Gaia or Goldilocks?
For its first half-billion years the Earth was a hot, dangerous and sterile world. This was the era of massive bombardment, the time when planets were created. Then the impacts stopped, the Earth cooled and life began. Ever since, our climate has been suitable for life, with our seas neither boiling away nor completely freezing. Four billion years of good weather is so surprising that it needs explaining, and that is what this book has been about. In my view, there are really only two games in town: either the Earth system itself maintains a rough equilibrium or our planet has been extraordinarily fortunate. It’s either Gaia or Goldilocks.
I hope it’s obvious by now that my preference is for Goldilocks, even though Gaian processes (broadly interpreted to include uncontroversial ideas such as volcanic weathering) must have played a substantial role too. However, even if I have been a little negative at times, there’s a lot that I admire about the Gaia hypothesis (even when interpreted narrowly). It is an ambitious theory, but there’s nothing wrong with ambition and James Lovelock’s big idea has played a significant role in stimulating the modern view of the world as a single system composed of many interacting and inter-related geological, biological and climatic mechanisms. I particularly admire its name.
‘Gaia’ was suggested by the novelist William Golding in an act of superb public relations. Gaia was an ancient Greek goddess whose name is sometimes translated as Mother Earth. She was the second of the gods to come into existence, following Kaos, and they were both part of an older generation that ruled before Gaia’s grandson, Zeus, overthrew his father, Kronos. Despite this regime change, Gaia retained her position of influence by adapting superbly to the changed circumstances. However, among the new gods on Olympus was Phaethon, who stole the Sun-chariot from his father, Helios, and drove it too close to Gaia. Only the intervention of Zeus, who struck Phaethon dead with a thunderbolt, prevented the complete destruction of Mother Earth.
As we’ve seen, modern science tells a remarkably similar story of a highly resilient Earth surviving dramatic changes in circumstances but destined to be scorched by the Sun. Our planet has gone through four main phases during its long history: first an abiotic time of massive bombardment, followed by the bacterial empire of anoxic stability, then the rise of an oxygen-rich atmosphere, leading finally to our own era of multi-celled plants, fungi and animals. At every stage Earth has serenely fitted in with each new regime as if nothing untoward were happening. However, in less than a billion years’ time the Sun will become too warm for life on Earth to continue. At the very least our planet will be severely overheated and, after a few more billion years, it may even become engulfed by the Sun as it evolves into a red giant. It’s unlikely that Zeus, or anyone else, will save Earth from terminal damage.
So, naming the idea that we live on an inherently benevolent and adaptable planet the ‘Gaia hypothesis’ was very appropriate. With tongue firmly in cheek I’m tempted to call the alternative idea, that we were just plain lucky, the Nemesis hypothesis. As I mentioned in the Prologue, Nemesis was the Greek goddess of undeserved good fortune whose main function was to restore balance by dealing out retribution to those who had got away with too much for too long. But Nemesis was also the mother of Helen of Troy. Nemesis therefore seems to me to be a highly apt designation, because if anthropic ideas are true, unearned luck has placed us on a world of extraordinary beauty.
Nemesis will exact her price in the long term, but fortunately not for hundreds of millions of years. Nevertheless, within a billion years even a zero level of carbon dioxide in the atmosphere will not be low enough to stop the ever-warming Sun from overheating our globe. Even worse, if the weathering-mediated feedback I discussed in Chapter 5 is sustained, then the carbon dioxide level in our atmosphere will continue to fall and, well before a billion years from now, it will become so low that plants will be unable to grow. Plants have begun to adapt to the very low levels of carbon dioxide we already have, but there is a limit to how far such adaptation can go. No conceivable plant could grow if there were no carbon dioxide at all in the atmosphere. Present models suggest that plant life might be able to limp along for another half-billion years or so but that’s about it. If, on the other hand, this negative feedback is unable to completely compensate for warming, carbon dioxide will not fall fast enough and our planet will become uncomfortably warm within a few hundred million years. Either way, the distant future for plant and animal life is bleak.
Even if our luck holds and some unexpected combination of processes succeeds in keeping temperatures down and carbon dioxide levels up, the Earth’s axis will become chaotically unstable in 1.5 billion years and this will certainly produce some very unpleasant conditions. So the idea frequently discussed in astronomy textbooks and science fiction stories, that the Earth will remain habitable for another 5 billion years, is hopelessly optimistic. If our distant descendants are going to survive they will need to take control and shield the planet from an increasingly hostile Sun. Indeed, if the thrust of this book is right and our 4 billion years of good weather has to a significant extent resulted from good luck rather than natural control mechanisms, we may have to take charge much sooner.
Still, from a human perspective that’s all in the extremely distant future. Are there reasons why the ideas discussed in this book should matter in the here and now? One reason for caring about anthropic thinking is that it may be a good, and rather neglected, tool for expanding our knowledge about the world around us. People have been thinking about anthropic ideas for a very long time but perhaps the earliest example of its use in a scientific debate was the late 19th-century controversy concerning the age of the Earth. The anthropic argument was simply that the Earth must be old enough to allow the evolution of human beings, that is, a young Earth is inconsistent with the observation that we exist. Although they wouldn’t have put it quite that way at the time, this argument led both geologists and biologists to conclude that the Earth must be hundreds of millions of years old. Physicists scoffed at this notion because their calculations showed that the Earth’s interior was too warm to be that old and, moreover, it was not possible for the Sun to shine for so long. Given this situation, Thomas Chamberlain, professor of geology at the University of Chicago, concluded in 1899 that ‘atoms [have] locked up in them energies of the first order of magnitude’. He also speculated that conditions at the centre of the Sun may allow this energy to be released. Chamberlain’s suggestion was years ahead of its time, decades before the discovery of nuclear fission gave a mechanism for keeping the Earth warm or the discovery of nuclear fusion showed how the Sun could shine for such an immense period of time. Nuclear power was invented by a geologist! Chamberlain’s prediction that new sources of energy would be found hidden in the Sun is one of the best examples I know of a successful prediction based on anthropic selection.
This was a rare success for anthropic reasoning, although there have been many uses of such ideas to explain already recognised facts. For example, it was known by the end of the 19th century that the Earth must be sufficiently large for its gravitational field to retain an atmosphere, and this explains, in retrospect, why the Earth is an unusually large rocky body. Similar arguments have been proposed to explain the location and mass of Jupiter, the existence of plate tectonics, the particular location of the Sun in the galaxy, and a great many other features of our planet and planetary system.
However, these ‘just so’ stories remain highly speculative and, in any case, do not provide the strong support for anthropic ideas that would result if new and successful predictions were made ahead of observations. Now is the best of times to make such predictions. Thanks to ongoing and planned exoplanet characterisation projects, we will have a great deal of new information concerning thousands of planetary systems within a few decades. We have an unparalleled opportunity to make anthropic predictions that can be tested within our own lifetimes. I’m going to finish Lucky Planet by sticking my neck out to make some.
The conclusion I draw from all that I have discussed in this book is that, statistically, we are most likely to be living on a second-best world in a second-best universe. This is a universe where planets as well suited to life as the Earth are rare but not vanishingly rare and where even simple life is a tough trick to pull off. We therefore live in a moderately habitable, rather than highly habitable, universe and this suggest some very specific predictions. The first one is easy: there will never be any flying saucers on the White House lawn. If good luck in an immense universe has played the kind of critical role I’ve been suggesting, then advanced civilisations elsewhere are inevitable – but they will also be so far away that we will never be able to communicate with them or even observe influences they may have on their galactic neighbourhoods. We are certainly not close enough for little green men to drop in when they happen to be in the neighbourhood. However, the trouble with this prediction is that it’s out of our hands. How long do we wait before realising that no visitors are coming?
What’s really needed are predictions that we can actively go out and test for ourselves. A good place to start is by looking for life elsewhere in the solar system. My hope is that we will soon find microscopic life living beneath the surface of Mars and my expectation is that its biochemistry will show it to be similar to Earth life. This will generate some interesting discussions as we debate whether this is evidence that there is only one way to make life or evidence for cross-contamination between the worlds. I expect a consensus to eventually emerge that the similarities are too great to be explained by a separate origin and we will then know that ‘Earth life’ exists on two planets in our solar system. Even that is a very exciting prospect and one that I may be lucky enough to live to see. We’ll then have to decide whether to make Mars off-limits so that its unique biosphere can develop without interference from mankind. I hope that we do.
The rest of the solar system is probably ours to exploit; I believe that the origin of life, like all the major steps leading to the emergence of intelligence, is a rare occurrence. Contamination of Europa or Enceladus by microbe-bearing meteorites from Earth is unlikely and an independent origin for life would therefore have to be taken very seriously if life is found in such places. An even more exciting prospect would be the discovery of life in the ethane lakes of Titan, because this would surely be so unlike Earth life that an independent origin would have to be the preferred explanation. However, if the thrust of this book is correct, even simple life is rare and my expectation is that the giant-planet moons of the solar system will turn out to be completely sterile. We can only tell by looking, though, and I really hope I’m wrong!
My final prediction is that the ongoing search for exoplanets will eventually show that the giant planets of our solar system are unusually far apart. If there really has been anthropic selection for an Earth with a stable axis, this implies that we live in a solar system whose orbits wobble unusually slowly, since the key to axial stability is avoidance of resonance, and this needs a gap between the relatively rapid axis-precession period of a planet and the slower periods at which orbits oscillate. This gap occurs only if major worlds are relatively small or relatively widely spaced so that orbits oscillate very gently. However, the gas giants in our solar system are not unusually light compared to known exoplanets, and so they must be more spread out than is usual. It will take many more decades of exoplanet searching to confirm this.
To be more specific, my expectation is that only about 1 per cent of planetary systems orbiting Sun-like stars will have a fundamental orbital oscillation period slower than the solar system’s 50,000 years. This prediction can be tested using current technology and that brings me back to OGLE- 2006-BLG-109L b and her sister world, the equally memorable OGLE-2006-BLG-109L c, mentioned in Chapter 3. OGLE stands for the optical gravitational lensing experiment, a Polish-run project to look for evidence of dark matter by monitoring the brightness of millions of stars in the galactic bulge (hence BLG in the planets’ names) and elsewhere. OGLE has almost incidentally proved to be very successful at detecting planets from the months-long brightening that occurs when one star passes in front of another so that its gravity acts like a lens and focuses the light of the more distant star onto the Earth. Deviations from a simple pattern of steady brightening over several weeks followed by equally steady fading indicate the presence of planets around the nearer star. And, unlike the RV method I discussed in some detail earlier, this gravitational microlensing is sensitive to planets orbiting relatively far from their star. OGLE’s 109th lensing event (hence 109L) of 2006 proved to be a particularly exciting example of planet detection by this technique. The results when analysed showed the presence of two worlds (hence ‘b’ and ‘c’). Planet 109L b turned out to be a little smaller than Jupiter and orbiting its half-solar-mass star at about half the distance Jupiter does from our Sun. Its companion, 109L c, is slightly smaller than Saturn and has an orbit about half the diameter of Saturn’s. So, OGLE-2006-BLG-109L looks like a solar system in miniature. This discovery of a mini solar system 5,000 light years from Earth demonstrates that we now have the technical ability to search for true analogues to the solar system, stars whose main companions are not hot Jupiters but gas giants in orbits comparable to the major planets of our own system.
OGLE-2006-BLG-109L is the most similar planetary system to ours yet found and, if solar-like systems are common, it should only be a matter of time before a truly comparable one is discovered. I think this search will fail though, because, as I said above, I believe that our planetary system is an unusually open one. In fact, no true solar system analogues have yet been found, although it will take many more decades before enough data has been collected to conclusively show that they are as rare as I’m suggesting.
There must be many other anthropic-based predictions concerning properties of extra-solar planetary systems that can be tested against data likely to become available as the 21st century progresses. If such predictions are successful, the increased credibility of anthropic ideas will profoundly change our understanding of the Earth and of our place in the Universe. My own motivation for beginning the research described in this book was that I felt that the anthropic principle, if true, should be one of the cornerstones of our understanding of geology. A good example of how anthropic reasoning can alter our view of important Earth processes concerns the issue of climate, which has been so central to my story. The observation that the Earth’s temperature has been surprisingly consistent over billions of years seems to cry out for an explanation in terms of stabilising processes. However, if climate stability is the consequence of anthropic selection then there is no grand stabilising process at all and searches for it will, ultimately, fail. That’s not to say that we shouldn’t look for it, it’s just that we shouldn’t necessarily conclude that we’re missing something if the search is unsuccessful.
Acceptance that the Earth is a very odd planet, and that this was necessary for the emergence of humans, also has a very obvious impact on the search for extraterrestrial intelligence. Quite bluntly, if there is significant anthropic selection for Earth properties, then we are probably effectively alone in the Universe. As I discussed earlier, the nearest extraterrestrial advanced civilisation could easily lie beyond the edge of the visible Universe and so be uncontactable. This is quite a disappointing conclusion for many. Indeed, one prominent, well-informed critic of anthropic ideas has admitted that his views may be coloured by having grown up watching the original Star Trek series. Maybe my own views have been coloured by slightly more recent films. I’ve thought for a long time that Alien was far more plausible than Mr Spock, so it’s quite possible that my subconscious doesn’t want aliens to exist.
There may be other psychological factors influencing people’s rejection of anthropic ideas. In particular, the concept of a balance of nature is deeply ingrained in western, and other, cultures. The idea that this balance may, at least in part, be an illusion is quite hard for many people to swallow. One particularly dangerous example of unsubstantiated belief in inherent balance is the rejection, by some, of the idea that humans are beginning to adversely affect the Earth’s climate. Opponents of the global warming hypothesis contend that some, as yet undiscovered, process will automatically compensate for mankind’s interference in the atmospheric composition. This seems to be based entirely on a naive belief that the Earth’s climate system is naturally stable, even though the geological evidence clearly shows that our climate has changed abruptly many times in the past; the last time just 11,000 years ago. Note that, if I’m right and the absence of really serious climate instability over the last half-billion years is just a fluke, we need to be very, very careful when we meddle with the atmosphere. We may just find out the hard way that planets with nasty climates are quite easy to produce.
Another psychological factor that, perhaps, makes anthropic selection hard to accept is that we are so used to our wonderful planet that we rarely stop to notice what an amazingly beautiful, unique and, dare I say it, miraculous place it is. If we didn’t take the Earth quite so much for granted it would become obvious, I think, that it is an extremely odd place. Many people may find the view of the Universe set out in this book rather bleak, but for me, the anthropic principle has merely served to sharpen my appreciation of our stunning home-world and how lucky I am even to exist.
So, I certainly believe that the possibility that the Earth is special should be taken very seriously by everyone and for all sorts of reasons, but in conclusion, I’d like to finish with the most important justification of all for considering this idea. It’s probably true.