we turn out to find that Earth-like worlds are even more common, then the focus shifts to the traits of the more common worlds. Modeling them as superhabitable is one way to tackle this question and reassert our mediocrity. But just as we shouldn’t credit attempts to assert our rarity that aren’t based in science, the same ought presumably to apply to assertions of our mediocrity. Here we have all these mini-Neptunian superearths, of which we are not an example. What to do?

    When defining superhabitability, then, the question of the type of life most common becomes a key one.

    The paper proposes that widespread shallow seas would result in a greater biological richness, and that seems reasonable enough. Yet my understanding was that continental drift and plate tectonics were a great driver of biodiversity.

    So is biodiversity, specifically, part of the definition of superhabitability? And if not, why not?

  • Heath Rezabek January 28, 2014, 13:26

    Meant to say, “Here we have all these mini-Neptunian superearths, of which we are not an example and of which our solar system has no example. What to do?”

  • andy January 28, 2014, 14:22

    There often seems to me to be an issue that the large catastrophes get an unfairly large amount of attention in these kind of discussions. Sure things like the Faint Young Sun Problem, Snowball Earth and the K/Pg impact are interesting, but the times in-between are also relevant.

    Are there any palaeontologists around these parts with expertise in, say, Triassic ecosystems who could say something about the biodiversity distribution of Pangaea when it wasn’t in the grip of the Great Dying? Or what about people who research the Palaeocene-Eocene Thermal Maximum – would they regard PETM Earth as more or less habitable than today’s world (and what are the implications for our future given that civilisation seems to be intent on rerunning the PETM upon itself)?

    It’s definitely a question that should be asked beyond the physics community – I note that the authors of the paper are both physicists. Not saying it isn’t a provocative and interesting contribution, but I sometimes worry that astrobiology has a tendency to end up as a physics echo chamber that fails to involve the people who have relevant expertise to bring to the discussion.

  • jim moore January 28, 2014, 16:36

    i have a question for the folks here. If your planet has a 2 g gravity. Can you get into orbit with chemical rockets?

  • Rob Henry January 28, 2014, 16:50

    Astronist says “The idea of Earth as “marginally habitable” seems bizarre, considering the quantity, diversity and durability of life that has inhabited it for the past several billion years.”
    And I am wondering if we are living on different Earths.

    On my one conversion of sunlight into biomass averages at least one and a half orders of magnitude ( by some figures I have seen, two) below our most efficient plant (sugar cane). The most obvious problem with my planet is mineral deficiency in the top layers of ocean surface. These only seem to circulate well when water temperature drops to near 4C or in the shallows. This doesn’t sound too bad a fault for a planet called Earth, but unfortunately mine is badly names and is mostly covered in kilometres of salt water. No doubt your stands better on that count, but I am sure you will be familiar with other problems such as vast deserts and mountain tops.

    Unlike some others here, I am doubtful that a super Earth with its thicker atmosphere and lower susceptibility to sputtering needs a magnetic field at all, nor whether supervolcanic events including ‘catastrophic magma outflows’ will have identical climatic effects to those seen on my Earth. Also, I can’t tell whether rotation or a non rotating biosphere is the more productive without studying a real example of one.

  • Alex Tolley January 28, 2014, 17:49

    @Heath

    The paper proposes that widespread shallow seas would result in a greater biological richness, and that seems reasonable enough. Yet my understanding was that continental drift and plate tectonics were a great driver of biodiversity.

    On Earth, the oceans are fairly contiguous. More importantly, the shallow seas are/were too. If a species can migrate and dominate a contiguous area, it should reduce biodiversity. I think there are papers showing this for trilobites. Oceanic species can also migrate easily. Because the oceans moderate climatic effects, the ranges of each species may be greater than terrestrial forms.

    It isn’t clear to me if there is an optimal point the distribution of land and sea to maximize biodiversity. What we think we know, is that speciation requires populations to be separated by a some sort of barrier so that genetic drift can drive the populations apart into separate species.

  • Alex Tolley January 28, 2014, 17:55

    @andy – I don’t have the data handy, but there are databases of paleontology with species counts. In fact this was used by Richard Muller as the basis of his hypothesis that extinction events are approximately cyclical, rather than random. The peak diversity of species appears just before teh extinction event, then recovers until the next one. In between each EE, we see an increasing number of species over the ~30 my between events.

    Once caveat is that species determinations can be tricky, so that we may be over or under counting them. Note the recent observation that several recent hominid species may simply be the natural physical range of just one!

  • Alex Tolley January 28, 2014, 18:08

    @Rob Henry

    The most obvious problem with my planet is mineral deficiency in the top layers of ocean surface.

    Indeed. Sunlight intensity is only limiting in the ocean depths, perhaps dense jungle floors and day/night cycles at extreme latitudes. Life is most abundant where the turnover of key mineral, e.g. iron in the ocean, is high. In many ecosystems, phosphorus may be the limiting factor.

    However I don’t think that this limits biodiversity. If it did, we would expect higher biodiversity by simply removing limits. Often we don’t see that, quite the reverse, e.g. toxic algal blooms.

    I also want to put in a word for the Gaia Hypothesis. The general concept that life shapes the planet which maximizes life should be considered – rather than life forced to adapt purely to the planet. Recall that Lovelock predicted Mars was dead because life had not been able to shape the planet to ensure its abundance. As a result, we are left with only the possibility that the lithosphere may still have microbial life thinly spread throughout its volume. Whether one agree with the GH or not, I do think that life tends to maximize its biomass and diversity (within the constraints of its environment).

  • Alex Tolley January 28, 2014, 18:22

    @jim moore. Probably not. A doubled gravity implies a 2 fold increase in orbital velocity. The rocket equation determines the relationship between mass ratios, exhaust velocity and terminal velocity. If we assume that the best we can do is approximately 4500 m/s exhaust velocity on both worlds, then the mass ratio for the 2g world is the square of that for the 1g world. For Earth the mass ratio is about 5.8, i.e. about 83% of the rocket is fuel. For the 2g world is is 34.6, i.e. over 97% is fuel. That would be extremely hard to achieve.

    You would probably want to use air breathing engines, nuclear engines or some other more potent or economical means of propulsion to reach orbital velocity on your 2g world.

  • Eniac January 28, 2014, 21:25

    As much as I like the superhabitability concept, statistically it is unlikely that Earth is far off the optimum. If there was a common kind of planet with much better conditions for life than Earth, most life-bearing planets would be of that kind, and we would be living on one. We must pick between two things: 1) we were very unlucky to end out on such a poor place, or 2) Earth is near optimum habitability and that is why we are here. In the absence of real evidence it is anyone’s pick, but Occam would surely have chosen 2).

    This argument gains in strength if you assume life is incredibly unlikely to form spontaneously: In that case there is only one life-bearing planet, and it is overwhelmingly likely to be one with near optimal conditions.

  • jim moore January 29, 2014, 11:16

    Thanks Alex,
    I was thinking that a higher gravity might prevent a substantial portion of technological civilizations from becoming a space based civilizations.

  • Michael January 29, 2014, 14:29

    @jim moore January 29, 2014 at 11:16

    ‘Thanks Alex,
    I was thinking that a higher gravity might prevent a substantial portion of technological civilizations from becoming a space based civilizations.’

    Conversely lower gravity and smaller massed beings for that matter increases the ease at which aliens can get into space substantially.

  • andy January 29, 2014, 16:16

    The peak diversity of species appears just before teh extinction event, then recovers until the next one. In between each EE, we see an increasing number of species over the ~30 my between events.

    Kind of missing the point here – this is still the view that the mass extinctions are the be-all and end-all of the history of life on Earth. What was it doing in the meantime? Throughout the history of this planet there have been successful ecologies that inhabited a rather different world to the one we have today. We should seek to understand these in their own terms instead of just in terms of mass extinctions happening to them. For example, in today’s world biodiversity is highest in the tropics, but was that also true on Pangaea? What were the polar seas like when the Earth did not have permanent ice caps? Etc, etc.

    I guess mass extinctions attract the most attention in the space community because several of them offer the chance of doing physics to them: the calculation of the solar luminosity at the time of Snowball Earth is rather tractable compared to figuring out what palaeoecologies were up to when they were minding their own business without big rocks falling out of the sky or massive volcanic eruptions poisoning the atmosphere. Plus the K/Pg impact gives space advocates something convenient to point to when they say we should be spending lots of money going to space: we should go there or we could end up like the dinosaurs (~10000 extant species, as compared to the ~5400 extant mammal species). But for the most part, life has been, well, living, rather than dying off in large numbers all at once and it seems slightly wrong-headed to restrict attention to the big catastrophes when a lot of the interesting stuff is happening at other times.

  • Alex Tolley January 29, 2014, 18:49

    @andy
    this is still the view that the mass extinctions are the be-all and end-all of the history of life on Earth. What was it doing in the meantime?

    I explicitly stated that biodiversity increases continuously after the extinction event. This means that the Earth never reaches its full potential diversity before it is reset with the next extinction.

    For example, in today’s world biodiversity is highest in the tropics, but was that also true on Pangaea?

    Probably. You could test this by looking at species located purely in the equatorial zones of Pangea compared to the higher latitudes. But obviously you could make the equatorial regions too hot and the peak diversity would move to 2 bands at higher latitudes.

    it seems slightly wrong-headed to restrict attention to the big catastrophes when a lot of the interesting stuff is happening at other times.

    I don’t think you can level that accusation at the vast majority of scientists studying the past.

  • David Grinspoon January 29, 2014, 18:54

    Hey there – I’m glad this question is getting some attention! Here is what I wrote in 2003, in Lonely Planets:

    If saguaro cacti were intelligent enough to theorize about life elsewhere, but not smart enough to think about observational bias, they would conclude that most of Earth is uninhabitable since most of it is not desert. (But they would be comforted to see that more of it is becoming habitable all the time.)
    While we’re searching for other Earths, we should also be asking what a more habitable planet would look like. More habitable than Earth? Dare I say it? Won’t lightning strike me down? What could possibly make a planet better than our perfect Earth?
    People are always giving the Earth a hard time about its slow development: “Look at that stupid world!” they say. “They ¬didn’t go multicellular until their star was half used up. No wonder they’re still in the primitive, factional, warring phase.”
    But, picture a planet that is similar to the Earth in many ways except that it formed originally with much less iron in the mix (perhaps orbiting one of those stars farther out in the galaxy having less of the heavy elements with which to make planets). Remember that on Earth iron hogged all the oxygen for the longest time and kept it out of the atmo-sphere, stunting our growth for billions of years. On this iron–depleted world, all other things being equal, oxygen would build up much faster in the air. If that’s what complex life needs, on that planet they’d have been finishing up grad school while we were still microbes in diapers.
    Another variable is the way planetary size affects atmospheric evolution. We do know that all planets are constantly losing hydrogen. The more hydrogen departs, the more oxygen is running free, not locked up in H2O. Over long periods, this hydrogenous exodus helps planets become oxidized. Now, the smaller a planet is, the faster it loses hydrogen. So, other things being equal, smaller Earth-¬like worlds might become oxidized sooner. If high atmospheric oxygen content is the key to animal complexity, then Earth may not be the fastest out of the gate by a long shot.*
    You could come up with countless other possible improvements, I’m sure. The examples I’ve just given assume a biosphere with essentially the same needs as ours, but an improved ability to meet them on time and on budget. What about life that thrives on very different conditions? Remember the extremophiles. Evolution is clever, or can pass itself off as clever through sheer persistence. Life is adaptable and resourceful. Those scary regions nearer the center of the galaxy are rich in alternative energy supplies. Who knows what grows there?
    I’m not complaining about Earth. This is my favorite world by far. I’m perfectly comfortable here and for the most part I’m glad things have worked out the way they have. I’m just saying let’s think twice before assuming that this is the only or best world that some kind of cogitatin’ creatures could call home.
    We’re just getting to the point where we can start to make decent predictions about the likely evolutionary paths of other worlds. This is an area of active research, funded by NASA’s exobiology and astrobiology programs, where meaningful new results should be forthcoming. Is Earth a late bloomer, a precocious planet, or typical? We don’t know yet, but stay tuned.
    In the next few chapters I’ll look in more detail at what we actually know about other planets in our own solar system and the rest of the universe. As we survey the lives of planets, think about what a planet really needs for life to arrive and thrive. Which qualities of Earth are simply local oddities to which life has adapted? Surely fate has dealt the Earth some of the cards a living planet truly needs. But surely also the wonderful adaptability of life, and its ability to shape its own environments, combined with our lack of imagination and perspective, could make a random draw seem like a rare winning hand.

  • andy January 30, 2014, 12:53

    I don’t think you can level that accusation at the vast majority of scientists studying the past.

    Indeed, that’s why I wasn’t. I was levelling it at something I see in a fair few astrobiology discussions which seem to regard life as an endless series of catastrophes and mass extinctions until eventually intelligence. The history of life on Earth is not a Michael Bay movie.

  • william collins January 31, 2014, 0:35

    The irony for me is that our own planet has a number of examples of intelligent life – octopi, wolves, dolphins, elephants, even evolved dinos (parrots), Neanderthals – that have not led to advanced technological
    civilizations. I suppose that scenario plays out at least as much or more than the emergence of big-brained creatures who launch spaceships into the cosmos.

  • Alex Tolley January 31, 2014, 11:20

    @andy I see in a fair few astrobiology discussions which seem to regard life as an endless series of catastrophes and mass extinctions until eventually intelligence.

    I don’t see what you do. In some cases, the major dominant faunal changes subsequent to an extinction event are important. As smart mammals, we probably needed the KT event to emerge. There is little evidence that the dinosaurs would have evolved the intelligence and physical form that we think is necessary to build a technological civilization.

    But when thinking about the diversity of forms that inhabit a planet, then the extinction events represent temporary (so far) setbacks. But in each case, biodiversity keeps increasing until teh next. There has been no plateauing of biodiversity at least in terms of numbers of classified fossils.

  • Rob Henry January 31, 2014, 16:55

    @Alex Tolley,
    My bet would be on a far high proportion of extant dinosaurs being tool users than mammals – that is certainly what I see with my own eyes and on nature documentaries. What makes you so sure that pattern would reverse if it hadn’t been for the K/Pg catastrophe?

    @andy, I am also puzzled by why diversity is so much higher in the tropics. I even recall that recent work shows most temperate niche innovations to be due to migrations from the tropics. This complete mystery emphasises to me why we should concentrate on measures of biosphere productivity in these discussions, and leave wild guesses about diversity to a time when we finally have a few clues as to its actual mechanisms.

  • Rob Henry January 31, 2014, 17:31

    @jim moore, I think Alex’s formula is a little to simple.
    Your question was “”If your planet has a 2 g gravity. Can you get into orbit with chemical rockets?””

    This is an incomplete question from which to derive rocket velocity as it gives no mechanism that would allow us to scale. If your question had “ If your planet of EARTH’S RADIUS has a 2 g gravity.” then its velocity would need to be 41% higher. If it on the other hand you had planets of EQUAL DENSITY then 2 would be correct. However, I have trouble believing a terrestrial planet of the necessary eight Earth masses wouldn’t be denser due to compression factors.

  • andy January 31, 2014, 20:38

    @Alex Tolley: you still seem to be missing my point – you’re continuing to insist on viewing everything as a broad trend through time.

    My point is this: in the past life has been present on a planet with very different continental distributions, with different mean global temperatures, different atmospheric compositions, etc. So we can pick a time and figure out what was going on at that time. Not as part of a trend towards whatever the next catastrophe is, but in terms of being a thriving ecology at the time on a planet which had different properties to present day Earth.

    For example, we might want to ask whether a planet with more oxygen is more habitable? Fortunately it seems the Earth has experienced such a situation, so maybe we could try to understand how the late Carboniferous ecosystems were behaving across the planet at the time, e.g. what and where the major biomes were, rather than merely regarding it as biodiversity increasing until Olson’s extinction came along.

  • william collins February 1, 2014, 1:08

    @ Rob: The most intelligent descendants of extant theropods appear to be – parrots. Dinos ran around the globe for several hundred million years without developing the ability to make tools. Toolmaking technology is not an inevitable step in the evolutionary path – at least not on Earth.

  • Alex Tolley February 1, 2014, 1:17

    @andy – I think I understand where you are coming from. Remember, ecosystems are not static, but dynamic, changing because life pushes them to change towards climax ecosystems, and these in turn will be further driven by evolution over time.

    Earth’s oxygen levels were higher in the Carboniferous and this gave rise to really large arthropods. But with increasing oxygen comes increased risk of fire which reduces the free oxygen. The increased oxygen was due to the forests fixing carbon, reducing the CO2 levels, which then resulted in a cooling world (albeit the Permian climate was probably heavily influenced by volcanic activity).

    Evolution by natural selection seems to result in increasing numbers of forms, much like market systems result in ever more products to serve ever more narrower niches, maximizing profit opportunities.

  • Michael February 1, 2014, 17:39

    @Rob Henry January 31, 2014 at 16:55

    ‘I am also puzzled by why diversity is so much higher in the tropics. I even recall that recent work shows most temperate niche innovations to be due to migrations from the tropics. This complete mystery emphasises to me why we should concentrate on measures of biosphere productivity in these discussions, and leave wild guesses about diversity to a time when we finally have a few clues as to its actual mechanisms.’

    In the tropics there are less environmental extremes such as heat, water and sunlight for example that would tend towards selecting species that could cope with those stresses. In effect there is plenty for everyone so to speak. So on average the more stresses there are in an environment the greater pressures put on living organisms to cope with those multiple stresses. Therefore there should be less organisms capable of dealing with all of these stresses at the same time.

    @Alex, Rob Henry is correct we also need to know the radius.

    ‘This is an incomplete question from which to derive rocket velocity as it gives no mechanism that would allow us to scale. If your question had “ If your planet of EARTH’S RADIUS has a 2 g gravity.” then its velocity would need to be 41% higher. If it on the other hand you had planets of EQUAL DENSITY then 2 would be correct. However, I have trouble believing a terrestrial planet of the necessary eight Earth masses wouldn’t be denser due to compression factors.’

    The orbital and escape velocity are dependant on the ‘gravity’ at the ~surface and the radius from the centre of mass. A 2 g terrestrial planet can have different masses and sizes due to their compositions, look at Jupiter, it has only 2.5 g yet it is 318 times as massive as the earth with only ~5 times the escape velocity of the earth. Jupiter has a lower escape velocity in relation to is mass because the ~surface is further away from the centre of mass.

    Escape velocity = the SQR of (2 x gravity x radius)

    So if the gravity of a world doubled and the radius remained the same (density doubles) then there will be a ~41% increase in escape velocity.

  • Jay February 1, 2014, 20:20

    Although it’s likely that some world somewhere is more suitable for life’s development than Earth, human evolution was shaped by Earth’s specific properties. For example, on a planet with significantly higher gravity than Earth, people are likely to develop back, knee, and ankle problems. A planet with less gravity is likely to have a thinner atmosphere and possibly more surface UV, neither of which are particularly good for us. And of course we depend on a number of crops that are only proven to be viable on Earth’s environment (and usually each crop only grows well on a small fraction of Earth’s total land area). And the crops are dependent on bees, worms, and nitrogen-fixing bacteria, each of which has its own dependencies, and so forth.

    Long story short: for humans, Earth is likely the only place in the universe where we can survive long-term without enormous technical infrastructure.

  • Alex Tolley February 2, 2014, 0:59

    My error on the radius, and indeed the change in orbital velocity needed.
    With the assumption of 2x mass and the same radius, using the 4500 m/s figure for the best chemical rockets, the mass ratio is ~12, so that is achievable, all things being equal. If the radius has to increase as well to retain the same density, then the mass ratio is ~34 and that is not achievable [today].

    The calculations assume no air resistance and ballistic flight. In practice, the rocket would be fighting the doubled gravity and reducing the effective acceleration and final velocity. This would increase the realizable mass ratios compared to the theoretical minimum.

    So to answer jim moor’s question, it may indeed be possible to get a rocket into orbit on a 2g world, assuming the density increased to keep the radius the same size as Earth’s.

  • Michael February 2, 2014, 12:28

    @Alex,

    ‘If the radius has to increase as well to retain the same density, then the mass ratio is ~34 and that is not achievable [today].’

    If the radius increases to keep the density the same, no self-compression, there is a reduction in the escape velocity as you will now be further away from the centre of mass. Any rotation of the world will reduce the orbital velocity required, provided it is in the direction of rotation.

  • Rob Henry February 2, 2014, 19:21

    Michael’s last answer is kind of confusing. He says
    “If the radius increases to keep the density the same, no self-compression, there is a reduction in the escape velocity as you will now be further away from the centre of mass.  “

    Now the model in simple. Surface gravity is here directly proportional to radius. The energy to lift from that planet to infinity is the same as lifting a test mass by that planets radius at its surface gravity in a uniform gravitation field. Thus the lift energy goes up with the square of radius, and the escape velocity is directly proportional. The reason that this is possible is that the mass goes up even faster, obviously with the cube.

    Michael also says “Any rotation of the world will reduce the orbital velocity required, provided it is in the direction of rotation. “

    Here he alludes to it being easier for a more massive body to retain angular momentum from its formation.

  • Alex Tolley February 2, 2014, 23:16

    Michael
    I applied the equations for gravity and escape velocity. In this case, to get a 2g world, the mass increases 8x, while the radius increases 8^2/3

    Unless I have done the computations incorrectly, the escape velocity doubles. This was the value Rob Henry seems to have.

    There are of course many factors that this simple BoE calculation leaves out, which would have an impact on the rocket performance. Rotational velocity at the launch site is indeed one, albeit a relatively small one compared to orbital velocity. But arguably, including this, reduces effective g at the surface. If you have different numbers, please share them, or email me, as these sorts of calculations interest me.

  • Michael February 3, 2014, 15:34

    @Alex Tolley February 2, 2014 at 0:59

    density = M/V
    V = (4/3)(pi) x (r ^3)

    sub into

    g (m/s^2) = (G x M)/(r ^2)

    I get

    g (m/s^2) = (4/3) x (G) x (pi) x density x radius

    Correct me if I am wrong

    So the gravity will increase in proportion to the density and radius

    So if you double the gravity (and the density remains the same) the radius must ‘double’ and therefore the mass by 8 times.

    ‘Michael

    I applied the equations for gravity and escape velocity. In this case, to get a 2g world, the mass increases 8x…’ Yes

    ..while the radius increases 8^2/3′ No, it doubles

    using V (e) = SQR (2 x g x r)

    we still arrive at a 41% increase in escape velocity for a 2 g world as opposed to a 1 g one.