by Peter D. Ward and Donald Brownlee
When this book was first published in 2000, it raised quite a stir. Its basic thesis is easily summarized: while bacterial life is probably common throughout the universe, complex multicellular life is probably much rarer. They present their case with great thoroughness, pointing out all the characteristics of earth that turn out to be rare in the universe and probably played an important role in permitting complex life.
The most obvious of these is the temperature zone that the earth occupies, falling between the freezing point and the boiling point of water. The presence of large quantities of water is considered to be a crucial factor in the development of life, so crucial that the presence of liquid water anywhere on a planet is considered to be an indicator of the possible presence of life.
A second factor that they deem to be of great importance is the presence of all the right chemical factors: lots of water; enough carbon dioxide in the atmosphere to keep the planet warm but not too hot; an initially oxygen-free atmosphere; and so on.
A third factor that they consider to be important is the presence and position of Jupiter in our solar system. Jupiter, it turns out, is so large that it deflects the orbits of the flotsam that poses the threat of mass extinction to life on earth. Of the five mass extinctions in earth history, at least two are held to be the result of the impact of a large object. Had there been more such mass extinctions, or had a really big object hit the earth, complex life might have been extinguished. Without Jupiter acting as a cosmic vacuum cleaner, there might well have been such impacts.
A fourth factor is the role of plate tectonics. Earth appears to be unique among the rocky bodies of the Solar System in having an active core: it is molten and in motion. This in turn drives the motions of the plates. The authors argue that these motions played a crucial role in driving the evolution of complex life, because these motions renew the mountain ranges. Without such mountain-building motions, the continents of the earth would be eroded down to nothing and the earth would end up covered in water. This point strikes me as rather weak when applied to other planets; conceivably another planet without as much water would erode down to a point where the oceans were too small to generate enough weather to continue erosion.
A fifth factor is the moon. They point out that the moon is uniquely large relative to the earth, and that the orbital motions of the moon serve to stabilize the tilt of the earth. Without the moon orbiting our planet, its axis of rotation would wander around as continents drifted and altered its balance. If the axis of rotation ended up pointing towards the sun, then half of the earth would be permanently in darkness and the other half would be baked by perpetual sunlight. The authors imply that such a condition would be destructive to life. I disagree; during such periods, life would retreat to the twilight area that gets just enough sunlight to remain at the optimal temperature. We’d see life evolving into forms that are best suited to different temperatures -- exactly as we do here on earth.
These are all good arguments and I am convinced that the development of life on earth was in fact a rather lucky occurrence. However, the authors have committed an oversight so huge that I cannot give the book, overall, much credence. They fail to come to grips with the fundamentals. Life is not fundamentally about water, or amino acids, or the correct temperatures, or even energy. Life is fundamentally driven by negentropy. Look at the earth’s biosphere. The fundamental source of life is sunlight, but it’s not the energy of the sunlight that makes life possible, it’s the negentropy. It’s the temperature difference between the earth and the temperature of the sunlight (about 5000ºK) that drives all life. I don’t have the space here to explain the thermodynamics of life, but I assure you that life is best understood in thermodynamic terms, and that negentropy is the driving force behind all life.
The authors never once mention thermodynamics or negentropy, or even touch upon these issues. This is absurd; it’s like arguing the possibilities of space travel without taking Newton’s Laws into consideration. Thus, when they declare that there might be life on Europa because it probably has water underneath its outer covering of ice, they fail to understand that there is no significant source of negentropy to drive the development of life. Yes, there’s a chance that there is something similar to our black smokers along seismically active portions of the ocean floor, which would be a source of small amounts of negentropy. But it’s a stretch.
If we instead turn our attention away from water, plate tectonics, and destructive meteors, and instead focus our attention on sources of negentropy, we get a completely different view of the possibilities of life. We immediately stumble over the problem raised by the planet Mercury. Here’s a planet harvesting huge quantities of negentropy on its sunlit surface, yet is lifeless. If negentropy were the driving force behind life, then Mercury would have lots of life. Oops.
This brings us to the second fundamental principle of life: it requires some system that can respond to negentropy in complex ways. A system using water as a solvent and organic molecules certainly fits that bill: organic molecules in water are capable of undergoing zillions of different reactions to produce zillions of different proteins. But to suggest that organic molecules in water are the only possible form of complexity capable of responding to negentropy -- that’s preposterous.
“OK, smart ass, if there are other systems, list them!”
I have only two such systems that I can cook up. The first is the system of plasma in a magnetic field inside a star. The motions of plasmas create magnetic fields, and magnetic fields affect the motions of plasmas. The interaction between these two is mind-bogglingly complex. This is why we’ve never been able to build a magnetic bottle to hold the plasma required for fusion energy: every magnetic field we construct gets distorted by the motions of the plasma, which changes those motions, which change the magnetic field... it’s simply too complicated to figure out. I see that complexity as fitting the bill for constructing complex life. That’s why I claim that there could well be life inside the sun. I believe that local motions of parcels of plasma could generate local magnetic fields that, interacting with each other, could easily generate complexities capable of reaping the available negentropy. Oddly, the weakest part of my claim is the paucity of negentropy. Remember, the thermal gradients inside the sun are low; there are only a few places, such as the boundary zones between radiative transfer and convective transfer, that might have negentropy sufficient to drive a form of life. Remember also that there is no reason to believe that such life forms would be our size; they could just as easily be the size of the earth, and in fact that seems more likely given the complexities of the solar magnetic field.
The second system I can imagine lies in the atmospheres of Jovian planets. There’s lots of negentropy inside these atmospheres, as indicated by the violent weather. Weather is the complex system that I consider to present a possibility for life. Any meteorologist can tell you just how complicated weather can get, and earth’s weather is driven by the evaporation and condensation of water. In the Jovian atmosphere, we have much more complexity. There’s water vapor, but there’s also ammonia, which also evaporates and condenses over a small range of temperatures. There’s all sorts of chemical complexity in there; a life form based on phase transitions coupled with chemical reactions strikes me as more fecund than what we have here on earth. And there’s certainly plenty of negentropy inside Jupiter’s atmosphere, coming from below and above.
All things considered, I believe that it’s more likely that life would develop in the atmospheres of Jovian planets than on the surfaces of rocky planets. Yes, our kind of life might well be rare in the universe. But there are lots and lots of Jovian planets in the universe, and some of them might well harbor advanced civilizations. Why then have not these civilizations contacted us? Perhaps because they’re just as blind to the possibility of life on our planet as we are to the possibility of life on theirs.