In Science Fiction by Brian Koberlein11 Comments

Aliens are probably the most common topic of science fiction. They are typically an extension of our hopes and fears. Wise parental figures, evil enemies, noble savages, fierce predators. They are often physically quite similar to us, with a bipedal gait, opposable thumbs,etc. We dream of life on other worlds. Reaching out to the stars and meeting an alien intelligence. But is that likely, or even possible.

The difficulty with that question is that we currently only have one example of life in the universe, and that is us, the commonly descended family of life on Earth. Beyond that, there is a quite a bit of guesswork. It is at this point that the Drake equation is often invoked.

The Drake equation is often interpreted as a way to calculate the number of intelligent civilizations in the galaxy. It was originally proposed at the first SETI conference by Frank Drake in 1961 as a way to stimulate discussion. Drake did not intend it as a prediction of the correct value, but more as a “what if” to consider.

The equation itself is basically a product of the rate at which stars form in our galaxy, how many stars have planets, how many planets they typically have, what fraction are habitable, what fraction of habitable planets form life, how many form intelligent life, then civilizations, and how long those civilizations last.

When it was first proposed there weren’t any known extrasolar planets. We now know that planets are quite common, and stars are more likely to have planets than not. Current estimates calculate that about 1 in 3 Sun-like stars have terrestrial planets in their habitable zone. That means there may be 100 billion potentially habitable planets in our galaxy alone.

Of course “potentially habitable” doesn’t mean “has life”, only that it has the right temperature and orbits a stable star. This is where we have to start speculating. For life similar to ours there needs to be liquid water on the planet. Earth has liquid water, and we know Mars had liquid water in its youth, but we don’t know how likely it is for a potentially habitable planet to have liquid water.

If there is liquid water on a planet of the right temperature, how likely is it that life will appear? On Earth we know that life appeared quite early in its history. This hints that life is fairly likely to appear, but with only an example of one, we can’t read too much into it. Early Mars had liquid water, and even if life does or did exist on Mars it is not robust, which could indicate early life is quite fragile. Again we’re faced with a lack of information.

When life appears on a planet, how often does intelligent, technological life appear? It did on Earth, but does that mean civilizations are a near certainty, or are we the product of extraordinary circumstances. And how long does a technological species survive? How long do you think human civilization will survive? Decades? Millenia? Eons?

So while habitable planets appear to be common, we don’t know anything about how common life might be. If we assume that Earth is a relatively average terrestrial planet, then it would imply that life exists on billions of worlds. One would expect at least some of them lead to technological civilizations, so there could easily be hundreds or thousands of civilizations in our galaxy alone.

This leads to a bit of a puzzle, since one would assume that a technological civilization would eventually start exploring the stars. Assuming humanity survives for a million years, it would seem likely that we will explore at least a portion of our galaxy. If not ourselves, then through our robotic proxies. If we are typical, then there are civilizations a million years behind us technologically, and ones a million years ahead. So if civilizations are common, then why haven’t they made contact with us? (Yes, there are those who think they have, and apparently are highly interested in our body cavities, but there’s no evidence for that.)

This is known as Fermi’s paradox. If intelligent life is common, then why don’t we see it? Several solutions have been proposed. Perhaps civilizations have a very short lifespan. Once they are capable of space travel they nuke themselves, or pollute their planet, or form an idiocracy and go extinct. Maybe there is a vast galactic civilization, but contact with Earth is forbidden until we’ve proved our worth. Maybe we’re in the cosmic equivalent of the outback, and no one has happened to stop by.

Or maybe we’re the only civilization in the universe. Perhaps Earth is extraordinarily rare. Perhaps the appearance of life, much less intelligent life, requires such an improbable chain of events that Earth may be the only example in the universe.

As it stands, we have only a single example of life in the universe. Only one planet in the universe with an extraordinary diversity of creatures. One example in a galaxy of billions of stars, in a universe of billions of galaxies.

If we were to find a distant planet like Earth we would be awed by its complexity and beauty. We would long to communicate with its intelligent species, and learn about its diverse culture.

Look around, make contact.


Miss the beginning of the series? You can find it here.


  1. On your point about how we categorize life, we only know about how it can exist on Earth. We aren’t sure on how exactly it can form, so who’s to say if life can exist off of different fundamental molecules, instead of water and carbon? It would be improbably as carbon, nitrogen, hydrogen and oxygen are our building blocks to life. What if life could form off of different building blocks? Or have different needs to survive. What if other lifeforms breathed off of methane? Life on other planets could have formed off of completely different principles, we only know of one case of how it can prosper, but what if it can exist in many forms?

  2. Personally, what really shatters the Drake equation for me is the ratio at which “intelligence” arises in a planet with life. Our own planet has had life for billions of years, and it has had complex, multi-cellular life for hundreds of millions of years. Yet so far it has only produced one “intelligent” species in such long time-span. And as we discover more and more exo-planets, I think the big issue will no longer be whether or not biological life is common, but whether or not intelligent life (the one we’re interested in talking with in the fist place), is common.

    I like what David Brin usually says to this point. He says that the real problem in all of this lies in the difficulty to do a qualitative jump from mere intelligence all the way into technological sentience. He points out how many “intelligent” creatures exist right now on Earth. Species like chimpanzees, dolphins, octopuses, gorillas, dogs, parrots (and perhaps even theropods in the dinosaur age), all have rather similar levels of intelligence yet all fail to go into the next step. Life on Earth is apparently good into making smart creatures, but it has found it really difficult to make technological ones. Perhaps that’s the bottle neck we should really be worried about.

    Just a thought 🙂 great article as always!

  3. What threshold should interstellar distances surpass, to become a plausible cause worth citing to the Fermi paradox? Aren’t we perhaps just fooled by overconfidence in our own eventual capacity to physically traverse to the closest stars? Or by a delusion that we possess a solid enough incentive to invest the efforts required?

  4. Brian Koberlein

    A friend suggested that if there are millions of planets with intelligent life, we would already have heard their electromagnetic transmissions. Is that a plausible evidence of the rarity of intelligent extra-terrestrial life?

    BTW, I really appreciate your posts.

    1. Author

      We wouldn’t necessarily hear them unless they directed a strong radio signal directly toward us. It’s often said that our radio signals have traveled light years, which is true, but there is also a great deal of radio noise out there, so it would be very difficult to detect across light years.

      1. I’ve heard we have started to use lasers to try and contact any intelligent life is this true? Your post are really educational and is there anything you don’t know? Lol

    2. Of course your friend is assuming that they are using radio signals to communicate. We are using radio signals about 100 years by now and we are already looking for more efficient ways to communicate.

      What if we find a better way to communicate within 50 years? This would mean we used radio signals only about 150 years. Comparing how long we (humans) walk on this planet to 150 years, 150 years would pass just like a blink of an eye.

      This would also mean our radio signals traveled only about 150 light years. Comparing to cosmic scale it is just a very small step. Of course it will be still traveling even we would stop using radio signals however adding the fact (as Brain Koberlein mentioned) across light years, it would be difficult to detect. Eventually after some point it will be just impossible to detect.

      P.S. Great post and a read Brain Koberlein. Thank you for sharing.

      1. Impossible to detect or impossible to decipher all the noise? How many of these possible habitable planets are within 100 light years that might of picked are signals up?

  5. To take it by the other end, the idea of contact itself deserves critical contemplation. Define a variable that as value has the typical way for an intelligent civilization like ours, to obtain positive sign of the existence of others. This helps to consider extremes of the range, ways for our expectations of the times and distances involved in contact, to be beaten. For instance, it can be imagined that a thousand years of parallel observations of patterns of stellar explosions in faraway galaxies, would be the typical way for an intelligent race to gain certainty of the existence of others – mired in interstellar wars occurring millions or billions of light years away. Even more extreme would be if “contact” occurred by way of reaching the conclusion that dark energy is a sign of ambient pollution of the Universe by Alcubierre drives – quintessence produced by some engineered process and then leaked in the environment over eons by countless civilizations all over the Universe!

    My favorite though is to substitute a mathematical equivalent to contact. Get closer to ET by figuring out possible alien curricula of mathematics education; assume variance in the possible physical environments of races with mathematicians to be such that the historical sequence of discovery of elementary mathematical notions differ, scrutinize known mathematics for junctures where we always teach ourselves one thing before the other but such that it would be imaginable for alien mathematicians to teach themselves the things in reverse order, and then raise bunches of such alien mathematicians as earthlings, as humans, after devising “biotopes” for them – adapted schools.

  6. I read an article about 40 years ago entitled “Where is Everybody?” written by Isaac Azimov, I think. Posed basically the same question, and answered it by citing the vast distances involved and the almost infinite number of places alien civilizations might explore besides where we might happen to live.

    There is also the funding issue… it is only fair to assume that aliens have other demands on their resources, and may face some hard choices on how to make best use of them. If they prioritize space exploration much as we do, no WONDER we haven’t seen much of them.

  7. The only civilizations we could make contact with would almost inevitably be at least a little, and probably a lot ahead of us. Maybe radio communication is, for them, “totally retro”. It would be like a tribe in the Amazon rain forest trying to make contact with us using drums, old example I know. It is also very possible that they consider a species so amazingly primitive that they still think smart phones are a really neat idea, to be far too boring to bother contacting. I have the greatest respect for Chimps and other primates, but I wouldn’t want to discuss Quantum Physics with one. So maybe one day, when they consider us worthy, read ‘interesting’ they make make contact or allow us to contact them.

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