The previous post in this series is here.
IN a book review of Heaven’s Touch by James B. Kaler – the reviewer reports this point made in the book:
The real surprise of the past few decades must be the vulnerability of the Earth to truly cosmic events – not only supernovae, whose “killing zones” may extend within 30 light years of us, but also gamma ray bursters whose emissions, beamed like death rays, could scour life from planets 6000 light years away. I have a soft spot for magnetars, ultra-dense neutron stars with the strongest magnetic fields in the universe, and Kaler covers these in detail. In 1998, the eruption of one such star 20,000 light years away generated X-rays so powerful that anyone in Earth orbit would have had the equivalent of a dental X-ray. Six years later, the Earth was irradiated by a magnetar outburst 100 times more powerful.
The reviewer is referring to the fact that cosmologists and astrophysicists have begun wondering, of late, just why we’re still here. They are beginning to understand that the universe is indeed a very dangerous place, and it seems less and less likely that life can begin on a planet and continue to grow and develop for nearly 4 billion years without simply being destroyed by stellar events in nearby star systems… “nearby” meaning 6,000 light years or so. Call it 36,000,000,000,000,000 miles or so. Within that range certain kinds of supernovae, gamma ray bursts, and more exotic things have the capacity to threaten whatever life may exist.
Even a standard, every day supernova may be dangerous within a hundred light years or so, depending on the size of the supernova. There may be three to five supernovae per century in Milky Way size galaxies. Estimates vary, but that’s a typical current guess. Modern astronomy hasn’t existed long enough to develop a baseline through direct observation of any single galaxy, but by observing 100 comparable galaxies for 10 years, astronomers can develop estimates that might be equivalent to watching one galaxy for 1000 years. So if in that period, observers see 30 supernovae, they can guess that 3 per century might be a reasonable estimate. But it’s early times yet. We’ll know a lot more in the coming decades.
A few minutes with a calculator will suggest that there have been maybe 100 million supernovae in the Milky Way galaxy since there was life on Earth. The galaxy is “only” about 100,000 light years in diameter. Supernovae will certainly be more common where stars are more densely packed. Even so, is it so unlikely that one of those supernovae could easily have exploded near enough to Earth to kill its lifeforms? Maybe more than once?
4 billion years is a LONG time. Remember when Carl Sagan soothed us with the naturalist fable about “billions and billions of years” and “primordial soup” as an explanation for life’s origin? Of course, now we know that was just a comforting materialistic bedtime story so children would go to sleep knowing that they really were just statistical accidents in space-time, and dream nice dreams about quarks and unlikely DNA. Now we know there was no “primordial soup.” We know that life appeared on Earth almost immediately from the moment earth’s temperature dropped below something like the interior of a jet engine.
And now the shoe is on the other foot. Instead of the huge length of time being an argument for the accidental, spontaneous origin of life in some kind of improbable tango dance of amino acids, that enormous timespan is getting very difficult to explain as even being possible for life to have continued without destruction by nearby stellar events. Supernovae are rare. But, as Carl Sagan would say, when you have billions and billions of years, anything can happen… and usually does. So why hasn’t it happened to Earth?
Here’s a current candidate for the job of terrestrial hitman. This sort of thing is probably very rare. But in 4 billion years, how rare does something have to be to happen now and then?
Just as important as the question of how life arose in a geological eyeblink after the end of the Late Heavy Bombardment is this: why hasn’t life on Earth been wiped out, over and over, in the 3.9 billion years since it began?
Maybe Someone is looking out for us?
Remember to say “thank You” during your bedtime prayers tonight. I know I will.
The next post in this series is here.
