08 April 2015

Lenton & Watson: predictions regarding life elsewhere

I found the following, slightly edited excerpt from Lenton & Watson's Revolutions that Shaped the Earth (Oxford, 2010), so interesting that I post it despite its length in hope that some few others might also find it compelling. I added a couple of notes in italics inside square brackets.

6.7 Predictions regarding life elsewhere

In this section we make some predictions from the assumptions of the anthropic model, concerning unknowns about life elsewhere. These are interesting in themselves, and since they are open to testing in the future as our knowledge increases, they could help establish or disprove the anthropic view of Earth history which we have been exploring.

So let us now review those fundamental assumptions and what they imply. The main one is that the pace of evolution on Earth, to ourselves, as complex, intelligent observers, has been constrained by the necessity to pass through a small number of intrinsically very unlikely events. These are sufficiently improbable that a priori, they would not have been expected to all occur during the limited time that the Earth will be inhabitable. However, on Earth, by lucky chance, they occurred considerably more quickly than was to be expected, which is how we came to be here and are able to ask these questions.

This idea has implications for how much life, of what kind, and around what kind of stars, we ought to expect to find in our part of the universe. These implications have some hope of being tested, because over the next few decades, astronomers and the space agencies will be putting huge effort into discovering and characterizing extra-solar planets, using telescopes both at the Earth's surface and in space. Ultimately, the goal will be to spectroscopically analyze the atmospheres of any planets that are found, looking for the raw materials we know to be necessary for life, such as water and carbon dioxide. But the investigators will also be hoping to detect ‘bio-signatures’ — in particular, ozone. This is thought to be diagnostic for oxygen, which itself is difficult to detect in a planetary atmospheres, since it does not exhibit visible or infrared absorption lines. Ozone however, is comparatively easy to detect and is expected to be present only when there is also significant oxygen.

The planet-finding missions of this century will build on ideas going back to the middle of the last one. Jim Lovelock was the first to suggest that analysis of planetary atmospheres could be used to diagnose the presence of life, an idea he developed with the philosopher Dian Hitchcock. They pointed out that the best bio-signature is not just one gas, but the presence of two that are in strong chemical disequilibrium with one another. They suggested that if you trained an infrared telescope on the Earth, you would be able to detect the simultaneous presence of ozone (hence oxygen) and methane in the atmosphere. Since methane and oxygen react with one another rapidly as carbon dioxide and water, you would be able to deduce that something must be producing them from their reaction products with equal rapidity, and this something must be life. (Actually, it's not just life, but [oxygenating] photosynthesis, that you would have diagnosed from the measurement. Some twenty-five years later, in one of his last papers, Carl Sagan and colleagues demonstrated that the technique works for the Earth, using the instruments on the Galileo spacecraft looking back to Earth while on its way to Jupiter.

One way or another, in the first half of the twenty-first century, we are going to get lots of evidence that bears on the habitability of nearby solar systems. There are some 250 star systems within thirty light years of Earth. Let's be optimistic and assume that we find many of them have planets in their habitable zones. What does the anthropic theory suggest we should find when we examine them with these planet-finder missions?

We suspect prokaryote life is not so common as to always arise on a planet within a habitable zone, but it involves at most one really difficult step (you'll recall, we can't really be sure it is critically difficult), and we consider that there is a reasonable chance that it will have arisen on some of the systems we will be able to observe. Pre-oxygenic photosynthesis is not apparently a critical step, and it arose relatively quickly on Earth after prokaryotes were established. This would ensure an energetic biosphere to exist, with a characteristic atmosphere full of exotic trace gases such as hydrogen sulfide and methane as well as carbon dioxide and water. We have some hope, therefore, of being able to detect a few planets that have this kind of biosphere, during the next few decades.

But oxygenic photosynthesis was a difficult step — it may be genuinely critical in the sense of the model, in which case it would occur on, at most, one in ten of the planets that took the first step, and more probably many fewer. In a sample of a few hundred planets we would be lucky to observe any that took this second step. We conclude that most likely we will not find any evidence for abundant oxygen on any of these target planetary systems.

There are some interesting predictions that also come from the anthropic model. For example, if complex life is rare, it is likely that Earth will be found to be one of the most favorable possible spots for it to have evolved, a cosmic Garden of Eden. This leads to an interesting question: Is there anything about our Solar System that marks it out as unusual compared to most others, and might make it particularly conducive to hosting complex life? Of course it has Earth, ideally situated in the habitable zone, and it may be that few other solar systems have such planets — but we don't know as yet what the distribution of planetary systems is, so we must put that aside for the moment. Is there anything else unusual about the Solar System?

As a matter of fact, there is: the Sun  is an unusually bright star, brighter than more than 90% of its neighbors. [A histogram of the 250 local stars within 10 parsecs (about thirty-three light years) of the Sun shows that]: the Sun is well out on the bright limb of the distribution.  This is a quite strong indication that most of the stars in our neighborhood, which are cool and dim type-M red dwarfs, with on average only about half the mass of the Sun, are really not as suitable for hosting complex life as our bright yellow type-G star. As this is all the more surprising because type M stars do have one factor that ought to make them more hospitable for complex life, and that is their long life span. Smaller stars burn their nuclear fuel more slowly, and the lifetime on the main sequence of type-M stars is typically (more than) twice as long as the Sun’s. According to our thinking and the evidence of our own planet, the shortness of the habitable period is a major factor limiting the chances that complex life develops. And yet we have awoken to find ourselves orbiting a bright, showy, short-lived firework of a type-G star, rather than a long-burning but dim and dull type-M.

We predict, therefore, that there is another factor or factors which make it difficult for life to develop to complexity around fainter stars, and explains why we don't find ourselves orbiting one. In fact there are several possible disadvantages to living around a type M star, but it's not clear as yet that any of them are so severe that they can tip the balance against them as good nurseries for life. Astrobiologists have recently been looking harder at type-M stars, as possible targets for SETI searches, for example, and the discussion below owes much to some recent papers from a symposium on the subject.

The habitable zone of a red dwarf is much closer to the parent star than is the Sun's — stars rapidly becoming much less bright as we go towards smaller size, and a star half the mass of the Sun emits only a few percent of its energy. A habitable planet around a M star would therefore have to huddle close to it for warmth, orbiting closer than Mercury to the Sun. Here, it will become tidally locked; its rotation slowed by internal energy dissipation until it equals the orbital, and the same face is always to the star. (The Moon of course is tidally locked to the earth, the reason why we always see its same face). Tidal locking will mean that there will be permanent huge temperature extremes on the planet, between the boiling daytime and the freezing, nighttime faces. If the atmosphere and ocean are not very efficient at transporting heat, this will mean that only a narrow strip around the terminator will actually be inhabitable, and we would expect that all the water on the planet would end up frozen out on the cold side by a 'cold trap' effect. However, with a sufficiently thick and mobile atmosphere, this fate could be avoided. [Note: it may not, according to some calculations, necessarily be the case that planets orbiting Class M dwarfs at the habitable zone distance will be tidally locked, but their rotational periods will be longer compared to their periods of revolution than is the case with Earth, possibly even longer than their 'year'].

Another problem may simply be that small stars tend to have small planets. The mass of the central star will certainly be related to the mass of the nebula that accretes around it, hence to the size of any planets that eventually form. As we have seen, it is critically important that it planet is big enough to hold onto a sufficiently thick atmosphere, and also to generating internal geothermal he to power plate tectonics (tidal dissipation would help there by adding a source of heat to the interior). We don't know enough about the planetary formation process to make very firm predictions, however. Current ideas tend to favor a picture of planetary formation as quite stochastic in nature, so while we certainly would expect a tendency for smaller stars to have smaller planets, perhaps there is nothing against a star half the size of the Sun having a rocky planet as big as the Earth.

There is a third problem for life around a faint star, as first pointed out by Ray Wolstencroft and John Raven. This one seems to be a really serious barrier to the development of complex life. Red dwarfs are the color they are because they are cooler than the Sun, typically only reaching about half its surface temperature. This means that the photons they emit are lower in energy. Chlorophyll makes full use of the high-energy photons emitted by the sun, absorbing strongly in the high-energy blue region of the spectrum, (as well as in the red, leaving the central, green wavelengths reflected — hence its characteristic color). But as we've discussed, for photosynthesis to split water, a high-voltage must be generated by the photo systems in plants, and even on Earth. This has required that two photo systems be coupled together. Under a much cooler star, very few high-energy photons would be available, and it is likely that three or even four such systems would have to be coupled together to accomplish the task of splitting water. But the evidence is that it was no simple task for evolution to arrive at water splitting photosynthesis even on Earth, so it may be much more difficult still to accomplish this under the light of a cooler star. What this might mean is that though type-M biospheres may evolve photosynthesis, they would find it nearly impossible to evolve the water-splitting variety, and, as we'll discuss in future chapters, without oxygen, there is very unlikely to be animals, let alone intelligent animals.

[At this point the authors might well have also mentioned the additional issue that red dwarf stars typically emit powerful flares, which, with the otherwise potentially habitable planets being as close as they are, could be highly disruptive to any life that might exist on the surface of those planets].

6.8  Summary

Our speculations in this chapter lead us to predict that simple (prokaryote) life might be moderately abundant in the universe at large — or possibly not, depending on just how difficult evolution to this first critical step [turns out to be]. Whether prokaryotes are rare or common, however, complex life will be rare. This idea has been named the Rare Earth Hypothesis, since it was put forward in a book of that name by Peter Ward and Donald Brownlee. Our analysis agrees very much with their thesis, though we find their arguments frustratingly qualitative. Ward and Brownlee argue their case based on the fortunate position of the Earth, our possession of a good-size moon, a friendly big brother planet in the shape of Jupiter, etc. The difficulty with their argument is precisely the 'self-selection bias' problem that we've tried to tackle in this and the previous chapter. We see that the earth has these attributes and (perhaps) that they have contributed to the evolution of complex life on her, but with only a single example of an inhabited planet, we don't see how to decide which of these properties are really necessary for us to be here, and which are not.

Maybe other solar systems have even more favorable circumstances? Anyway, we are subscribers to the Rare Earth Hypothesis, and expect to be born out when we eventually start to get data from other star systems on the atmospheres of Earth-like extrasolar planets. However, while waiting a decade or two that it is going to take before this data begins to come in, we hope that the more formal approach that we have taken here can provide some theoretical support for the 'Rare Earth’ view.

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