Complex life in the universe may be much rarer than previously thought

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.”

21 thoughts on “Complex life in the universe may be much rarer than previously thought

  1. This analysis lacks imagination. The deepest criticism I have is that it limits the definition of life or emergence of intelligence to what we are familiar with. Something we might ultimately classify as a living intelligence could arise in a configuration that is not susceptible to radiation. Without even going that far, it precludes the possibility of biotic mechanisms for radiation resistance in complex organisms.


    1. Thanks for stopping by. The problem with that type of observation is you can make it about any analysis. We never know everything we don’t know. Any logical extrapolation is limited by what we have experience with. There could be life or intelligences out there composed of dark matter, but we have no way to assess that possibility. We just to have to humble about the limitations of these types of analyses.


      1. That’s a fair critique, but even sticking with what we’re familiar with, I think things needn’t be so grim. If we posit that complex life arose from microbial life, and that microbial life can evolve to to have biotic mechanisms for radiation resistance (such as Deinococcus radiodurans with Fenton chemistry), and that such events as described would eliminate microbial life lacking some means of resistance, why is it necessary that any life arising from those remaining microbes must not have any mechanism for surviving bursts of radiation? Sticking purely within the realm of what we have observed with terrestrial extremophiles paired with biomes with selective criteria that necessitate evolution of highly radiation resistant phenotypes, the analysis is still unimaginative.


        1. To add a little clarification, radiation resistance was never a prerequisite for complex life as we know it. It was never an evolutionary advantage, and a resistance mechanism might be a disadvantage in the absence of selective criteria (the need to survive radiation). To say that having not observed such a phenotype in an environment in which said phenotype was never the difference between survival and extinction demonstrates with any modicum of finality that gamma ray bursts preclude evolution of complex life as we know it (or close enough to not be the hand-waivey tautology you called out) is unrealistic in my opinion.


        2. They were pretty open that a GRB wouldn’t sterilize a planet, just kill off most, if not all, of the complex life. Think about what would be left if only the extremophiles were left on Earth. I didn’t quote it, but the article notes theories that the Ordovician extinction might have been from a GRB, where 80% of species were wiped out, but not 100%.

          The problem is that a GRB only happens every few hundred million years, so it’s almost always going to be a disruptive event for an environment. Now, if the life were in an environment where it was constantly bombarded by radiation, where it evolved in that environment, I might could see it being hardened against the results of a GRB. But we just don’t know how successful a complex biosphere could be in such an environment.

          Liked by 1 person

          1. This didn’t seem to go anywhere, which is unfortunate. Perhaps, “This analysis lacks imagination,” wasn’t the best opening line, but I do agree with DisposableScientist’s point. That we’re so badly damaged by ionizing radiation is one thing. Assuming all forms of life are is quite another.

            It may well be that, as you approach galactic center, the background radiation levels get higher providing a stress on any organisms that try to evolve there. We’re also protected by a powerful magnetic field — what if life tried to evolve on a planet with a much weaker field that allowed in some radiation from the local star?

            SAP, I know you believe consciousness is just a process. What about that same process in some crystal or metallic form not terribly bothered by hard radiation?


  2. It sounds like a plausible explanation of the Fermi Paradox, assuming that they got their gamma ray physics right. I find it a much more optimistic scenario than the usual “intelligent species always kill themselves or each other” explanation. Unless there’s a gamma ray burst close to us, life on Earth has a real chance of surviving.


    1. I think complex life has a good 500 million to 1 billion years ahead of it on Earth. After that, the increasing heat from the sun will make conditions progressively harsher. Past that point, it seems unlikely that anything other than the extremophiles will be around. Assuming of course, some intelligent species doesn’t find a way to alter Earth’s orbit, or that they haven’t migrated themselves and a substantial portion of the animal kingdom elsewhere.


  3. I’ve heard that there is a “Goldilocks Zone” in galaxies just as there is in solar systems. Too far in, and there’s too much radiation. Too far out, and the star systems are metal-poor. We happen to be in that goodness zone that’s just right.


    1. “SAP, I know you believe consciousness is just a process. What about that same process in some crystal or metallic form not terribly bothered by hard radiation?”

      I do believe that consciousness is an information architecture, just like an operating system or a word processor. We don’t understand that architecture yet, but I tend to think we will, eventually.

      So, given that, I would think it could arise anywhere there’s a sufficient substrate. The problem is that substrate itself has to evolve and needs adequate chemistry, at least until a species has a chance to move to a post-biological stage (assuming that’s possible). There seems to be disagreement among astrobiologists on whether life could plausibly evolve using silicon, which as I understand it is considered the only realistic alternative to carbon. And from what I understand of the chemistry, silicon life might take a much longer time to evolve.

      Which is all a long winded way of me saying, I don’t know 🙂

      On Goldilocks zones, I’ve heard the same thing. This analysis seems like a particular instance of that overall theory. Of course, this might effectively just be a Goldilocks zone for life that is anything like us.


      1. Indeed. And, as I think you know, I do come down on the Rare Earth Hypothesis side (I’ve been kind of playing Devil’s Advocate here because, as you say, we just don’t know), and it wouldn’t surprise me at all if the chances of intelligent life arising in a galaxy aren’t less than 1 per galaxy.

        Liked by 1 person

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