The excitingly named Kepler 22b, a planet believed to have been discovered orbiting a star a mere 600 light years away, is being hailed as a “New Earth”. But sci-fi fans shouldn’t get too excited just yet: as always with these stories, the likelihood is that we have not met the neighbours. Or, if we have, they probably aren’t very exciting conversationalists.
I spoke to Dr Lewis Dartnell, of the Centre for Planetary Sciences at UCL, about how likely it is that there is life, or better yet intelligence, on Kepler 22b. The answer, essentially, is that we don’t know. “There are big hurdles that life has to get over, and we don’t know how big a hurdle the origin of life itself is,” he says. “You simply can’t tell with a single datum – you can’t do stats when N=1.” And, of course, at the moment, the number of planets known to have life of any form is precisely one: Earth.
There are efforts to find more data. “The interesting thing will be when we go to Mars and Europa and see whether there are bacteria there. It would be enormously significant if life is found there,” says Dr Dartnell. “But the next step, once Kepler has looked at a lot of planets, will be to see what their atmospheres are made of, using infrared spectroscopy.
“If one or two of them have oxygen in the atmosphere, it may be a transient thing – like Venus, undergoing a runaway greenhouse effect – but if we find, say, 20 Earth-like planets,all with the signature of oxygen in their atmosphere, then that would be very unlikely. Life would be the more reasonable explanation.”
So, as yet, we don’t know. Perhaps life is very unlikely. But, on the other hand, perhaps it’s not: perhaps simple reproducing cells, like Earth’s bacteria, are fairly likely to develop. But there are major bottlenecks between bacteria and interesting, complex life like fungus, plants and science journalists. All those, and pretty much everything big enough for us to see with our naked eyes, are made of eukaryotic cells: that is, cells with a nucleus, and a vast array of other microscopic machinery, unlike the simple, “prokaryotic” bacteria.
As Nick Lane pointed out in his fantastic book Life Ascending, the “mind-boggling complexity” of the eukaryotic cell is not easy to achieve. It’s a “genetic chimera”, a combination of two genomes: a monster created in a single, sudden freak occurrence, not the slow accumulation of tiny differences which evolution usually involves. Specifically, one form of simple cell took up residence inside another, creating a hybrid. This happened about two billion years ago, and was, to put it mildly, not all that likely. As Lane puts it: “The eukaryotic cell only evolved once because the union of two prokaryotes, in which one gains entry to the other, is truly a rare event, a genuinely fateful encounter.” Lane even goes so far as to say that it was so unlikely that it may not have happened anywhere else in the universe: and without the higher energy production available to prokaryotes, through the internalised bacteria known as mitochondria which function as little electrical power stations, complex life cannot evolve.
So, even if life has arisen on Kepler 22b, it’s far from a given that it proceeded to do anything more interesting than bacteria do (of course, prokaryotes do interesting things: but they do interesting things like surviving in inhospitable places, such as volcanoes, or eating implausible foods, such as concrete. They don’t, eg, hunt giraffes, or grow to 350 feet tall, or investigate their own origins). And while eukaryotic cells are probably the biggest bottleneck between bacterial life and civilisation, they’re far from the only one: multicellularity, the development of muscle and movement, and, of course, the evolution of a complex nervous system and consciousness.
That’s not to say it wouldn’t be incredibly exciting to find even a bacterial ET. But much as we tend to think of evolution as a sort of inevitable march to intelligence, it’s actually a miracle piled on top of a miracle that we are how we are. Kepler 22b may well not have been so lucky.