What Makes a Planet ‘Superhabitable’?

by Paul Gilster on January 27, 2014

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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{ 65 comments }

Alex Tolley February 3, 2014 at 18:38

@Michael
while the radius increases 8^2/3′ No, it doubles yes – my error due to haste.
So radius = 2x, mass = 8x. – makes sense as mass increases as r^3.
Escape velocity

Ve_earth = sqrt(2GM/r) [1g]
Ve_super = sqrt((8*2GM)/(2r)) [2g][8x mass, 2x radius)
= sqrt (4*2GM/r) = 2*sqrt(2GM/r)
Ve_super = 2*Ve_earth

Using your alternative formula:

Ve_earth = sqrt(2gr)
Ve_super = sqrt(2*2g*2r) [2x g, 2x r]
= sqrt(4 * 2gr)
= 2* sqrt(2gr)
Ve_super = 2*Ve_earth
———————————————————————

Michael February 4, 2014 at 14:09

I see, I should have doubled the radius at the same time as the mass went up x8, my mistake.

So there we have it, doubling the g doubles the escape velocity.

Eniac February 4, 2014 at 23:26

william collins:

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.

Not for a given individual species, that’s right. But for Earth as a whole, it very well may be. Many past failed attempts (or close calls, if you will) are no indication that eventual success is not likely (or inevitable, if you will). Quite the contrary: many close calls indicate persistence, and a high likelihood to make it over the barrier, eventually.

Rob Henry February 5, 2014 at 1:10

Michael says “doubling the g doubles the escape velocity”
I say, how about “doubling g doubles the PRODUCT of the escape velocity AND the root of the ratio of old to new compression factors”

That should make us all happy

Jay February 5, 2014 at 20:32

Eniac: Tool use is one thing, advanced industrial societies are another. An industrial species needs 1) brainpower, 2) decent manipulators, and 3) gregariousness. Not being aquatic probably helps, since fire and electricity are both hard to use underwater. There are probably a few more that I’m not thinking of right now. Importantly these traits are mostly independent; a big brain doesn’t help you grow hands or vice versa. The odds of a single species winning all three jackpots may be pretty low.

Eniac February 7, 2014 at 0:07

Importantly these traits are mostly independent; a big brain doesn’t help you grow hands or vice versa.

This is highly debatable. Gregariousness fosters a large brain, the main job of which is to deal with others of the same species.

The necessity of manipulators is highly overrated. Monkeys are far better equipped in this respect, yet it is us with the feet degenerated into walking pads (and no tails!) that made the cut.

The real hallmark of intelligence is information processing. As our species learned to transmit more and more information amongst themselves and to their descendents, a threshold was reached in which information was accumulated faster than it was forgotten or lost through death. That is the beginning of the human story, not some monkey grasping a bone to hit a pig over the head with. That was merely a beneficial side effect.

Alex Tolley February 7, 2014 at 10:46

@Eniac

The real hallmark of intelligence is information processing. As our species learned to transmit more and more information amongst themselves and to their descendents, a threshold was reached in which information was accumulated faster than it was forgotten or lost through death. That is the beginning of the human story, not some monkey grasping a bone to hit a pig over the head with.

The question is why that happened and is it a likely evolutionary step. Brains are expensive in energy, especially ours. One hypothesis is that cooking allowed our guts to be smaller, allowing us to grow larger brains. (Cooking may also have allowed us to reduce the large jaw muscles allowing our craniums to expand). So this requires fire (at least on Earth) unless there are a lot of hot springs or volcanoes around to heat food.
In terms of driving brain size, another hypothesis is that it is sex selection – females selecting males for traits that require large brains. Sex selection often picks a random trait. If so, then our large brains might just be chance. As we have become a cultural, and now also technological, species, selection is likely to favor smarter brains. However this is also controversial, as it appears that the brain/body ratio has peaked. This may be because we can offload some function to technology (e.g. memory is stored as books) and repurpose those functions.
Bottom line: we should be careful of “just so” stories that show us humans as the “peak of evolution” as a consequence of developing big brains through some “inevitable” evolutionary force.

Jay February 7, 2014 at 17:59

Eniac: Ants and bees are gregarious, yet possess hardly any brainpower at all. Squid, on the other hand, are rather clever yet solitary. Whales have both large brains and gregarious behavior, but no manipulators. I’d say there are at least three separate, distinct traits necessary to start a civilization.

Eniac February 8, 2014 at 0:04

Alex:

Brains are expensive in energy, especially ours. One hypothesis is that cooking allowed our guts to be smaller, allowing us to grow larger brains.

I don’t buy this. A lot of animals are burdened with heavier and less useful appendages, mostly from sexual selection. And they do not cook.

If so, then our large brains might just be chance.

All of evolution is chance. But the dice are thrown over and over again, which can lead to inexorable inevitability.

Bottom line: we should be careful of “just so” stories that show us humans as the “peak of evolution” as a consequence of developing big brains through some “inevitable” evolutionary force.

I am not sure what you mean by “Just so”. The hypothesis of inevitability is the opposite of “just so”, it is better described by “no matter how”.

And it is not right to speak of a peak. A peak needs two sides. All we know is we are on a rapidly rising curve, with no signs of a decline on the other side. From past experience, evolution does not have a peak. It is an ever accelerating march forward. In fact, evolution by chance and selection is just now being replaced by intelligent design, which is many orders of magnitudes faster, and even more directed. Among the many things made irrelevant by that is the brain to body size ratio of our biological bodies, or the brain’s demand for energy (which is quite puny, actually, less than a laptop, I would guess).

Eniac February 8, 2014 at 0:09

Ants and bees are gregarious, yet possess hardly any brainpower at all. Squid, on the other hand, are rather clever yet solitary. Whales have both large brains and gregarious behavior, but no manipulators. I’d say there are at least three separate, distinct traits necessary to start a civilization.

Yes, there are many independent examples of each of these three traits. Even in combination, as you point out in the example of whales. It is really only a matter of time until all three show up in the same species. As has indeed happened, eventually.

Eniac February 8, 2014 at 0:43

Alex: In case you have doubts about why higher intelligence is an enormous evolutionary advantage, go to the zoo and check who is inside the cage and who is outside. Or check who is eating who. Or who is bulldozing who’s habitat. Clearly, we have gained enormous advantage from our brains, totally out of proportion to the expense of keeping them. Nevertheless, what seems like a unique upheaval to us, in the geological view, is merely another one of these threshold events that have happened countless times on Earth. No more or less astounding than the advent of polypeptides, DNA, photosynthesis, the Eukaryotic large genome, sexual reproduction, etc., etc.

In my view, all of these were dams to be broken by a rising flood, not rare chance events that were unlikely to happen and without which life would have stagnated somehow. Evolution and stagnation are fundamentally incompatible.

Alex Tolley February 8, 2014 at 12:18

@Eniac

Brains are expensive in energy, especially ours. One hypothesis is that cooking allowed our guts to be smaller, allowing us to grow larger brains.

I don’t buy this. A lot of animals are burdened with heavier and less useful appendages, mostly from sexual selection. And they do not cook.
Wrong end of the stick. The issue is energy budget. More energy costly physiology requires a balancing advantage for selection to bear the cost. One method to reduce costs is to reduce other traits. In our case, cooking allows us to extract for useful nutrients and energy from food using a smaller gut. Given how energy expensive our brains are, I agree that gut reduction is more than a small part of the adaptation.

If so, then our large brains might just be chance

All of evolution is chance. But the dice are thrown over and over again, which can lead to inexorable inevitability.
I totally disagree. When you say inexorable inevitability I hear the fallacious argument about evolutionary direction to intelligence. It just is not so.

I am not sure what you mean by “Just so”. I mean post hocarguments. They tend to be quite common in evolutionary psychology. One has to avoid teh general proposition that because you observe A, that A must have an evolutionary advantage, and therefore concoct argument C to explain it.

And it is not right to speak of a peak. A peak needs two sides. Perhaps a poor choice of word, although it is usually used in the context of reaching a local or even global fitness peak in the adaptive landscape. In this case, the tendency is to see humans at some top of the evolutionary fitness landscape. Indeed your comments seem to confirm that view in terms of our power to dominate the world. But just remember, when we are gone (within 1 million years), the insects will still be around, as will the bacteria and a host of other “lowlier” organisms.

Among the many things made irrelevant by that is the brain to body size ratio of our biological bodies, or the brain’s demand for energy (which is quite puny, actually, less than a laptop, I would guess).

From Wikipedia: Although the human brain represents only 2% of the body weight, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucose utilization.
Human basal metabolic rate is around 100 W (it depends on body mass). Therefore the brain is using 20W. That is a high cost in terms of supporting the brain. [A Macbook Pro sucks between 43W (idle) and 205W (max CPU usage). An iPad sips power at just a few watts.]

Even for just mammals, our encephalization quotient is anomalous – 3x that of our close cousins, the Chimpanzee. Evolution has rolled the dice for 0.5 billion years since the Cambrian explosion, and only once has any organism attained such an EQ. If life inevitably throws up this feature, then we are back to considering the Fermi paradox.

Eniac February 9, 2014 at 0:44

Alex: You give a lot of good arguments. However, inevitability of intelligence is not the same as “direction to intelligence”. As I have said, Intelligence is but one of many breakthroughs that have waited billions of years to be stumbled upon by evolution. You could hold up each of them as an unlikely chance event, with no more or less validity. It is by throwing the dice billions of times that they all eventually happened.

Evolution has rolled the dice for 0.5 billion years since the Cambrian explosion, and only once has any organism attained such an EQ.

Only once? Breakthroughs happen only once by nature, they are irreversible and change the rules. The Earth is never again going to be left to insects or bacteria. Whatever we are currently wreaking is as likely to ever go away as photosynthesis. Or life itself, for that matter.

If life inevitably throws up this feature, then we are back to considering the Fermi paradox.

So be it.

Except, it is not really a paradox. It would be a paradox if there wasn’t one breakthrough that happened without evolution to drive it, the one that could most easily be a real rare chance event: abiogenesis, the origin of evolution itself.

Eniac February 10, 2014 at 22:42

About the EQ: Note that, apparently, bottlenose dolphins have reached a level more than half ours, independently. Can it still be an oddity if there are at least two instances of it, so close together in geological time?

Other perceived breakthroughs also happened more than once, despite not having done so for billions of years before. There are, I think, thirteen or fourteen documented instances of multicellularity arising, independently. There are at least two independently evolved mechanisms to convert light to chemical energy: The reaction center and the rhodopsin family of proteins. There are two very important instances of symbiotic endocytosis: mitochondria and chloroplasts. The eye is known to have evolved multiple times independently.

If I was a better biologist I could go on and on ….

Let’s face it, evolution encourages and preserves higher complexity, because it allows organisms to do more with less. This results in an overall up-trend, making more and more complex breakthroughs achievable, with time. Inevitably.

Ronald February 11, 2014 at 13:05

I am a late-comer in this thread due to a busy schedule lately, but:

Truly fascinating topic that I have often dwelled upon myself, maybe a realm on the edge of science and science fiction.

Of course, (super)habitability and the meaning of being ‘more habitable’ has to be clearly defined and quantifiable, otherwise it has no more meaning than ‘more human’ or ‘more alive’. I can understand Astronists’s indignation.

I understand from the post and the referred article that (greater) habitability, besides the well-known definition of liquid surface water, also refers to the potential duration of life on planets, or its habitable lifespan. And the factors that control this. I agree, also because this can be quantifiable.

I can understand Eniac’s argumentation against the earth being only marginal on the basis of probability distribution. However, consider the following;

Our planet Earth is far from optimal for long-term life and so is the Sun-Earth combination:
- Earth is alarmingly close to the inner edge of our HZ and will probably leave it (at least for higher life) in about 500 million years. This means that the entire window of opportunity for higher life, from the Cambrian diversification to its future end will have been only about 1 gy, or about 10% of the entire lifespan of our Sun.
- As we move up the stellar spectral scale toward larger, hotter stars, the (habitable) lifespan and this window of opportunity for (higher) life decreases rapidly, so I guesstimate that the hottest stars suitable for planets with higher life are around spectral types F9/G0.
- However, there is plenty of room toward the cooler end of the spectrum, with habitable stellar lifespans increasing spectacularly, before issues like tidal locking (and possible stellar flares) become an issue, at least to K2 or beyond. If quantifiable habitability factors together form a normal distribution, this would imply that the optimum would be somewhere around G5/6, or maybe even toward the later G/early K.
- And then there are other factors determining habitability such as planet size (partic. with regard to duration of plate tectonics an geological lifespan), the extent and depth of the oceans, biological biomass and productivity, etc. etc.
All these factors could easily be influenced very positively by starting out with a slightly different star and/or planet.

I agree with Alex Tolley (and Lionel) that the optimal would probably be a longer-lived stable solar type star (from G5 – K0), a slightly larger terrestrial planet with a slightly denser atmosphere, relatively more land (50/50?) distributed among various large continents, and an equitable moderately warm wet climate. And very few major disruptions that lead to mass extinction, the great eraser.

We seem like inhabitants of a small desert oasis that believe that their home is optimal, not on the basis of scientific analysis, but simply because they do not know otherwise and are not aware of tropical rainforests and coral reefs.

But first of all we need to clearly and unambiguously define and quantify (degree of) habitability. I can presently only think of a few quantifiable parameters, foremost:
- Habitable lifespan, both for all life and higher life. The longer and more stable, the greater the window of opportunity for life on a planet. This depends on the stellar type and planetary characteristics (see following). I would consider this one of the most important parameters, if not thé.
- Planet size, though bigger is not always better, there is probably an optimum range.
- Average temperature and temperature fluctuations.
- Amount of water: again, there may be an optimum range.
- Atmospheric density and composition: again, there is probably an optimum range.
- Biodiversity: number of species, etc.
- Biomass and biological productivity.
- …?

A nice thought closely related to this topic is that there may be many (potentially) habitable, even ‘superhabitable’ planets available out there with no life yet or only very primitive life. Ready for the picking and, where necessary (additional) terraforming.

BTW, if Alph Cen A and B are indeed at least 4.85 gy old or older, this would mean that Alph Cen A has increased in brightness significantly (as indeed seems to be the case at some 1.54 * solar lum.), so that any planets that were in the HZ long enough to be life-bearing are probably no longer in it. But this is no issue for Alph Cen B.

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