Galactic Census Report: How Many Civilizations Are in Our Galaxy?

Hello, friends!  Have you heard the news?  Scientists have determined that there should be at least thirty-six alien civilizations in our galaxy right now.

Here’s the actual research paper from The Astrophysical Journal (warning: paywall).  As you might imagine, this research is based on some key assumptions.  And the authors do make it clear that they are making assumptions.  Reasonable assumptions, they argue, but still… ASSUMPTIONS!!!

The first major assumption is this: any Earth-like planet with an Earth-like chemical composition that happens to have an Earth-like orbit (i.e.: habitable zone) and has been around for an Earth-like period of time (approximately 4.5 billion years) has a reasonably good chance of developing Earth-like intelligent life.

The second major assumption is this: once a civilization advances to the point that it can start broadcasting its presence to the rest of the universe, that civilization will also have advanced to the point of being able to destroy itself.  Earth-like intelligent life has a tendency, the authors argue, for self-destruction.  Maybe it’ll be nuclear weapons, or maybe a climate catastrophe of some kind.  Or, I don’t know, maybe an increasingly globalized society will make itself more vulnerable to some sort of global pandemic.

Obviously the authors make other assumptions as well, but those two are the big ones.  When plugging numbers into a modified version of the Drake Equation, the most pessimistic assumptions yield an estimate of 36 civilizations in our galaxy (with a margin of error that could push that number all the way down to 4).  The most optimistic assumptions yield an estimate of 928 civilizations (with a margin of error that could push the number all the way up to 2908).

As I’ve said before, scientific papers should never be taken as proclamations of absolute fact.  That’s especially true for papers like this one.  Scientific papers are part of an ongoing back-and-forth conversation in the scientific community.  What do we currently know?  How much do we still have to learn, and what should our expectations realistically be?  That’s what this paper from The Astrophysical Journal is really about: setting expectations for SETI research.

So what should our expectations be, based on those two key assumptions the authors made?  Well, even under the most optimistic scenarios, our nearest neighbors are predicted to be hundreds (if not thousands) of lightyears away—far enough away that they’d be very, very, very difficult to find using our current technology, and establishing two way communications would be virtually impossible.

So maybe we’re not alone in the universe, but we may as well be.

5 thoughts on “Galactic Census Report: How Many Civilizations Are in Our Galaxy?

  1. Given what we know about Earth’s evolutionary history, I’m not sure it’s reasonable to even assume complex life will always develop (it took 4 billion years on Earth), much less a civilization producing intelligence. If we adjust for that, the picture becomes much bleaker.

    Liked by 2 people

    1. I agree. My main take away from that paper was that even with the most generous of assumptions, our chances of finding another civilization out there are vanishingly low. And then if the rare Earth hypothesis is even partially true, the galaxy really starts to feel like an empty and lonely place.


      1. It seems reasonable to think that simple life is pervasive, although a lot will depend on what we ultimately find in the solar system. Complex life may be few and far between, although if only one in ten biospheres have it, that might still be a lot in the galaxy.

        But yeah, likely lonely in terms of other civilizations.

        Liked by 1 person

  2. A couple of decades ago we were being told the galaxy is teeming with life. Despite a massive radio search we have found no evidence of it.

    It is good to see scientists have since toned down their over-optimistic expectations.

    The paper you refer to was I believe from a team of English scientists from Nottingham University. They found that the probable number of communicating extra-terrestrial intelligent civilizations is likely to be 36, with an “uncertainty” between 4 and 211.

    In 2018 a team from Oxford University published a paper in which they conclude: “We find a substantial probability that we are alone in our galaxy, and perhaps even in our observable universe.”

    Even if they don’t agree, it is good to see scientists dialling back on their expectations.

    Whilst primitive life seemed to form fairly quickly on Earth, that does not mean it occurred elsewhere – and even if primitive life does form elsewhere, there is no pre-ordained inevitability that it will eventually develop an evolutionary branch that becomes an advanced society with the ability to communicate beyond their own environment.

    I lost my initial optimism a long time ago and expect other intelligent civilisations to be either rare or non-existent. That is all the more reason to continue the search and so it is disappointing that Seti@home has ceased offering work to citizen science after twenty years.

    Liked by 2 people

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