The Common Europa Hypothesis

Hello, friends!  So Europa month ended a while ago, and I haven’t done much blogging since then.  Sorry about that.  I’ve been distracted by other writing projects.  But I now have some blog time in my schedule again, and I’m ready to blog about some new topics.  Except… I can’t help myself.  There’s one more thing I want to say about Europa.

I have this crazy idea.  I haven’t found much scientific literature to support me on this, but I still think this idea makes scientific sense.  I think that Europa—or rather, Europa-like worlds—may offer a solution to the Fermi Paradox.

For those of you who don’t know, in 1950, Italian physicist Enrico Fermi famously asked “Where is everybody?” in reference to extraterrestrial life.  Fermi argued that alien life should be all around us.  Almost everywhere we look in the cosmos, we should find alien beings waving back at us.  And yet, as of 1950, no real evidence of alien life had been found.  And as of today, in 2021, the situation remains much the same.

One possible answer to Fermi’s question came in the form of the rare Earth hypothesis.  Earth-like planets must be few and far between.  To be clear, when I say Earth-like planets in this context, I mean planets that meet the same Goldilocks parameters as Earth: not too hot, not too cold; not too big, not too small; not too wet, not too dry; et cetera, et cetera.  Planets that are so Goldilocks-perfect must be vanishingly rare in our universe.  Like, you could probably count on one hand how many Earth-like worlds exist in our whole galaxy.  So if life needs an Earth-like environment to survive, that may explain why alien life has been so frustratingly hard to find.

But then there’s Europa, the sixth moon of Jupiter.  Could there be life on Europa or on a Europa-like world?  And when I say a Europa-like world, I mean a world that looks like this:

A Europa-like world is a world with an ocean of liquid water covered up (and protected) by a thick shell of ice.  The mantle and core are hot, much like Earth’s, and hydrothermal vents on the ocean floor offer heat and nutrients to any potential life forms that might develop.

With respect to life on Europa herself, I’m 50/50.  There are good reasons to think Europa is habitable, and there are good reasons to think Europa falls just a little bit short of habitability.  But only a little bit.  Conditions on Europa are either just right for life or almost right.  So even if Europa misses the mark on habitability, another Europa-like world could easily hit it.

And here’s the important thing: while truly Earth-like worlds are rare, Europa-like worlds seem to be quite common.  There are at least two of them here in our own Solar System: Europa (obviously) and Enceladus, one of the moons of Saturn.  And there may be more.  In my research, Ganymede (moon of Jupiter), Dione (moon of Saturn), Titan (moon of Saturn), Ariel (moon of Uranus), and Triton (moon of Neptune) have all come up as places with certain suspiciously Europa-like qualities.  Even Pluto may have some liquid beneath her surface.

I’m choosing to call this idea the common Europa hypothesis, as a nod to the rare Earth hypothesis.  I think Europa-like worlds are common, both here in the Solar System and all across the cosmos.  Even if only 1% of these Europa-like worlds support life, that could still end up being an enormous amount of alien life out there.

Getting back to Enrico Fermi’s original question: “Where is everybody?”  Well, between the rare Earth and common Europa hypotheses, perhaps we have an answer.  Aside from us Earthlings and the lucky few who get to live on Earth-like planets, everybody is swimming around in Europa-like subsurface oceans, beneath thick layers of ice.


I suggest reading Exoplanets by Michael Summers and James Trefil.  Among other things, there’s plenty of discussion about all the surprising yet plausible places Europa-like worlds might exist.

19 thoughts on “The Common Europa Hypothesis

  1. I hope there is alien life in our universe and its peaceful. We’d better be ready if its not but lets think about what some of the great mind have said about aliens and there after. Gene Roddenberry was a believer in humanity, aliens and peace also agnostic. Stephen Hawking said the brain was like a computer and once it breaks, then you have Asimov who doesn’t believe in heaven or hell just nothingness once the computer breaks. I’m hoping aliens know something we dont.

    Liked by 2 people

    1. I’m not sure, but being in the habitable zone is not enough for a planet to be habitable. Proxima b, for example, is technically in the habitable zone of its star. But Proxima b is tidally locked, and its star is a flare star. And I’ve read about other exoplanets that fit neatly into habitable zones but sound more like Venus when you start reading about their temperature and chemical composition.

      I guess it’s really a question of how rare is rare and how common is common. Putting numbers on these things is hard. But I think, however many truly Earth-like worlds exist out there, the Europa-like worlds must vastly outnumber them.

      Liked by 1 person

      1. Definitely just being in the habitable zone isn’t enough. I should have been more careful in my wording. It seems like there are three situations with decreasing numbers.
        1. In the habitable zone
        2. In the habitable zone with conditions suitable to life of some type (liquid water, minerals, etc)
        3. In the habitable zone with conditions suitable to current Earth life

        I suspect 1 is largish and 3 is small, perhaps vanishingly small. What I’m wondering about is 2.

        But I agree that, just based on our own solar system, it’s hard to argue that Europa type environments aren’t more common.

        Liked by 1 person

      2. I really like that list. That’s a great way to articulate things. And I agree with you: the number of type 2 worlds is hard to pin down. Especially when you add in the possibility (the remote possibility, perhaps, but still a possibility) of life that uses a liquid other than water. There could be ammonia habitable zones or methane habitable zones to think about too.

        Though I still think that Europa-like worlds would outnumber the type 2 and type 3 Earth or semi-Earth-like worlds. The habitable zone for a world like Europa is anywhere beyond the frost line of a star.

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  2. With respect, I think an element of wishful thinking has crept in with regards to Europa.

    With every planet all but ruled out, the best we can hope for in the Solar System is some form of primitive micro-life on one of the larger satellites. It’s a very long shot and of course it is worth investigating.

    However, whilst there is a hypothesis that the current conditions below the surface of Europa “may” be considered conducive to supporting life forms, it does not necessarily mean that the conditions are (or were ever) conducive to the actual genesis of life in the first place.

    It’s not that long ago that astronomers were predicting swamps and dinosaurs on Venus. 😎

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    1. Fair points, though I think the speculation about Europa is based on somewhat firmer footing than the whole swamp monsters of Venus stuff was. Astronomers today have a pretty good read on Europa’s chemical composition, while astronomers back in the day had absolutely no clue what Venus’s clouds were made of.

      I’ve read speculation that life on Earth might have begun around a hydrothermal vent. If so, then I would think life could get going around a hydrothermal vent on Europa or a Europa-like world, too. I realize that’s speculation on top of speculation: speculation about Europa based on speculation about ancient Earth. But still, a plausible means of abiogenesis is there. That’s not a guarantee that it happened, but it’s plausible to think it could have happened.

      As I said, I’m kind of 50/50 about whether or not Europa is habitable. And even if Europa is habitable, that does not necessarily mean that she’s inhabited. The more important thing, at least for the purposes of this post, is that Europa-like worlds are probably more common than Earth-like worlds. Which means, I think, that there are more opportunities for life to develop beneath Europa-like ice shells than on the open surfaces of Earth-like worlds.

      Liked by 1 person

    2. Actually, astrobiologists have been speculating about the possibility of life in Europa since 1979. Since then, they’ve expanded the list of potential “ocean worlds” to include Ganymede, Callisto, Enceladus, Titan, Dione, Rhea, Triton, and even Pluto and Charon. And this is based on ongoing observations, whereas speculation about Venus was based solely on Earth observation.

      And Mars isn’t ruled out, except for the possibility that life exists on or near the surface. This isn’t a product of wishful thinking, it’s the result of investigation.

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  3. It totally makes sense that, whether there’s life on our Europa or not, Europa-like worlds might be more typical homes for life than Earth-like planets, just given the numbers. I’d forgotten there was actually some possibility of liquid water under Europa’s ice! I was just reading Weird Life by David Toomey, where he talks about a lot about Titan-like worlds as being potential habitats for life that slurps liquid methane instead of water. But we only know for sure that water-loving life exists, so the possibility that Europa-like worlds could be teeming is pretty exciting!

    Liked by 1 person

    1. I haven’t read that one. Sounds interesting. There’s a compelling case to be made for life on Titan as well, and it wouldn’t surprise me if Titan-like worlds also turn out to be more common that Earth-like worlds. Although, as you say, we already know for sure that water-based life works. Methane-life is still, from our perspective, purely theoretical.

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