Panspermia and the Drake Equation: Looking good

One by one, the empty boxes in the Drake Equation are being filled in with actual numbers, and it’s looking good. So good that Yuri Milner is spending $100 million of his own money over the next 10 years to fund the search for non-human civilizations orbiting other stars. But it’s a pity that the Philae lander from the European Space Agency’s Rosetta mission didn’t have more time to look for life on Comet 67P/ChuryumovGerasimenko.

Yuri Milner is a Silicon Valley billionaire who was working on a PhD in theoretical physics at the Russian Academy of Sciences before he moved to the United States and got rich. His money will buy thousands of hours of radio-telescope time each year to look for radio transmissions from other star systems.

This represents at least a tenfold increase in the amount of work being done on finding intelligent life elsewhere in the galaxy, and Yuri Milner is no fool. Why does he think it’s worth spending this money now?

Probably because the Drake Equation is finally coming into its own. It has seven terms, and American astronomer Frank Drake could not give a value to any of them when he first wrote it in 1961. It was just a formula that would let us estimate the number of civilizations in the Milky Way galaxy when the relevant data eventually became available.

To fill in the first three terms, we needed to know how many stars there are in the galaxy, how many of them have planets, and how many of those planets are in the “habitable zone” where liquid water can exist.

In 1961 the estimate was 100 billion stars. Now it is 400 billion, of which 300 billion are essentially similar to our Sun.

Until 1992, we didn’t even know if other stars had planets circling them. Now we can estimate that at least 40 per cent of them do, although the real answer may be almost all of them. ( We still cannot detect planets much smaller than Earth).

As for how many planets are in the “habitable” zone, not too close or too far from their parent star, the answer is probably one or two per star.

Using the data acquired in the past 20 years, NASA now estimates that there are 144 billion habitable planets in our galaxy. Not all of them will harbour life, of course, but that is a very encouraging number.

Other questions remain, however. How many “habitable” planets will actually have life on them? On how many of those planets will an intelligent species appear? How many of those intelligent species will build civilizations that use electromagnetic communications? And how long, on average, would those high-tech civilizations last?

We don’t yet know the answers to any of those questions, but we do know that organic compounds are common even in interstellar space, and that they are continuously raining on our own planet. So the standard assumption is that they somehow combined on Earth to form the first single-celled creatures, and evolution did the rest.

But if it were easy for those organic compounds to combine into complex microbes and viruses, then you would expect it to have happened here a number of times. There would be several or many unrelated genetic lineages on Earth — and there aren’t. All life here has a common ancestor.

So it must be very rare for life to develop spontaneously. If it actually happened here, it would mean that we are a miracle, and pretty much alone in the galaxy. But maybe the miracle happened on another of those 144 billion planets, billions of years ago, and life been spreading through the galaxy ever since — not as alien beings on starships, but as microbes and viruses on meteorites and comets.

This is the “panspermia” hypothesis, first proposed by astronomers Sir Fred Hoyle and Dr Chandra Wickramasinghe in 1974. Dissatisfied with the notion that Earth was unique, they suggested that not only organic compounds but actual microbes and viruses could travel through interstellar space, dormant but still viable in the liquid water that they suspected was present in the interior of many comets.

It sounds weird, but it is just as plausible as the rival hypothesis of an independent origin of life on Earth. Comet 67P/ChuryumovGerasimenko was the first-ever opportunity to see if this hypothesis holds water (so to speak). The Philae lander did detect 16 different organic molecules as it bounced along the comet’s surface, but it ended up in the shadows without power to pursue its investigations further.

Pity, but there’ll be another comet along in a while. And if it turns out that Hoyle and Wickramasinghe were right, then most of those 144 billion planets will have life on them. The history of evolution on Earth tends always to greater complexity, so a fair proportion of them would have intelligent life on them.

How many of them have hightech civilizations on them at the moment, of course, depends on how long the average technological civilization survives. Our own hitech civilization has survived, so far, for about one century.