At least, according to a couple of astrophysicists: Complex life may be possible in only 10% of all galaxies | Science/AAAS | News.
The universe may be a lonelier place than previously thought. Of the estimated 100 billion galaxies in the observable universe, only one in 10 can support complex life like that on Earth, a pair of astrophysicists argues. Everywhere else, stellar explosions known as gamma ray bursts would regularly wipe out any life forms more elaborate than microbes. The detonations also kept the universe lifeless for billions of years after the big bang, the researchers say.
…The sheer density of stars in the middle of the galaxy ensures that planets within about 6500 light-years of the galactic center have a greater than 95% chance of having suffered a lethal gamma ray blast in the last billion years, they find. Generally, they conclude, life is possible only in the outer regions of large galaxies. (Our own solar system is about 27,000 light-years from the center.)
Things are even bleaker in other galaxies, the researchers report. Compared with the Milky Way, most galaxies are small and low in metallicity. As a result, 90% of them should have too many long gamma ray bursts to sustain life, they argue. What’s more, for about 5 billion years after the big bang, all galaxies were like that, so long gamma ray bursts would have made life impossible anywhere.
This is sobering when considering how much life might be in the visible universe. It doesn’t really change the possibility of life on the exoplanets in our neighborhood of the galaxy. I still tend to think we’ll find evidence of life in the light spectrum reflected off one of those exoplanets within a few decades. And there’s this caveat in the article:
But are 90% of the galaxies barren? That may be going too far, Thomas says. The radiation exposures Piran and Jimenez talk about would do great damage, but they likely wouldn’t snuff out every microbe, he contends. “Completely wiping out life?” he says. “Maybe not.” But Piran says the real issue is the existence of life with the potential for intelligence. “It’s almost certain that bacteria and lower forms of life could survive such an event,” he acknowledges. “But [for more complex life] it would be like hitting a reset button. You’d have to start over from scratch.
Most of my regular readers will know that I already tend to think that microbial life is the most prevalent in the universe, that complex life is rare, and that, due to the Fermi Paradox, intelligent life is profoundly rare. Having biospheres periodically purged every few hundred million years throughout most of the universe probably just makes complex and intelligent life orders of magnitude rarer yet.
I usually say that our closest neighboring civilization may be in another galaxy. If these findings stand, it might be more likely that they’re hundreds of millions, if not billions, of light years away. Of course, it’s also possible that civilizations arise more often than I’m thinking, but that virtually all of them get wiped out from a gamma ray burst before they get a chance to spread.
Either way, the chances of us ever meeting any of them appear to be increasingly unlikely.
The article finishes with some possible advice for SETI:
The analysis could have practical implications for the search for life on other planets, Piran says. For decades, scientists with the SETI Institute in Mountain View, California, have used radio telescopes to search for signals from intelligent life on planets around distant stars. But SETI researchers are looking mostly toward the center of the Milky Way, where the stars are more abundant, Piran says. That’s precisely where gamma ray bursts may make intelligent life impossible, he says: “We are saying maybe you should look in the exact opposite direction.”