Sciency Words A to Z: Rare Earth Hypothesis

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, R is for:


Once upon a time, it was believed that the Sun, Moon, planets, and all the stars revolved around the Earth.  This was known as the geocentric theory.

Copernicus, Galileo, Kepler, and others set us straight about our planet’s physical location in space.  However, it is still sometimes asserted that Earth is special or unique in other ways.  Such assertions are often referred to in a derogatory sense as “geocentrisms.”

It’s tempting to dismiss the Rare Earth Hypothesis as just another geocentrism.  The idea was first presented in 2000 in a book called Rare Earth: Why Complex Life is Uncommon in the Universe by Peter Ward and Donald Brownlee.  In that book, Ward and Brownlee go through all the conditions they say were necessary for complex life to develop on this planet.  Crucially, they point out all the ways things could have gone wrong, all the ways complex life on Earth could have been prematurely snuffed out.

In other words, we are very, very, very lucky to be here, according to Ward and Brownlee, and the odds of finding another planet that was as lucky as Earth must be astronomically low.  Sure, there might be lots of planets where biology got started. Simple microorganisms may be quite common.  But complex, multicellular life like we have here on Earth—that’s rare.  And intelligent life forms like us are rarer still.  Perhaps intelligent life is so rare that we’re the only ones.

My favorite response to the Rare Earth Hypothesis comes from NASA astronomer Chris McKay.  In All These Worlds Are Yours, McKay’s argument is described as the Rare Titan Hypothsis.

Imagine intelligent life has developed on Titan (such a thing seems unlikely, I know, but there may be something living on Titan).  Titanian scientists look through their telescopes and soon realize that no other world in the Solar System is quite like their own.  Earth, for example, if too hot for life as the Titanians know it, and there’s far too much of that poisonous oxygen in the atmosphere anyway.  Furthermore, water would wreak havoc on what the Titanians would consider a biomolecule.

Perhaps a pair of Titanian scientists then decide to publish a book.  They list all the conditions required for complex life to develop on Titan, point out all the ways Titanian life could have been snuffed out prematurely, and argue that the odds of finding another Titan-like world must be astronomically low.

Personally, I think there’s some validity to the Rare Earth Hypothesis, but McKay’s point is worth bearing in mind.  There could be many different ways for life to develop in our universe.  Earth is but one example.  Planets that are just like Earth may indeed be rare—extremely rare—but there’s no reason to conclude that Earth-like life is the only kind of complex life out there.

Next time on Sciency Words A to Z… oh my gosh, we’ve finally made it to S!  It’s finally time to talk about SETI!

20 thoughts on “Sciency Words A to Z: Rare Earth Hypothesis

  1. It looks like the time window for life to exist on Earth will be about 5 billion years. Within another billion or so, the increasing luminosity of the sun will make life impossible.

    I think the fact that it took 70% of that time window for complex life to develop tells us that it is probably at least somewhat rare, although perhaps not as rare as the Rare Earth Hypothesis proponents think. As you note, there may be many alternate pathways, even on Earth like planets, for complex life to arise.

    What is probably profoundly rare is intelligent life capable of building civilizations. There are probably multiple pathways for it too, but so far on Earth, a species with both the intelligence and dexterity to sufficiently manipulate the environment has only happened once, and but for a few chance events, may never have happened at all.

    Liked by 3 people

    1. The real question in my mind is how rare is rare? Is Earth part of the 1%, or the 0.01%, or the 0.0000001%?

      In a sense, the Rare Earth Hypothesis is just another version of the Drake Equation. The number of civilizations that we can communicate with is limited, but right now it takes way too much guesswork to determine an exact number.

      Liked by 3 people

  2. There is of course a sense in which other creatures have manipulated the environment and built civilizations long before us. Bees build hives and live in colonies. They mine pollen from flowers and manufacture honey in purpose-built factories. They even developed division of labour to make the process more efficient. Those hives look like cities to me. Then there are birds, ants, termites, etc. But none of them build space ships or radio telescopes. Why would they? So it’s always possible that even quite complex intelligent life could exist on many planets, but none of them would do what we expect from “aliens”. And it’s equally possible that we don’t do the kinds of things that intelligent aliens do.

    Liked by 5 people

      1. You’re right, even if Earth-like planets are rare, there may still be a lot of them. And remember that Rare Earth book came out in 2000. We’ve discovered a whole lot of exoplanets since then, so the odds of finding another Earth may be better than initially thought.

        Liked by 3 people

    1. Fair points. I’ve been reading a lot of old SETI research for this series, and it’s been interesting how much scientists in the 60’s and 70’s thought they knew about how aliens would behave. They were very confident that alien civilizations would communicate by radio, and they even thought they knew which radio frequencies the aliens would most likely be using. More recent research seems much more openminded about what aliens might be doing, but this presents a problem because we don’t really know what we should be looking for.

      Liked by 2 people

      1. That would nicely explain why we haven’t picked up their transmissions yet. I haven’t read up on the neutrino communications idea, but I’m wondering what the advantage to that would be for the aliens—aside from keeping up from listening in, of course.


      2. They are tricky to detect but can pass through almost everything at light speed and Neutrino are only very weakly influenced by gravity. A neutrino beam could easily pass through 1000 light-years of lead, or so I’ve read. In 2011 a lab thought they discovered that the ghostly particle were faster than light, but that finding was subsequently debunked by several other groups. Still a great way to talk compared to EM waves.

        Liked by 1 person

    2. The structures that bees, ants, and birds build, not to mention beavers, are amazing. And ants even farm! But this activity appears to be entirely instinctual. They didn’t design those structures, natural selection did. So they don’t really have the ability to improve on them, at least other than through additional evolution.

      But definitely there’s no guarantee another civilization would be at all interested in the things we are. They would be products of evolution, which implies some inevitable similarities, but evolution always surprises us with the bizarre (from our point of view) solutions it comes up with.

      Liked by 2 people

    1. I’m no scientist either, but the general idea makes sense to me. It’s when they get into specific numbers that I think the Rare Earth Hypothesis runs into trouble. There’s just too much we don’t know yet, and even our best educated guesses are still guesses.


  3. Indeed, Earth might be rare, though it doesn’t really seem to be, last time I heard something they already knew several exoplanets with water on them. So far there are at least three other places in our own solar system that show signs of possible life forms. If Titan really does have any kind of life at all, life would probably be swarming all over the universe, since even water and oxygen would be insignificant. To me it seems life is just complex matter and a natural way to develop from dead molecules just as molecules develop from atoms, whenever the conditions are there. But what do I know, I just write fiction 😀
    Nice illustration, by the way. Humans have been thinking that way ever since we started thinking, it seems. Then suddenly the elephants show off their ability to simple arithmetic calculations.

    Liked by 1 person

    1. It really is a question of how rare is rare, isn’t it? That Rare Earth book came out in 2000, and my understanding is that a lot of the specific details in that book have turned out to be incorrect. We’ve found more exoplanets than the authors were expecting, for example. So the odds of finding alien life are still low, but not as ridiculously low as the Rare Earth Hypothesis suggests.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.