There’s been a lot of news lately about the estimate of habitable planets in our galaxy. It sounds like there may be 40 billion planets in the habitable zones around their stars. This is cool stuff! The nearest one might be only 12 light years away.
It’s very tempting from this news, to conclude that life is pervasive in the galaxy, and that extraterrestrial civilizations themselves must also be pervasive. However, it should be remembered that we don’t know two things. First, given the right conditions, how likely is life to arise? Second, once life has arisen, how likely is it to evolve into intelligent life with a technological civilization? These are some of the components of the famous Drake Equation.
All we currently have to go on for any guess at the answers is our own planet. Now, any scientist or researcher will tell you that drawing conclusions from a sample size of one is perilous. And they’d be right. However, it’s the only thing we currently have to work with. There is some hope in the fact that science has mostly demonstrated that we’re pretty ordinary, and that Earth may be a somewhat typical planet in its star’s habitable zone. And anyway, this kind of speculation is fun.
Life started relatively early on Earth. The earliest fossils date to around 3.5 billion years ago and are generally judged to be too sophisticated to be the earliest versions of life, indicating that it more than likely started within a few hundred million years of the Earth cooling sufficiently. This would seem to indicate that, given the proper conditions, the chance of life starting is fairly high. Of course, life starting on Earth might have been a freak accident, both in terms of the event itself and its timing. However, if we are actually typical, then life, at least simple microscopic life, probably is indeed pervasive in the galaxy. Even if we’re conservative here and say that only 1% of habitable worlds have life, that would still leave 400 million of them.
Things become murkier when we talk about complex life. Complex life, that is, multicellular animal life, didn’t arise in the first 80-85% of the planet’s history. In other words, on geological time scales, animal life is a recent event. We really have no idea how likely or unlikely it was. Still, it did arise several hundred million years ago, and that seems like a good indicator that, although it might be somewhat rare, it wouldn’t be nonexistent.
But when we talk about intelligent life, things become more stark. If we’re generous with the label ‘intelligent’, then it didn’t arise on Earth until it was 99.9% of its current age. If we narrow from intelligence to civilization (again being generous), then we’re at 99.99978% of Earth’s current age. In other words, intelligence and civilization are very recent phenomenons, a blink of the eye in geological and astronomical time. We can’t really determine anything about the probability of intelligence arising, only that it took a very long time on our planet. If we’re conservative here, then intelligent life is probably exceedingly rare. But is conservatism called for? Am I being just too pessimistic?
Anyone who thinks they have reason to be optimistic, has to consider Fermi’s paradox; if extraterrestrial civilizations are pervasive, then where is everyone? The Earth is around 4.5 billion years old, but the universe is around 13.8 billions years old. If civilizations really are prevalent, then the first successful ones should have arisen long ago and colonized the entire galaxy. If so, we should see evidence of that colonization at some point in Earth’s geological history. But there is none, at least not any that is widely accepted, despite what UFO enthusiasts or the Ancient Aliens people say.
There are many possible answers to Fermi’s paradox. Here are a few:
- It’s in the nature of intelligent life to destroy itself. This seems a bit pessimistic, but I don’t see any way to dismiss it as a possibility. Note that ‘destroy’ doesn’t necessarily mean blow ourselves up; we might just stagnate once all of life’s challenges are conquered, as speculated by Sean Carroll.
- Interstellar travel is impossible, or so monstrously costly that no one bothers. Possible, but given the time scales, even if travel at a (relatively) pokey 1% of light speed is the best anyone can hope for, that’s enough for a species’ self replicating probes to colonize the galaxy within 100 million years or so.
- Everyone obeys a prime directive (à la Star Trek). Possibly, but ask yourself how likely such a regime would be to hold across thousands of purported civilizations, across billions of years. Again, we have no evidence of colonization in the history of the planet.
- Earth is in a backwater portion of the galaxy. Please see discussion for the second item.
- Berserkers. Some intelligence exists in the galaxy that hunts down and destroys intelligent life, either to kill potential competitors or as a malfunctioning process; it is only a matter of time before it finds us. A dark possibility, but I suspect one that is basically a fear projection onto the universe.
While some of these can’t be dismissed completely, ultimately I think we have to consider Occam’s razor, and look hard at the simplest explanation, the one apparently backed up by Earth’s history, that intelligent life is exceedingly rare. So rare, that the nearest extraterrestrial civilization is so far away that it hasn’t had time to reach us yet. We may be alone in our galaxy, at least for now.
- Galaxies can’t be overrun by bored, suicidal extraterrestrials (motls.blogspot.com)
- Novel Strategy May Help Target Extraterrestrial Intelligent Life (talesfromthelou.wordpress.com)
- Billions and Billions of Planets (norberthaupt.com)
- First step into the unknown (lifepastinfinity.wordpress.com)
- Discoveroids Disparage SETI Again (sensuouscurmudgeon.wordpress.com)
- They aren’t talking (aliensilence.wordpress.com)
- Can Giant Telescopes Help Answer Fermi’s Paradox? (33rdsquare.com)