Habitability is the measure of highest value in planet-hunting. But should it be?
Kepler and the other planet-finding missions have begun to bear fruit. We now know that most stars have planets, and that a surprising percentage will have Earth-sized worlds in their habitable zone–the region where things are not too hot and not too cold, where life can develop. Astronomers are justly fascinated by this region and what they can find there. We have the opportunity, in our lifetimes, to learn whether life exists outside our own solar system, and maybe even find out how common it is.
We have another opportunity, too–one less talked-about by astronomers but a common conversation among science fiction writers. For the first time in history, we may be able to identify worlds we could move to and live on.
As we think about this second possibility, it’s important to bear in mind that habitability and colonizability are not the same thing. Nobody seems to be doing this; I can’t find any term but habitability used to describe the exoplanets we’re finding. Whether a planet is habitable according to the current definition of the term has nothing to do with whether humans could settle there. So, the term applies to places that are vitally important for study; but it doesn’t necessarily apply to places we might want to go.Whether a planet is habitable according to the current definition of the term has nothing to do with whether humans could settle there.
To see the difference between habitability and colonizability, we can look at two very different planets: Gliese 581g and Alpha Centauri Bb. Neither of these is confirmed to exist, but we have enough data to be able to say a little about what they’re like if they do. Gliese 581g is a super-earth orbiting in the middle of its star’s habitable zone. This means liquid water could well form on its surface, which makes it a habitable world according to the current definition.
Centauri Bb, on the other hand, orbits very close to its star, and its surface temperature is likely high enough to render one half of it (it’s tidally locked to its sun, like our moon is to Earth) a magma sea. Alpha Centauri Bb is most definitely not habitable.
So Gliese 581g is habitable and Centauri Bb is not; but does this mean that 581g is more colonizable than Bb? Actually, no.
Because 581g is a super-earth, the gravity on its surface is going to be greater than Earth’s. Estimates vary, but the upper end of the range puts it at 1.7g. If you weigh 150 lbs on Earth, you’d weigh 255 lbs on 581g. This is with your current musculature; convert all your body fat to muscle and you might just be able to get around without having to use leg braces or a wheelchair. However, your cardiovascular system is going to be under a permanent strain on this world–and there’s no way to engineer your habitat to comfortably compensate.
On the other hand, Centauri Bb is about the same size as Earth. Its surface gravity is likely to be around the same. Since it’s tidally locked, half of its surface is indeed a lava hell–but the other hemisphere will be cooler, and potentially much cooler. I wouldn’t bet there’s any breathable atmosphere or open water there, but as a place to build sealed domes to live in, it’s not off the table.
Also consider that it’s easier to get stuff onto and off of the surface of Bb than the surface of a high-gravity super-earth. Add to that the very thick atmosphere that 581g is likely to have, and human subsistence on 581g–even if it’s a paradise for local life–is looking more and more awkward.
Doubtless 581g is a better candidate for life; but to me, Centauri Bb looks more colonizable.
A definition of colonizability
We’ve got a fairly good definition of what makes a planet habitable: stable temperatures suitable for the formation of liquid water. Is it possible to develop an equally satisfying (or more satisfying) definition of colonizability for a planet?
Yes–and here it is. Firstly, a colonizable world has to have an accessible surface. A super-earth with an incredibly thick atmosphere and a surface gravity of 3 or 4 gees just isn’t colonizable, however much life there may be on it.
Secondly, and more subtly, the right elements have to be accessible on the planet for it to be colonizable. This seems a bit puzzling at first, but what if Centauri Bb is the only planet in the Centauri system, and it has only trace elements of Nitrogen in its composition? It’s not going to matter how abundant everything else is. A planet like this–a star system like this–cannot support a colony of earthly life forms. Nitrogen is a critical component of biological life, at least our flavour of it.
In an article entitled “The Age of Substitutibility”, published in Science in 1978, H.E. Goeller and A.M. Weinberg proposed an artificial mineral they called Demandite. It comes in two forms. A molecule of industrial demandite would contain all the elements necessary for industrial manufacturing and construction, in the proportions that you’d get if you took, say, an average city and ground it up into a fine pulp. There’re about 20 elements in industrial demandite including carbon, iron, sodium, chlorine etc. Biological demandite, on the other hand, is made up almost entirely of just six elements: hydrogen, oxygen, carbon, nitrogen, phosphorus and sulfur. (If you ground up an entire ecosystem and looked at the proportions of these elements making it up, you could in fact find an existing molecule that has exactly the same proportions. It’s called cellulose.)
Thirdly, there must be a manageable flow of energy at the surface. The place can be hot or cold, but it has to be possible for us to move heat around. You can’t really do that at the surface of Venus, for instance; it’s 800 degrees everywhere on the ground so your air conditioning spends an insane amount of energy just overcoming this thermal inertia. Access to a gradient of temperature or energy is what makes physical work possible.
Obviously things like surface pressure, stellar intensity, distance from Earth etc. play big parts, but these are the main three factors that I can see. It should be instantly obvious that they have almost nothing to do with how far the planet is from its primary. There is no ‘colonizable zone’ similar to a ‘habitable zone’ around any given star. The judgment has to be made on a world by world basis.
Note that by this definition, Mars is marginally colonizable. Why? Not because of its temperature or low air pressure, but because it’s very low in Nitrogen, at least at the surface. The combination of Mars and Ceres may make a colonizable unit, if Ceres has a good supply of Nitrogen in its makeup–and this idea of combo environments being colonizable complicates the picture. We’re unlikely to be able to detect an object the size of Ceres around Alpha Centauri, so long-distance elimination of a system as a candidate for colonizability is going to be difficult. Conversely, if we can detect the presence of all the elements necessary for life and industry on a roughly Earth-sized planet, regardless of whether it’s in its star’s habitable zone, we may have a candidate for colonizability.
The colonizability of an accessible planet with a good temperature gradient can be rated according to how well its composition matches the compositions of industrial and biological demandite. We can get very precise with this scale, and we probably should. It, and not habitability, is the true measure of which worlds we might wish to visit.
To sum up, I’m proposing that we add a second measure to the existing scale of habitability when studying exoplanets. The habitability of a planet actually says nothing about how attractive it might be for us to visit. Colonizability is the missing metric for judging the value of planets around other stars.
This raises the ethical question of at which point do we as a race change the environment of an alien world’s biology in order to suit our needs?
Do we engage in biological genicide to seed a planet with Earth-life, or do we adapt ourselves to suit the exoplanet’s environment?
Or do we move on to another planet that is more “colonizable” as Schroeder suggests and totally build a habitat from scratch?
Hat tip to Centauri Dreams.
According to biologist and science-fiction author Peter Watts, the dumping of a hundred tons of iron sulphate into the ocean off the islands of Haida Gwaii is a double edge sword and it could be working.
And most of all, nobody really cares.
[...]Proximately, the gambit seems to have paid off: the resulting bloom covered ten thousand square kilometers and greatly exceeds the penny-ante impact of more “legitimate” experiments. Whether it will actually increase salmon yield remains an open question, but it seems a reasonable expectation; the project was inspired by a paper in Fisheries Oceanography which connected the dots between volcanic ash-fall, diatom blooms, and record salmon catches. As to the potential long-term carbon-sequestration impact, nobody knows.
In fact, not only does nobody know, nobody even seems to give a shit. They’re too busy pointing fingers. Discovery News regards Russ George, the entrepreneur behind the project, as a “Geoengineering nut“. David Suzuki decries the effort as “stupid”. Scientists and lawyers fill endless column inches with quotes about bad experimental design and the breaking of international treaties. The UN is gravely concerned, and has granted the Harper governmentan actual award (“The Dodo”) for its role in this fiasco; the Harper government, those champions of the environment, has in turn condemned the entire affair and is “investigating” (although their misgivings have been a bit muted by credible reports that they knew about the project in advance and did nothing to stop it, which makes them complicit).
For my part, I’m not going to argue those who point out that the project was poorly planned, that phytoplankton blooms are often toxic, and that even when they aren’t local eutrophication often leads to anoxic “dead zones”. (Iwill observe that some of these charges tend to cancel each other out: you can’t both buy into Jay Cullen’s complaint that strong eddy circulation compromises experimental design while at the same time worrying aboutAlyssa Danigelis‘s specter of neurotoxic dead zones.) I have no trouble believing that Russ George isn’t interested in anything other than turning a fast buck (although if there are laws on the book that make it illegal to profit from climate-mitigation research, you have to wonder if its author had ever spent more than two minutes observing human behavior).
In terms of environmental damage, however, I can’t help noticing that right around the corner from Haida Gwaii, the city of Victoria BC flushes the raw sewage of eighty thousand people directly into the ocean. I can’t help noticing a thousand-square kilometer dead zone off the Oregon coast, or the seventeen-thousand-square-kilometer dead zone in the Gulf of Mexico, or the continent-long daisy-chain of dead zones skipping merrily up the eastern seaboard. If I squint hard enough I can just barely keep myself from noticing the salmon farms along our coasts that not only generate their own local anoxic zones but which also spread disease, parasites, and bad genes to wild populations. (I trust I don’t have to remind you all of past and ongoing oil spills.) All of these impacts arise directly from human activity — and while few would claim to like any of these things, I find it curious that the one-time dumping of a load of nutrients into the open ocean would provoke such outrage while all these other, vastly more severe impacts get off with a shrug and a what-are-you-gonna-do?
The fact is, the Haida-Gwaii patch is vastly bigger than any similar project heretofore attempted. It’s way out intoHere There Be Dragons territory, and you know what? It’s a fucking data point.
Bad experimental design? Let me remind you of another badly-designed experiment: that time about a decade back when a bunch of religious fanatics ploughed into the World Trade Center to prove that their invisible sky fairy was tougher than ours. Those guys didn’t check their flight plans with the research community at all, but that didn’t stop the scientists from making some serious inroads into the impact of jet contrails on climate change. (Granted, that particular inroad turned out to be a dead end. That’s science for you.)
This is nature, damn it. It’s a complex metasystem, if you think it’s ever going to let you run a “controlled experiment” in the laboratory sense then I’ve got some voting machines in Ohio to sell you. If you make the perfect into the enemy of the potentially-adequate you’ll never stop running simulations, because there is no perfect. Meanwhile, outside the window, Nature’s rolling her own D20. One day she’s going to kick over that anthill you’ve been too chickenshit to poke at all this time, and then where you gonna be?
This plankton stuff is small potatoes anyway; you want something to get scared about, stop looking out to sea and look up instead. Climate change is hitting the poles and the tropics especially hard — and the tropics are just chock full of small poor countries already sinking, increasingly impatient as the so-called developedworld sits on its ass and mumbles oxymoronically about clean coal. I wouldn’t blame them in the least if they got tired of waiting and started their own stratospheric geoengineering program out of self-defense — and it would be kind of nice if we had a bit of real-world data on that front, too, before it happened.
Make as many caveats as you like. Be cautious in your extrapolations, by all means. Remember that correlation is not causation, keep alternative hypotheses firmly in mind, scrawl Nature Is Not A Petri Dish onto a piece of duct tape and stick it over the Far Side cartoons yellowing on the wall. Be Adaptive in your “Management”. But use the goddamned data you’ve got. Don’t piss and moan because someone without all your degrees, someone more interested in bucks than biology, went out and took the first step when you were too fucking timid. Do it better.
Forget the world at large; Russ George’s sins pale into insignificance even set next the city of Victoria. The difference is, we can learn from his.
We’ve already kicked the whole world off-balance. We’re running out of time to figure out which way it’s falling.
Whether one adheres to the concept of the Kardashev Scale of Civilizations or not, of which becoming a Class 1 depends on human beings being able to control all processes of the planet; environmental and energy-wise, it doesn’t matter because we already have started down that road according to Watts.
It depends on us now to balance out these forces before Nature itself will surely balance things out.
And leave us in the dust-bin of planetary history.
Global Warming, whether one considers it caused primarily by humans, or as a natural process determined by cyclical solar activity, is potentially a huge problem for the human race regardless of its cause.
One possible cure for GW is geoengineering. What is geoengineering you ask?
Well, read this post from The New Yorker:
Late in the afternoon on April 2, 1991, Mt. Pinatubo, a volcano on the Philippine island of Luzon, began to rumble with a series of the powerful steam explosions that typically precede an eruption. Pinatubo had been dormant for more than four centuries, and in the volcanological world the mountain had become little more than a footnote. The tremors continued in a steady crescendo for the next two months, until June 15th, when the mountain exploded with enough force to expel molten lava at the speed of six hundred miles an hour. The lava flooded a two-hundred-and-fifty-square-mile area, requiring the evacuation of two hundred thousand people.
Within hours, the plume of gas and ash had penetrated the stratosphere, eventually reaching an altitude of twenty-one miles. Three weeks later, an aerosol cloud had encircled the earth, and it remained for nearly two years. Twenty million metric tons of sulfur dioxide mixed with droplets of water, creating a kind of gaseous mirror, which reflected solar rays back into the sky. Throughout 1992 and 1993, the amount of sunlight that reached the surface of the earth was reduced by more than ten per cent.
The heavy industrial activity of the previous hundred years had caused the earth’s climate to warm by roughly three-quarters of a degree Celsius, helping to make the twentieth century the hottest in at least a thousand years. The eruption of Mt. Pinatubo, however, reduced global temperatures by nearly that much in a single year. It also disrupted patterns of precipitation throughout the planet. It is believed to have influenced events as varied as floods along the Mississippi River in 1993 and, later that year, the drought that devastated the African Sahel. Most people considered the eruption a calamity.
For geophysical scientists, though, Mt. Pinatubo provided the best model in at least a century to help us understand what might happen if humans attempted to ameliorate global warming by deliberately altering the climate of the earth.
For years, even to entertain the possibility of human intervention on such a scale—geoengineering, as the practice is known—has been denounced as hubris. Predicting long-term climatic behavior by using computer models has proved difficult, and the notion of fiddling with the planet’s climate based on the results generated by those models worries even scientists who are fully engaged in the research. “There will be no easy victories, but at some point we are going to have to take the facts seriously,’’ David Keith, a professor of engineering and public policy at Harvard and one of geoengineering’s most thoughtful supporters, told me. “Nonetheless,’’ he added, “it is hyperbolic to say this, but no less true: when you start to reflect light away from the planet, you can easily imagine a chain of events that would extinguish life on earth.”
There is only one reason to consider deploying a scheme with even a tiny chance of causing such a catastrophe: if the risks of not deploying it were clearly higher. No one is yet prepared to make such a calculation, but researchers are moving in that direction. To offer guidance, the Intergovernmental Panel on Climate Change (I.P.C.C.) has developed a series of scenarios on global warming. The cheeriest assessment predicts that by the end of the century the earth’s average temperature will rise between 1.1 and 2.9 degrees Celsius. A more pessimistic projection envisages a rise of between 2.4 and 6.4 degrees—far higher than at any time in recorded history. (There are nearly two degrees Fahrenheit in one degree Celsius. A rise of 2.4 to 6.4 degrees Celsius would equal 4.3 to 11.5 degrees Fahrenheit.) Until recently, climate scientists believed that a six-degree rise, the effects of which would be an undeniable disaster, was unlikely. But new data have changed the minds of many. Late last year, Fatih Birol, the chief economist for the International Energy Agency, said that current levels of consumption “put the world perfectly on track for a six-degree Celsius rise in temperature. . . . Everybody, even schoolchildren, knows this will have catastrophic implications for all of us.”
The human race might have no choice but to try geoengineering by the end of the 21st Century if the prognosis of a six degree Celsius rise in temperature holds true.
But if we are to become a true Kardashev Level One civilization, humans must have total control of the energy outputs of the planet.
And that includes the climate.
Hat tip to Boing Boing.