My Times column on the paradox that planets seem to be abundant, but signs of life are rare:
The search for another world that can sustain life is getting warmer. We now know of 1,879 planets outside the solar system. A few weeks ago, we (the planetary we, that is: no thanks to me) found Earth’s twin, a planet of similar size and a habitable distance from its sun, but 1,400 light years from here. Last week we found a rocky planet close to a star just 21 light years away, which means if anybody lives there and tunes in to us, they could be watching the first episode of Friends.
Also last week the Philae lander’s results showed that the comet it is riding on has organic (carbon-based) molecules in its dust, the ingredients of life. Even in our own solar system we know of a moon, Titan, where it rains methane, and another, Europa, with an ice-covered ocean. In short, it is getting ever more likely that there are lots of bodies like Earth in our own galaxy alone: with liquid water and the right sort of temperatures for the carbon chemistry of the kind that life runs on here.
Which only underlines Enrico Fermi’s famous question, first delivered over lunch at Los Alamos in 1950 during a conversation about UFOs: “Where is everybody?” His point was that if there are billions of habitable planets, and many have had billions of years to produce intelligent life forms, then the chances are that some of them must have had time to broadcast to, or even visit other solar systems. So why is there not a whisper in the ether of their version of Top Gear, let alone a glimpse of a tentacled Clarkson careering through the air in a flying saucer, or even a bit of ancient, rusty wreckage to show where he once crashed?
The Fermi paradox gets ever more baffling, as the evidence grows of other habitable planets. The silence is beginning to seem ominous. Robin Hanson, the chief scientist of the prediction market research firm Consensus Point, advises: “Take a minute to look up at the dark night sky, see the vast, ancient and unbroken deadlands, and be very afraid.” He thinks there may be an obstacle ahead of us that has caused every previous planetary civilisation to collapse before colonising the galaxy: nuclear war, or something equally horrible.
He calls this argument the great filter, and defines it thus, “The sum total of all of the obstacles that stand in the way of a simple dead planet (or similar sized material) proceeding to give rise to a cosmologically visible civilisation.” Have we got past the great obstacles, or are there still some insuperable ones ahead?
We’ve certainly got through at least five big filters. First, life evolved. Nick Lane in his magnificent new book The Vital Question thinks that a peculiar feature of all earthly life — that it traps energy in the form of protons pumped across membranes — indicates that it began at warm alkaline vents on the floor of the early ocean. Gradually that energy came to be used to make information, in the form of genes, and the machinery to replicate it.
Genetically and biochemically, there is only one form of life on this planet, which might imply that it’s a rare and lucky accident, but then later forms would have struggled to compete with the first one, so it’s not clear how difficult this step was and how many planets with the right conditions failed to take it. That life did not then die out thanks to a global freeze-up or fry-up may have depended — David Waltham argues in his book Lucky Planet— on the good fortune of having a relatively large moon that stabilised our spin and moderated our climate: another possible stroke of luck that other planets perhaps did not share.
Next, after a couple of billion years, creatures bigger than microbes emerged, once (Nick Lane argues) an energy-per-gene limit was breached by the invention of the mitochondrion, a specialised energy-generating microbe living inside another cell. This gave us large and complex cells of the kind found in plants, animals and fungi, and eventually multicellular creatures making and using oxygen. It took a very long time to achieve this step, so many planets may have been filtered out at that stage, and be stuck with microbes only.
Earth, then, had complex life forms for more than half a billion years before anything remotely intelligent enough to develop technology appeared. For 140 million years, the dinosaurs achieved large size and nimble agility without ever threatening to do much mental or physical (as opposed to genetic) innovation. Their evolutionary experiment was cut short by a meteorite, and the mammals took another 60 million years even to start on technology. Even then, it was only one species, a primate, that became technological, rather than the almost equally large-brained dolphins. So that was a strong filter: intelligence without technology is clearly possible.
Even once we had big brains, technology, language and culture, human ancestors spent a few million years stuck in hunter-gathering, before something triggered an explosion of cumulative culture in one sub-species on one part of one continent, Africa, about 200,000 years ago. I have argued before that this step was enabled by the invention of exchange and specialisation, which made us capable of “cloud intelligence” so we could collaboratively build devices too complex for individual minds to comprehend. Only then did we experience rapid cultural evolution and innovate to the point where we could start exploring space.
It is, in other words, very likely that most planets would have failed to clear all these hurdles, which may explain the silence. Many may teem with microbes; others could be rich in fossils of life that died out; a few possibly host herds of agile, even ingenious, creatures, some of which communicate in languages; one or two might have got to the point of inventing weapons of war before some catastrophe intervened. But almost none have reached the point where they could send messages and spacecraft out of their atmospheres.
Is it not incredibly lucky that we live on a rare planet that did make it? Well, no, because whoever lived on such a planet would say that about themselves. It is for this reason that I am not persuaded yet that the most severe filter, the one that stops most planets colonising the galaxy, comes after the stage we have reached. I think that’s unnecessary pessimism. And with that I am off on a summer holiday.