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”)