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Sir Fred Hoyle

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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 ten years to fund the search for non-human civilisations 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/Churyumov-Gerasimenko.

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 civilisations 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 percent 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 twenty 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 civilisations that use electromagnetic communications? And how long, on average, would those high-tech civilisations 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/Churyumov-Gerasimenko was the first-ever opportunity to see if this hypothesis holds water (so to speak). The Philae lander did detect sixteen 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 high-tech civilisations on them at the moment, of course, depends on how long the average technological civilisation survives. Our own hi-tech civilisation has survived, so far, for about one century.
To shorten to 725 words, omit paragraphs 5, 6, 7 and 8. (“To fill…star”)

Things We Know Now

15 April 2005

Things We Know Now

By Gwynne Dyer

Things we know now that we didn’t know twenty years ago:

We know that most stars have planets: in the past decade astronomers have identified around two hundred planets circling nearby stars. They are all gas giants like our own Jupiter and Saturn, because only massive planets like that can be detected by our present techniques, but most stars are probably also surrounded by smaller, rocky planets like Earth that we cannot yet detect. How likely is it that our own solar system, which contains four gas giants and five smaller, rocky planets, is unique in a universe otherwise made up solely of gas giants?

We know that there are abundant quantities of organic molecules, the chemical building blocks of life, in interstellar space, and the hypothesis that life on Earth was seeded from space, first advanced by astronomer Sir Fred Hoyle, gains ground by the day. But if that is how life emerged on Earth, then why not on many of those trillions of other planets in this enormous universe?

We also know that relatively local events like asteroids or comets crashing into planets can devastate the entire biosphere: there are five known “extinction events” in the history of life on Earth. And now we know that distant cosmic phenomena like collapsing stars can have just as great an impact. Dr Adrian Melott of Kansas University and his colleagues have just made a convincing case that the first of those events on Earth, the Ordovician extinction of 440 million years ago, was caused by a ten-second burst of gamma rays emitted by a dying star several thousand light-years from here.

Unlike the two mass extinctions known to have been caused by asteroid strikes, 251 million years ago at the end of the Permian period and 62 million years ago when the dinosaurs disappeared, the Ordovician one happened at a time when most life on this planet was still in the seas and nothing had even developed a backbone yet. About 60 percent of the marine species then in existence suddenly vanished from the geological record, but there is no asteroid collision associated with this upheaval.

Until recently, the only explanation we had for the Ordovician was the sudden onset of a ice age, but that didn’t really make a lot of sense. Even severe environmental stress and loss of habitat caused by falling sea levels shouldn’t have killed off sixty percent of existing species — and besides, why did the planet suddenly tumble into an ice age after a long period of stable, warm climate? So along come astronomer Adrian Melott, palaeontologist Bruce Lieberman and their colleagues at Kansas University and NASA with a much more plausible — and worrisome — explanation.

Stars above a certain size have a life cycle that ends with collapse into a black hole — and as they collapse they emit a pulse of energy, mostly made up of gamma rays, that is so intense that it carries all the way across the universe. It is a highly directional pulse, however, so you will only detect it if your home planet happen to lie within the cone of radiation from the particular star in question.

Down here on the Earth’s surface, astronomers only detect about one gamma-ray pulse a month: the thick blanket of atmosphere muffles most of the weaker, more distant ones. But these stellar collapses are happening all the time here and there in the universe, and satellites simultaneously scanning all parts of the sky for these brief bursts of radiation would see about a dozen a week.

We are caught in the cone of gamma radiation from one dying star or another about a dozen times a week. Most of them are safely millions of light-years distant — but it has been calculated that if such an implosion occurs within six thousand light-years of us, and happens to emit its beam of gamma radiation in our direction, it would strip the protective ozone layer off our planet and leave all life on the surface exposed to deadly ultraviolet radiation for up to five years. It would also fill the upper atmosphere with nitrogen oxides that absorb the sun’s heat and could easily push the Earth into an ice age.

Dr Melott and his friends believe that this double-whammy is what caused the Ordovician extinction 440 million years ago. What actually happened — first a rapid die-off of many species that were presumably killed by UVB radiation, then an ice age to finish the job — fits the profile of a gamma-ray event very closely. They also calculate that such an event is only likely to hit the Earth two or three times per billion years, so it won’t have an immediate impact on real-estate prices. But the larger pattern that emerges from all this is not pretty.

We appear to inhabit a universe in which there may be trillions of planets that broadly resemble the Earth, and many if not most of them may be home to life of one sort or another. Nobody has a clue how many might harbour consciousness or intelligence, now or in the future, or how many have done so in the past, but even that number could easily be in the billions. And we have reason to suspect that each year hundreds or thousands of these planets are hit by close-range bursts of gamma rays from collapsing giant stars that cause mass extinctions.

We know a lot more about the universe than we used to, and the knowledge is not very comforting.


To shorten to 725 words, omit paragraphs 5 and 8. (“Unlike…upheaval”;and “Down…week”)