Yesterday I mentioned in passing the concept of super-habitability. I’m happy to say that ‘Handbook For Space Pioneers‘ introduced this concept, although it may have passed unnoticed. In this book, which describes the eight habitable planets available for human settlement in 2376 CETC (Common Era Terrestrial Calendar), the final planet, Athena, 77.6 light years from Earth in the HR 7345 system, is actually more habitable than Earth. The book rates the different planets according to their habitability using the ‘Von Roenstadt Habitability Factor’, with the least habitable, Mammon, having one of 0.79. This is a largely desert planet with a thin atmosphere which was largely settled due to its rare earth metals, which are useful in matter-antimatter reactors. Earth has a factor of 1.0, and Athena one of 1.09. This is because Athena is not only similar to Earth but lacks a polar continent, meaning that more of its land is hospitable to human settlement. Therefore it was this book that came up with the idea. Ahead of its time.
The search for exoplanets tends to be biassed in various ways as I’ve previously mentioned. One of these is that the ideal planet to be detected would be a super-Jupiter orbiting very close to its rather dim star, orbiting parallel to the line of sight. This is indeed a very common type of planet to be found but not at all Earth-like. It also gets a bit depressing after a while because the planets concerned may make their systems unsuitable for more hospitable planets further out. This situation has been somewhat remedied more recently and many somewhat Earth-like planets have now been found.
However, it’s been suggested that we may be setting the bar too low here. I mentioned the Copernican principle yesterday, also known as the principle of mediocrity, that it’s often informative to look at the world with the assumption that there’s nothing special about us as a species, and perhaps more closely, apply the same principle to ourselves as individuals (e.g. “accidents always happen to someone else until they happen to you” – this is not some kind of exaggerated humility and self-effacement). When we look for Earth-like planets, we are in a sense only looking for worlds which are good enough, and also good enough for humans, as opposed to worlds which are fantastic. That is, Earth is all very well, and yes it is precious and rare and all the other stuff, but there could in principle be worlds which are even more hospitable to life than this one. Super-habitable planets, in other words.
I feel a sense of hesitancy here because it’s like insulting my own mother, but the fact is that this planet may be particularly suitable for humans, but it could have, and has been in the past, more comfortable for life than it was when we first evolved as anatomically modern humans. At that point, it was plagued by regular ice ages, had larger deserts and had little in the way of continental shelves, meaning that fishing, for example, would’ve been very difficult without boats in most places. Not that I care, as a vegan, but there is a theory that I accept that we went through an amphibious phase when we depended on sea food, which could be relevant to the development of the our brains and ability to speak. Even now there are issues with this planet regardless of the influence of humans on it. Looking beyond humanity confronts us with the fact that although this planet is really quite a nice place to live, it may not be ideal for other life. There are two aspects to this as well. One is “life as we know it”. That is, life which breathes oxygen, drinks water, uses sunlight to synthesise food, is carbon-based and depends on biochemistry, which may not be the only kind of life. We don’t know that there cannot be other forms of life which are very different, to which Earth would seem a very hostile place. For instance, it’s easy to imagine aliens detecting the high level of corrosive and hyper-reactive free oxygen in our atmosphere and concluding that this is not the best abode for life because they have simply never encountered body chemistry which actually requires that. They might feel the same way about this place as we would about a planet whose atmosphere was high in elemental fluorine or chlorine and had seas of aquæous hydrofluoric or hydrochloric acid. Or, if there were life forms based on plasma, and I strongly suspect this can happen, they might wish to avoid anywhere with even a trace of liquid water in its atmosphere or on its surface and only survive in the very driest deserts here. But the current popular conception of superhabitability is to be biocentric in the sense of looking for the ideal conditions for life to begin, develop and thrive on it. Perhaps surprisingly, Earth is not currently ideal for this, and it isn’t even entirely due to human technological development. Moreover, Earth has been more habitable in the past that it currently is.
Seventy-one percent of Earth’s surface is underwater, and of that most of it is so deeply so that no daylight ever reaches the ocean bed. This doesn’t rule life out at all of course – there is life in the deepest part of the ocean – but it does reduce the energy available and there is less diversity down there than elsewhere. It relies on food raining down from further up, sometimes including dead whales, which provide a huge amount of nutrition for quite some time. The water column above the beds is also rich in life, getting richer the closer to the surface it gets, and there are of course ecosystems around hot water vents at the bottom of the ocean. However, the euphotic zone, where there’s enough light for photosynthesis to take place, is only two hundred metres deep, and that’s where the ocean teems with life. Shallow seas are much richer in life and more diverse for that reason. Hence, withough being disrespectful of the ecosystems of the trenches, abysses and abyssal plains, more than half of the surface of this planet could be said to be “wasted”. Two ways in which things could be different here are for the seas to be shallower. The shallow sea biome is quite rare nowadays on Earth, but in the past has been much more extensive, and would mean that plants could grow on the bottom of the sea and support many other organisms. The same could apply to another possible situation, where the Sun was either brighter or we were closer to it, meaning that although there might still be extensive abyssal plains, the light would penetrate further down. Alternatively, photosynthesis, which currently operates using red light, could possibly be based on shorter wavelengths which penetrate further. A planet with a red photosynthetic pigment, for example, might have seaweed growing deeper. It’s possible that chlorophyll only appeared on this planet because a preceding purple pigment used by another taxon of organisms meant that only red light was available to plants, and if that second stage hadn’t happened, “plants” would now mainly be purple.
Stars are also different colours. From the viewpoint of photosynthesis, this could also have implications for the colour of the pigments. Habitable planets were long thought only to be possible for worlds associated with F-, G- or K-type stars but it’s now thought that red dwarfs may be more suitable (for a summary of spectral types, look here). K-type stars last longer on the Main Sequence and are orange, so I presume a planet orbiting one would be in a constant “rosy-fingered dawn” situation, though much brighter, but this gives life longer to evolve than it does here. Our own planet will be able to support microörganisms for up to 2 800 million years in the future, but these will to some extent resemble the æons when only microbes existed here. Large multicellular life is likely to become extinct only about 800 million years hence when there won’t be enough carbon dioxide in the atmosphere to support photosynthesis, which will happen due to the increasing brightness of the Sun. This is also driven by plate tectonics, as carbonate rocks are forced from the sea bed into the mantle where they react and release carbon dioxide while forming silicates. This means that a fainter, more long-lived star or more active continental drift would make this planet more hospitable to life for longer.
A larger planet would often have a number of advantages for life. Here I’m talking about rocky planets rather than gas giants. This is where the biocentric approach becomes more evident. The definition of a “superhabitable” planet here is to do with how suitable it would be for life of the kind we’re currently familiar with rather than humans, because large, dense planets would have higher gravity than Earth. LHS 1140b, for example, may be superhabitable, but not for us. It’s seven times the mass of Earth and 40% larger in diameter, giving it a surface gravity more than three times ours. This is beyond the capacity of human beings to survive unaided without some form of modification, but its dense atmosphere may provide it with a greenhouse effect which heats it to a temperature compatible with life as found here. Larger rocky planets will take longer to cool, meaning that there will be more active plate tectonics and more carbon dioxide recycling, although there may be a limit here because higher gravity would slow it beyond a certain point due to the weight of the continental plates. Larger planets, particularly fast-rotating ones, would have stronger magnetic fields and therefore be more easily able to hang on to their atmospheres and protect the surface from ionising radiation.
When Earth has been warmer, there has been more biodiversity, although just to comment on anthropogenic climate change that doesn’t work out now because it’s a rapid change and also unstable. Evolution might eventually fill in the gaps of course. This planet has a tendency to edge into colder conditions rather than hotter ones, for instance the current spate of ice ages and the Snowball Earth scenario in the late Cryptozoic Eon, but most of the time it’s been hotter on average than it is today. Consequently we’d’ve done better if we were somewhat closer to the Sun, although it’s not clear how much, at least to me. Another circumstance where there’s been more diodiversity and larger animals has been when oxygen was higher in the atmosphere, although this has a limit above which there would be devastating fires and other oxidative damage. The highest partial pressure of oxygen ever on this planet has been 350 millibars, amounting to 35% of the atmosphere, but in a denser atmosphere this proportion would be smaller because the absolute quantity of oxygen is what matters, not the proportion.
The final helpful criterion is a combination of about the same water cover combined with a larger number of landmasses. Earth currently has six continents, one of which is circumpolar. At other times it’s had as few as one. I’m not sure why this is considered advantageous but I can make a few guesses. It would mean that evolution would take place in isolation on each of the continents, as it has here with Australia, South America and the rest of the non-polar continents taken together, and that arid areas would tend to be smaller, although rain shadow deserts would still exist. Those, however, would be less common as well because there would be fewer mountain ranges due to fewer continents crashing together. There would also be more land near the coast than there currently is here, and more shallow sea areas on continental shelves. I’m reminded of Douglas Adams and his description of Ursa Minor Beta as consisting largely of beaches.
Some of these characteristics are impossible to detect with existing technology. For instance, although I think there is a way of finding continents and mapping them crudely by measuring fluctuations in brightness and colour, only a very few planets have even been imaged as points of light so far and they’re much larger than Earth. There’s probably a way of coördinating and processing telescope images taken in different parts of the Solar System to simulate the effect of an enormously magnifying lens, but I’m just guessing there. Nonetheless, there are certain known planets which do appear to satisfy some of these conditions. One is Kepler-442b. Incidentally, many of these planets have rather boring, monotonous names at present because they were all found by the same project. This planet, also known as KOI-4742.01 orbits its K-type sun at a distance of 61 200 000 kilometres once every 112.3 days, has a mass 2.3 times Earth’s and a diameter of around 17 100 kilometres. This gives it a surface gravity twenty-eight percent higher than ours, which is just about bearable. Without other factors being involved, its surface temperature would be about -40, but it could easily be warmer due to the Greenhouse Effect, low albedo and so forth. This last factor is of course the way in which it isn’t super-habitable, if it’s so.
It’s possible that there are more super-habitable planets than strictly Earth-like ones because there are more orange dwarfs than yellow dwarfs. Nine percent of stars in the Galaxy are of this kind as opposed to seven percent of the Sun’s spectral type, and several of the criteria simply follow from a planet being more massive than Earth. For instance, higher gravity could make the oceans shallower and give the planets more tectonic activity, and as a personal interjection continents on larger planets have more chance of being widely separated and having different evolutionary histories. A planet 1.4 times the diameter of the Earth whose surface is 29% covered in land, i.e. proportionately the same as Earth’s at the moment, has almost twice as much land, which if the average continent size were the same would be represented by a dozen continents. Likewise, we currently have five oceans here but there could be more there, although oceans are not well-defined and in a sense there is only one ocean. Planets with more than fifty percent land coverage could have landlocked oceans, and even worlds with less land due to quirks of geography, but this won’t apply to planets of this kind.
What might such a planet be like? Here I’m going to choose an example which satisfies all the criteria. There will be many others, if they exist at all that is. The average temperature would be somewhat higher than Earth’s and there would be no permanent ice caps at the poles, although snow-covered mountains and glaciers could easily exist. The climate would be warm and humid, with more cloud cover than Earth. Tropical rain forest vegetation would cover much of the land and there would be smaller desert areas. The oceans would be shallower and there would be copious vegetation on the sea beds. Atmospheric oxygen content would be higher in absolute terms but lower in terms of percentage than Earth’s. The denser atmosphere would mean stronger winds and more devastating rotary storms such as tornados and hurricanes. Also, these planets are likely to be more similar to each other than strictly Earth-like planets would be. The general picture is a little similar to the way Earth was at the climax of the age of dinosaurs or during the Carboniferous, although the higher gravity would limit the size of terrestrial megafauna.
These larger planets have a kind of momentum to them, which allows them to continue with fairly stable climatic conditions for many æons. By contrast, Earth-like planets run the risk of becoming permanently ice-covered, tipping into runaway Greenhouse effects and ending up like Venus, losing much of their atmospheres or relying more on large moons to maintain their magnetic fields, and therefore perhaps the ones that don’t have them or something else to raise internal tides only have thin atmospheres and no land life.
However, this does raise another question in my mind. As far as we know, we are the first technological civilisation to arise on this planet, although there’s tantalising evidence that there may have been advanced industrial processes here during the Eocene, maybe not from terrestrial species. If that’s so, it means that the periods of time during which Earth was more hospitable to life than it is today didn’t give rise to tool-using species. Therefore, is it possible that a more hospitable habitat is like the Swiss with their cuckoo clocks? Would stability end up providing little challenge to species and prevent it from evolving humanoid intelligence? Are these planets too comfortable? And there’s another question. Although these planets seem stable, they would be nearer a set of limits for biological habitability. Our planet has issues, but maybe that’s a sign of it being able to endure change. If something happened to one of these planets, like a large asteroid impact or a change in the luminosity of the sun, would this proce too much for life to handle? For instance, on this planet the life could be replenished by that surrounding undersea vents even if it died elsewhere, but I’m not sure whether there’d be many extremophiles – organisms who prefer hostile conditions – on such a world. Also, these planets are superhabitable by the needs of life as we know it in general, but not by human standards. We’d do okay, probably, on a planet which was somewhat larger and warmer than Earth although we’d have to be wary of the storms, but these super-habitable worlds wouldn’t be pleasant places to live for us. There’s a potential second set of hypothetical planets which are super-habitable for us, for instance with the Galactic Association example of Athena, a planet with no polar continent. Some of the characteristics would be similar, such as smaller but more numerous continents, but the set of criteria is not the same.
Taking this the other way is even more speculative because we’re only aware of one example of how life can arise and evolve, which is based on chemistry, organic matter, water and to a lesser extent oxygen for respiration. If there are other ways for life to exist, there could be whole other sets of “habitable” environments – I hesitate to say “planets” here because for all I know this would be happening on the surfaces of neutron stars, in nebulæ or the photospheres of “ordinary” stars. This is rather more difficult to discuss, but if life can exist in different ways it would seem to multiply the probability of life in the Universe, and also of the types of world which could support it. But I can only really leave this as a possibility thus far, or at least without making this post really long.
To conclude then, all this looks rather cheerful as regards the possibility of life as we know it in the Universe, but it also kind of pushes us Earthlings into a corner, as if all of this is true, we’re quite atypical of life in the Universe and our planet’s a bit weird. So who knows?