Ranking Exoplanet Habitability

by Paul Gilster on November 23, 2011

Our notions of habitability are built around environments like our own, which is why the search for planets with temperatures that support liquid water at the surface is such a lively enterprise. But as we saw yesterday, it is not beyond possibility that many places in our Solar System could have sub-surface oceans, even remote objects in the Kuiper Belt. And that raises the question of how we assess astrobiological environments, an issue studied by Dirk Schulze-Makuch (Washington State University) and Abel Mendez (University of Puerto Rico at Arecibo), working with an international team of researchers in a paper suggesting a new approach.

Schulze-Makuch makes the situation clear:

“Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star. For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are habitable in principle. Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life.”

To avoid overlooking potentially habitable worlds as we discover more and more exoplanets, the authors propose two indices that help to provide a quantitative look at a given exoplanet’s chances for habitability. The first is an Earth Similarity Index that screens exoplanets in relation to all the factors that make our planet hospitable to life. The second is a Planetary Habitability Index, which describes chemical and physical parameters that may allow life to exist under conditions that vary markedly from Earth. Think Enceladus, or Europa. Think the exomoon of a gas giant. Think, in other words, as speculatively as possible.

The Planetary Habitability Index is based on ‘the presence of a stable substrate, available energy, appropriate chemistry, and the potential for holding a liquid solvent,’ as the paper’s abstract notes. But it’s also based upon hypotheses about life’s viability in extreme environments that we’re as yet unable to test. Acknowledging this, the authors see their index as an ongoing work that can be updated as technology and knowledge about astrobiology advances. Interestingly enough, they apply their metrics to the provocative Gliese 581 system, finding that both GJ 581c and GJ 581d show an Earth Similarity Index comparable to that of Mars, and a Planetary Habitability Index somewhere between that of Europa and Enceladus.

Future space instrumentation should be able to tell us whether an Earth-class planet shows the signature of life, but how do we use those instruments to size up an icy exomoon when we can’t make the call on far closer worlds like Europa? What will change the game is finding proof in our own Solar System that life can occur in just this kind of extreme environment. Such a demonstration would make the Planetary Habitability Index far more interesting — and accurate — telling us that life can adapt to places utterly unlike our own planet. Until that occurs, constructing the PHI seems like an intriguing but premature exercise.

The paper is Davila et al., “A Two-Tiered Approach to Assessing the Habitability of Exoplanets,” accepted by Astrobiology (abstract).

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Ole Burde November 23, 2011 at 15:35

It would be nice to know more about exoplanet habitability in a general sort of way , but what I really would like to know , is a lot more about EXACTLY how this is going to happen , when Kepler has done its job .
Lots of universities and whatever-organisations seems to be engaged in finding more and more exoplanets , which is ofcourse a good thing, but where is the next REAL step forward , worthy of being a logic continuation of Keplers breakthrough ?
If there is no such FOCUSED continuation getting started NOW , we are going to loose a lot of time . As the apollo program showed ,loosing focus on what you want , can be a disaster , and it may take generations to reinvent the ability to focus on the real target again . If anybody should have any doubts , the real target is to identify a planet where humans can live more or less naturally .

andy November 23, 2011 at 15:47

Talking of habitable planets, another super-Earth located close to the HZ of its star appeared in an arXiv preprint today. Gliese 667 Cc, located around the tertiary of the Gliese 667 triple system has a minimum mass of 4 Earth masses and receives roughly 90% of Earth’s present-day insolation. (Orbital distance appears to have been mistyped in the current version of the preprint though, should be 0.12 AU not 0.28 AU…)

The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample

A. A. Jackson November 23, 2011 at 20:26

I always have conflicting thoughts about ‘zones of habitability.’ We have a single empirical example, us. We know the ingredients for that, a Goldilocks zone and some physical and organic chemistry. Then what becomes the elephant in the room? Evolutionary biology. This model of biology (notice I do not use the misunderstood term ‘theory’, model is a much better term). is a fearsome thing.

In the Permian-Triassic extinction event 96% of all marine species and 70% of terrestrial vertebrate species became extinct. It took anywhere from 4 to 6 to maybe 40 million years for Biotic recovery. Yet, why did it recover at all? Life has recovered from all the mass extinction events over the last 500
million years.

We were and are still , blessed with the Goldilocks zone… but is that all there is? Surely there must be astrophysical environments where the Black Box of evolutionary biology does not work. Yet, that damned Black Box, that inscrutable Blind Watch Maker, what is its range? How circumscribed can it be?

Look at environments here from deep ocean heat vents to Antarctica. As Copernicus taught us, every time we have conceived something unique about us humans we have been shown to be dead smooth wrong.

Dale McConnell November 23, 2011 at 22:15

Life, and the possibility of such, seems to be considered in such remarkably constrained terms these days.

Is it beyond us, carbon based, oxygen reliant, life forms to consider that other sentient beings might be considering similar quandaries as to what constitutes life, and the basis upon which it can exist?

We have, for decades, if not centuries, focused our attentions & imagination/scientific endeavours on the myriad possibilities of similar planets, systems & environments upon which our specific life type can exist.

If we have learnt nothing, we are but one chance computation of a successful combination of chemicals. Either that or we were lucky in the fact we were hit by a comet exhibiting the requisite components for our particular form of life on a rock exhibiting the requisite components to sustain it.

If the idea of a ‘life-form’ based on another complex molecule has not been ignored, it has surely been side-lined by the same community of thinking that is now suggesting that neutrinos, a relatively new realisation (scientifically speaking) , cannot possibly behave beyond the theory of a century old idea/ideal.

Interpretations of results differ. Without such, science would not progress. This is to be applauded.

Differing interpretations of similar results, validated by simply referring to previous conclusions, should be questioned. Such is the the progress of the human intellect, and therefore the scientific method.

Point in question, if not already plain, is the recent FTL neutrino experiments from CERN.

Though I offer no scientific argument to either party, the idea that the anomaly in the results is possible due to a lack of decay (based on a similar theory) of the particle, merely extenuates the point that current thinking is somewhat blinkered.

If point 1 (FTL) is possible and goes against current knowledge, then an argument against it, centred around (or loosely based upon) a similar theory, is surely flawed, due to being based upon the same (or similarly founded) theology.

In good conscience, we cannot deny a theory which disproves another based upon a sub-set (a theory borne out of the previous thinking) of that which it denies.

In conclusion, I ask that we, as a whole, neither dismiss, nor deny, that which we believe to be outside of physical lore and that which appears to be beyond our current realm of understanding.

Keep thinking.

Kind regards,

Dale McConnell

Michael Simmons November 24, 2011 at 1:31

My random thoughts:

Intelligent Life in a sub-surface ocean would have a lot of troubles advancing out of them. i.e. building radio and optical telescopes, rockets, computers and so on underwater.
When they break out of the ice they are directly confronted with the hostile vacuum of space and freezing temperatures.
They won’t have adapted the correct limbs etc for life outside of the water (thinking of dolphins here) and would need to re-evolve into a more suitably form.

Intelligent Life on super earth’s are going to find it very hard to start a space program due the the enormous amount of energy require to escape their gravity. Try building a large optical/radio telescope in that type of gravity.
How would our “put man on the moon” have gone in the 60′s had we been on a 4x earth mass planet?

I do believe methane based etc (titan) very cold life might possible, but its metabolic rate is going to be extremely slow and this will mean extremely slow evolution. Intelligent Life in these environments made not have had enough time to evolve since the big bang

I think different chemistry life is possible, but our life is vastly more common simply because it is made of the most common elements.

Re the infinite adaptability of life, our form of life had to wait until the atmosphere contain lots of O2 to power the metabolism required for active animal life. It had billions of years of a CO2 based atmosphere in which to solve the problem of a CO2 based complex animal life form and it failed completely. An this was on a planet where everything else was for the most part near perfect.
Realise that when you burn something, half of what’s burning is the O2 in the atmosphere. It’s a fuel source all around us and powers our metabolism.

Regardless of what places life might exist elsewhere, our environment is an absolute paradise for life that can evolve space faring intelligence.

Barring interstellar probes, our chances of finding and communicating with non-space faring intelligences is very low.
Looking for habitable planets that might harbour non-space faring intelligences might be ignite the imagination, but it won’t return any real tangible benefits.

Eniac November 24, 2011 at 15:27

Intelligent Life in a sub-surface ocean would have a lot of troubles advancing out of them. i.e. building radio and optical telescopes, rockets, computers and so on underwater.

Intelligent Life on super earth’s are going to find it very hard to start a space program due the the enormous amount of energy require to escape their gravity.

These are very good points. They have important bearing on the Fermi Paradox, which requires the life we are not seeing to become spacefaring, or at least “transmitting” in some way. On the current topic, though, about which planets support life, the question whether such life will become spacefaring or not is secondary.

I would caution against concluding prematurely that water is somehow an inferior environment for developing technology. That is true for our kind of technology, but there may be a large variety of technology we have not conceived of that is useful under water, and under water only. Most of our own early technology for a long time employed animals and plants, which can be done under water just as well as on land. One exception is fire, for which I cannot now come up with an underwater equivalent, maybe just because of a lack of imagination. Ocean currents may provide more energy under water than wind does on land, but it would be difficult to develop metallurgy without fire of some sort.

I think different chemistry life is possible, but our life is vastly more common simply because it is made of the most common elements.

On Earth, and presumably most other rocky planets, Silicon and other metals are far more common than carbon. According to this principle, then, life built on metals and oxygen should be more common. It seems that the natural chemistry of metals and oxygen is not suitable for spontaneous abiogenesis, although we can’t know for sure with silicon. It may happen with our help, one day, because mechanical life would be made of these elements.

Rob Henry November 24, 2011 at 19:06

If this is the first serious paper to decouple a habitability index from an Earth-like index, I predict that it will go down in history as one of the first steps of twenty first century science back towards the supremacy of the Copernican Principle.

How good a biosphere is the Earth in reality. To give just one hint that it is shockingly deficient in some respects, note that our oceans have much less cloud cover than our land and are not limited by the availability of water, yet they cannot even average 60% of the productivity of our land. This is because our mid oceans are deserts of mineral deficiency. Thus it is easy to imagine a planet with better hydrology on land and nutrient recycling at sea being half an order of magnitude more efficient, and I could imagine a full order of magnitude increase in productivity is possible.

Could Venus, before it lost its water, have actually been “better” than the Earth – even though we would also have to factor in the shorter period that it could remain in this state. Is hydrogen actually a better gas for intelligent life in the sea than oxygen (it is certainly less toxic). And how do me measure the suitability of extreme anomalies such as Io.

I know that most of you will think me mad, but I have become obsessed by my inability to rule out the prospects for life on Io. Electricity seems an almost comically ideal source of energy for life, yet it is delivered unto Io. For most planets or moons, high levels of ionising radiation would spell sterilisation, but here its high rate of flowing lava (a couple of orders of magnitude higher than Earth’s) just means that high energy chemicals are delivered into protected depths at rates that Europan’s could only dream of. Also, if there really is as much sulphur there as some think, then we have the perfect solvent for high temperature life. So what really are the prospects for life on Io?

amphiox November 24, 2011 at 23:31

Regardless of what places life might exist elsewhere, our environment is an absolute paradise for life that can evolve space faring intelligence.

Well earth seems nice and all, but there are any number of ways it could have been even nicer for space faring intelligence.

A smaller planet with a lower g would have made the space program even easier.

A thicker atmosphere may have made early aviation easier, providing a boost to technological advancement.

A double planet system with a habitable large moon would have provided a much stronger economic impetus for space colonization.

Indeed a red dwarf system with the planets bunched closer together, would also have provided a much stronger economic impetus for space colonization. Consider how close Gliese 581c and d are to one another, compared to Earth-Mars, and imagine a similar system with two even more habitable planets so closer to each other, and an intelligent civilization arising on one of them.

stephen November 25, 2011 at 11:35

Larry Niven had The Smoke Ring, in which a gas giant orbits a neutron star just outside its Roche limit. The gas giant’s atmosphere is pulled loose into an independent orbit, the Smoke Ring. Life evolves and thrives…it’s a neat idea….but how accurate and plausible is it? Thanks.

andy November 26, 2011 at 19:07

If this is the first serious paper to decouple a habitability index from an Earth-like index, I predict that it will go down in history as one of the first steps of twenty first century science back towards the supremacy of the Copernican Principle.

I think you seriously overestimate the importance of this. It isn’t like contemplating whether life can exist in non-Earthlike environments (for however you want to define “Earthlike”) hasn’t been done before, nor is it the case that things like, say, the methane analogue of the water zone haven’t been studied.

In essence all this is doing is representing a complex issue with an arbitrary ranking. This is not history-making at all.

bigdan201 November 27, 2011 at 1:39

I’ve always considered surface gravity to be an important consideration for habitability. But gravity follows an inverse square law based on distance. It could be that a super-earth would not have prohibitive gravity, given a large enough radius.

Rob Henry November 27, 2011 at 3:22

Andy, I believe that you missed that none before has seen the forest that has been obscured by the trees. It is a natural to contemplate that life is possible in unusual environments, but hard to spot the fact that we might be one of them.

spaceman November 28, 2011 at 9:16

I welcome the attempt at classification and clarification aimed at in this paper on ranking habitability.

Aphiox said:

“…imagine a similar system with two even more habitable planets so closer to each other, and an intelligent civilization arising on one of them.”

Indeed, I too have wondered about the implications of there being two life-bearing planets in the same solar system orbiting close enough to one another not to overly disturb the other’s orbit and close enough so that life from one planet could easily adapt to life on the other. I have also, like Amphiox, taken this line of thinking a step further by imaging that intelligent life develops on one the planets and develops the capability to first explore and then settle the other planet. Would this type of system be more conducive to the development of spaceflight than our own?

Lastly, I cannot underscore the importance of the discovery announced by the HARPS team and mentioned by Andy earlier in this thread. It now seems that we can be sure that although giant planets are significantly rarer around the small red stars whose numbers overhwhelmingly dominate the galaxy, smaller planets seem to be no less common around the M-dwarfs than they are around solar-type stars. Seems likely that the nearest solar system to our own has a red dwarf primary.

The Bobs November 28, 2011 at 18:46

To those of you who believe that there may be some non-carbon based life, please tell me about any large, complex molecules that are not carbon based. There aren’t any. There won’t be any in the future. The periodic table is what it is, there will be no changes to it, except for the meaningless addition of some synthetic elements of high atomic number.

Only silicon has any potential to form large molecules, but it is well known that it does not have the ability to form many of the types of bonds that are necessary for the complex chemistry of life. In order to add complexity to silicon, you have to add carbon and turn it into silicone.

Eniac November 29, 2011 at 0:00

@The Bobs: The prominence of carbon in the chemistry of macromolecules could be a consequence of rather than a cause of its use in biochemistry. The reason all naturally occurring macromolecules are carbon based is because they are biological, and biology is carbon based. We do not have examples of other large complex molecules, because there is no way to synthesize them. The claim that they cannot exist is myopic and, as far as I know, completely unsupported by hard evidence.

To those of you who believe that there may be some non-carbon based life, please tell me about any large, complex molecules that are not carbon based. There aren’t any.

This statement surely requires some qualification. There are polyoxometalates, phosphazenes, boranes, and sulphur compounds that can form macromolecules of unlimited size. Likely there are others. For some of these, water is not a good solvent, but then, water is not the only solvent around, either. The reason these macromolecules are not well known or understood is simply that they are hard to synthesize and there is no lifeform around to do it for us.

If we had ever only seen diamond, graphite, CO2 and carbonates, we would probably play with a few semi-complex organic molecules and polymers just as we do with the above and never dream of the rich organic chemistry that makes life possible.

See these links to read more about “large, complex molecules that are not carbon based”:

http://en.wikipedia.org/wiki/Polyoxometalate
http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry
http://en.wikipedia.org/wiki/Boron_nitride#Boron_nitride_nanotubes

The combination of boron and nitrogen is in many ways equivalent to carbon, they even form nanotubes.

And, of course, any compound that can form crystals can also form “large, complex molecules”. A crystal is really just a very large macromolecule, and could be used for richly varied complex chemistry if assembled atom by atom into desired shapes with strategically placed defects and dopants.

Ronald November 29, 2011 at 4:24

@spaceman: “Indeed, I too have wondered about the implications of there being two life-bearing planets in the same solar system orbiting close enough to one another not to overly disturb the other’s orbit and close enough so that life from one planet could easily adapt to life on the other. (…). Would this type of system be more conducive to the development of spaceflight than our own?”

Good examples of this among solar type stars are Zeta 1 and 2 Reticuli (resp. G2, G1) at about 6000 AU or 0.1 ly separation, and 16 Cygni A and B (resp. G1, G2) at about 840 AU separation.

The Bobs November 29, 2011 at 12:34

@Eniac, the examples you gave are large, but not complex. They have a simple, repeating structure.

You have not met the requirements I stated. You gave examples of macromolecules that have useful structural properties. I was clearly referring to complex chemistry, not complex structure.

andy November 29, 2011 at 14:19

Andy, I believe that you missed that none before has seen the forest that has been obscured by the trees. It is a natural to contemplate that life is possible in unusual environments, but hard to spot the fact that we might be one of them.

This has also been contemplated before. I recommend reading Evolving the Alien by Ian Stewart and Jack Cohen (published in the US as What Does a Martian Look Like? – presumably because the US market cannot handle the mention of evolution without a substantial proportion of people undergoing a Jeebus-inspired mental breakdown)…

I stand by my point that this paper is neither staggeringly new or original. Certainly not the paradigm-shifting revolution that you implied previously.

Rob Henry November 29, 2011 at 15:07

Eniac, you say “The combination of boron and nitrogen is in many ways equivalent to carbon”, but I fail to see how it is any thing but better. Carbon is exalted, not just for its ability to concatenate, but also for its ability to form multiple bonds. It seems to me that boron can form every sort of bond that carbon can, but it is the only element which can form such unusual Lewis acid base bonds, and it does it par excellence with nitrogen (as you alluded to).

Michael Simmons November 29, 2011 at 21:45

@Eniac
I said “I think different chemistry life is possible, but our life is vastly more common simply because it is made of the most common elements.”

I agree their is probably all kinds of exotic different chemistry life.
However
http://en.wikipedia.org/wiki/File:SolarSystemAbundances.png
Is there somewhere where boron is anywhere near as abundant as carbon.
Would these places be as common as those where carbon is abundant?
Ditto other possibilities?

Do the others support a high metabolic rate?
Do they have an equivalent for photosynthesis that would pump some other reactive gas into the atmosphere?

@amphiox
“A smaller planet with a lower g would have made the space program even easier.”
But might not support tectonic plates and hence a carbon cycle => CO2 rich atmosphere (which several billion years of evolutionary failure here indicates won’t support complex animal life forms)

“A thicker atmosphere may have made early aviation easier, providing a boost to technological advancement.”

Most of the O2 in the atmosphere comes from photosynthesis. A thick atmosphere might block that. And chances are it would have a lot of CO2 in it that would take a lot longer to remove and replace with O2.

“A double planet system with a habitable large moon would have provided a much stronger economic impetus for space colonization.”

Well yes.. but we don’t even know the odds of one earth like habitable planet, none the less two in the same system.

Places better than ours probably do exist, But I’m betting they are must more rarer than ours.

@Rob Henry
More potassium and other rare elements used by life would make earth a better planet.
I just think that we have done very well for ourselves.

Rob Henry November 30, 2011 at 5:16

Andy, what a wonderful book that was of Stewart and Cohen, though perhaps a little too exuberant. Even so, I read it twice.

How very right you are that it emphasised a belief that our type of life might exist in a universe fill of other types of life. Unfortunately it did not take that possibility seriously enough to propose any sort of objective test whereby we might test such speculation.

Eniac November 30, 2011 at 22:45

@Eniac, the examples you gave are large, but not complex. They have a simple, repeating structure.

The examples we know, yes, because otherwise they would be impossible to make. Do they have to be simple, or repeating, in principle? No, of course not. Which principle would that be? Polyethylene is simple and repeating, but that does not mean all carbon based structures have to be.

You have not met the requirements I stated. You gave examples of macromolecules that have useful structural properties. I was clearly referring to complex chemistry, not complex structure.

Reread your comment. You asked for “large, complex molecules”, not chemistry. And, of course, many elements have more complex chemistry than carbon. Metals are particularly notorious for their multiple valence states and complex electronic configurations. It is that same property that makes them essential parts of many of the most critical processes in biochemistry.

Eniac November 30, 2011 at 23:16

I agree their is probably all kinds of exotic different chemistry life.
However
http://en.wikipedia.org/wiki/File:SolarSystemAbundances.png
Is there somewhere where boron is anywhere near as abundant as carbon.
Would these places be as common as those where carbon is abundant?
Ditto other possibilities?

Do the others support a high metabolic rate?
Do they have an equivalent for photosynthesis that would pump some other reactive gas into the atmosphere?

Good questions, all. Boron is actually only 10-20 times less common than carbon on Earth, arguably enough for a sizable biosphere. Silicon is more abundant than carbon, as previously mentioned. About metabolic rate and photosynthesis we can not say much for lack of examples, but once you can form arbitrarily large molecules, you should be able to do pretty much the same things with any number of substrates. The key ingredients to processes like photosynthesis, respiration, and water splitting are in fact metals that transcend the C-N-O triumvirate and would likely be able to do the same jobs irrespective of the nature of the structural matrix in which they are embedded. We have iron, magnesium, manganese, zinc, copper, selenium, etc. all playing key roles wherever the going gets tough for less versatile ordinary organic compounds.

Ronald December 4, 2011 at 10:07

Paul,

this news just in (did not know where to put it):
“Caltech-Led Team of Astronomers Finds 18 New Planets, Discovery is the largest collection of confirmed planets around stars more massive than the sun”

http://media.caltech.edu/press_releases/13476

andy December 5, 2011 at 20:10

Good questions, all. Boron is actually only 10-20 times less common than carbon on Earth, arguably enough for a sizable biosphere.

Elemental abundance is not the only question: we also need to know where the boron ends up. Formation of various hydrocarbons and organic compounds seems to occur fairly readily in nature, do we have evidence that the right kind of boron-nitrogen molecules will form anything like as readily?

Eniac December 7, 2011 at 0:49

@Andy: You are, of course, correct. Let me point out here that I do not claim that any other chemistries are more likely to serve as the basis for life than carbon. Most certainly are not. What I do believe is that most are suitable in principle, and that we can’t be sure that there isn’t one with comparable likelihood to carbon, perhaps given a different environment.

Even that would not have led me to post on this well-argued subject, had it not been for the very categorical claim that carbon is the only element able to form large complex molecules. I am allergic to categorical claims, especially if I believe them to be false.

Ronald December 8, 2011 at 10:56

Latecomer to this interesting discussion, I would agree that the PHI is rather premature to say the least, if not even speculative, if not outright contradictory in terms;
First of all I would argue that the more valid and testable ESI is probably a smaller subset of the larger PHI.

Secondly:
“The Planetary Habitability Index is based (…) appropriate chemistry, and the potential for holding a liquid solvent,’ as the paper’s abstract notes. But it’s also based upon hypotheses about life’s viability in extreme environments that we’re as yet unable to test.”

Therein lies precisely its weakness and contradiction: there are certain (quite a few) things we have learned about the biochemical prerequisites for life (C, H2O, …). We define life and hbaitability on the basis of this and with good reasons.
Even if life could exist under totally different (biochemical) conditions as some here argue (Si-based, other solvent, …), this ispresently so (almost) totally unknown and speculative that it hardly seems to make any sense to base something as concrete as an *index* upon it.

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