How many other planets have intelligent life?
Understanding the drake equation
Is there alien life on other planets? Intelligent life?
I think the answer to both questions is probably yes.
There are somewhere between 100 and 400 billion stars in our galaxy.
Some of those stars are like the sun. About 1 in 5 of the sun like stars probably have a habitable planet, something that’s rocky, about the right size, and orbiting at a distance where life could form.
Over-all there might be about 10 billion habitable planets in our galaxy. That gives 10 billion chances for intelligent life to begin.
10 billion is close to the number of people on Earth — if we discovered them all, you could have one of the galaxy’s habitable planets named after you, and so could everyone else on Earth.
Is there life on the planet with your name?
If the odds of a planet evolving life are higher than 1 in 10 billion, then there’s at least one other planet in the galaxy with life on it.
The same is true for intelligent life — if the odds are at least 1 in 10 billion, then it has evolved on one of those planets (or will evolve in the future).
It could be even more than 10 billion habitable planets. Most of the stars in our galaxy are red dwarfs, smaller than the sun:
Those can have planets, too, but we’re not sure if they’re habitable — those have a few problems. The main issue is those planets are too close to the star and that makes them tidally locked, like the moon is to the Earth. One side of the planet would always be hot and bright, the other side dark and cold.
So, it’s at least 10 billion planets, it could be more like 40 billion if those ones can support life, as well.
Going a step further, there are 2 trillion galaxies in the known universe.
That gives life a lot more chances to evolve: 10 billion times 2 trillion chances.
As long as the odds are greater than 1 in 2*10²² of life evolving on a habitable zone planet, life exists somewhere else in the universe.
That’s a tiny number. I don’t think Earth could be that unique. It stands to reason that there is probably life somewhere else, and that some of it is intelligent.
Is any of that life close enough for us to find it?
We can mostly ignore what’s happening in other galaxies, because they’re just too far away to make contact with.
The andromeda galaxy is 2.5 million light years away.
If you could make a spacecraft that flies near the speed of light, it would still take more than 2.5 million years to get there.
Suppose you could build a radio telescope that can detect transmissions from Andromeda (an unlikely proposition) and you found alien signals with it. Those signals were sent 2.5 million years ago. There’s no guarantee the aliens who sent them are still alive. If we tried to have a conversation with them, every message and response would take 5 million years.
There might be some rare ways that we could detect aliens in another galaxy, but only if their civilization was extremely advanced. Suppose that they built a dyson sphere, a gigantic structure that could collect all the light coming from one star. We might be able to find that — the dyson sphere would look different than other stars, maybe we could see that.
But we wouldn’t be able to spot another Earth, that far away. So let’s focus the search only on our galaxy.
We’re back to just those 10 billion planets, or so. How many have life?
The Drake equation
In 1961, astronomer Frank Drake came up with a simple equation to estimate that number.
I’ll present it in a slightly different form as the original. This is the version favored by Carl Sagan:
N is the number of civilizations in our galaxy, and:
- N∗ = Number of stars in the Milky Way Galaxy
- fp = the fraction of those stars that have planets.
- ne = the average number of planets that can potentially support life per star that has planets.
- fl = the fraction of planets that could support life that actually develop life at some point.
- fi = the fraction of planets with life that go on to develop intelligent life (civilizations).
- fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
- fL = fraction of a planetary lifetime graced by a technological civilization
We’ve already guessed at the first 3 terms, and said there are about 10 billion planets. But we also need to know how many develop life, how many develop intelligent life, how many develop civilizations with radio telescopes and rocket ships. And, we need to know how long civilization lasts.
What are the odds of life starting at all?
One group estimated that 1 out of 5 stars have a habitable planet. They defined that as any planet between 1 and 2 times the size of the Earth, orbiting anywhere from 1/2 as close as the Earth to the sun, or twice as far.
Defining that “habitable zone” is difficult, because we only have one habitable planet in our solar system. Most planets in our solar system probably don’t have any life.
Venus would be within that estimated “habitable zone”. We’ve launched probes to the surface of Venus. The probes usually only send back a few pictures before they overheat. The surface of Venus is 864 degrees Fahrenheit (462 Celsius), under a dense greenhouse atmosphere:
Mars would also be within that “habitable zone”, but it’s too cold and too dry. Robots can survive there and explore for a long time, but none of the experiments done by those robots have found a convincing signal of life.
If you had another Earth-like planet, at the same distance from a star, with continents and oceans, the odds might be much better.
An optimistic guess would be that simple life develops on 50% of those “habitable planets”. I think that estimate is likely much too high — in our solar system it seems to be 33%. But we don’t know, maybe Mars or Venus had simple life at some point in the past.
It’s hard to estimate because we don’t know how life starts. Some of the first steps can be reproduced in a lab — simple gasses mixed with electrical sparks create amino acids, the building blocks of protein. The same thing would happen from lightning on a new planet. We don’t know how those go on to form into DNA and enzymes and protein synthesis.
But, if you add water, sunlight, and millions of years, maybe that’s inevitable. There are a huge number of molecules all trying out different combinations. On Earth, simple life showed up relatively soon after the planet cooled to an acceptable temperature.
How many will have intelligent life?
There are still a bunch of other steps that still have to happen before you get intelligent life. Simple cells need to become complex cells. That happened slowly on Earth, but also happened more than once in history, so perhaps it’s also inevitable.
Other key steps have happened multiple times in Earth’s history. For instance, life has transitioned from single celled to multi-cellular dozens of times in the past.
The optimistic view is still that, given enough time, you’ll get complex life any time you have the right starting conditions, like water and mild temperatures.
Other people are more pessimistic. They think that you might need additional factors for it to work out.
Maybe you need tides or tide pools. In that case, you could only count planets with a moon.
The Earth is tilted slightly, causing seasons, but not too tilted too much. Maybe that’s necessary.
The Earth has a magnetic field to shield radiation. The Earth is also inside the orbit of Jupiter, and Jupiter’s gravity reduces the number of asteroids that can hit us.
The most pessimistic take is the “rare Earth hypothesis”, which says that Earth has many unique features that other planets do not, and all of those were necessary for life.
If you do get complex multicellular life, then I think intelligent life is also likely to show up.
I say that because it has evolved multiple times on Earth.
The first highly intelligent life form on Earth might have been the octopus.
Those are already alien, compared to us. They have a very different nervous system — about 2/3rds of its neurons are in the arms, and only 1/3rd in the brain. The arms can act autonomously.
We also know their intelligence evolved independently from ours.
Octopuses existed at least 300 million years ago, likely longer. The common ancestor between an octopus and a human would be some tiny worm, 500 million years ago, with a very primitive nervous system. So the nervous system can grow from simple to advanced, multiple times.
Life has also gone from “low intelligence” to “high intelligence”, on multiple occasions. There are some moderately intelligent birds like ravens. Last common ancestor with us would be some kind of reptile.
There are several smart land mammals, from elephants to apes. We have dolphins and orcas in the ocean. All of those diverged from less intelligent mammals.
Since that has happened many times, it seems fair to say that it’s inevitable that intelligence increases over time.
It’s less clear that intelligence always reaches a human level, or something beyond.
In all the 300 million years in the ocean, octopuses didn’t build much. Dinosaurs had a hundred million years but never built a technological civilization. Other mammals had 60 million years. Only humans have built a technological civilization, with rocket ships and radio telescopes.
Finding octopodes on another planet seems much more likely than finding something like humans. I don’t see any reason why they’d always have 8 arms, but maybe you’d still find smart, tentacled sea creatures.
Drake’s group guessed high odds that life would start and that it would become more intelligent, but only 10% odds that it would reach a human level or beyond, with a technological civilization.
How long will civilization last?
The Earth has been around for 4.5 billion years. Humans civilization has been around for maybe 10,000 years and we’ve had advanced technology for less than 100 years.
The planet could sustain us for another billion years, maybe more. But it’s not that obvious if civilization can sustain itself.
We’ve had nuclear weapons for 79 years. We haven’t had a full scale nuclear war yet, but we could still have one at any time.
And we could still invent something deadlier than nuclear weapons.
Some of those alien civilizations will blow themselves up or die out quickly, some might last a long time.
For us to detect aliens, they have to exist near the Earth, and also at the same time.
The same is true in reverse — if someone in history looked at the Earth, they would have concluded it was uncivilized for most of those 4.5 billion years.
Putting it all together: estimating N in the drake equation
Start with 10 billion habitable planets, and let’s guess:
Odds of some life = 50%
Life becomes complex and then intelligent = 20%
That intelligent life develops advanced civilization = 10%
I think those are all optimistic numbers, they could be much lower. But let’s see where the math leads. That gives 100 million planets that will become civilized, at some point.
Each planet lasts maybe 10 billion years, but it’s only civilized for some lifespan. How many will be civilized at the same time?
That gives us a simple equation:
N = civilization lifespan / 100
If the average advanced civilization lasts for 100 years (about how long we’ve had one), there’s only civilized planet in the galaxy right now, and that’s us. We’ll blow ourselves up soon, but another civilization somewhere else will just be hitting its prime around then.
If civilization lasts for 200 years, then there are 2 planets right now. That scenario is almost more depressing than the first — there are 2 planets in the galaxy, both are currently struggling with climate change before they nuke themselves to oblivion. Even if we knew where the other one was and sent messages, both civilizations would be over by the time the messages got there.
Frank Drake’s group guessed at all the parameters and came up with similar results (they agree with these numbers to within a factor of 2). They were optimistic about life starting but unsure about civilization length — they thought that might last 1,000 years on the short end or 100 million years in the optimistic case.
Other people have since given a wide range of estimates for all these numbers. The pessimistic case is that there’s only us, either because life doesn’t develop often or because civilizations end quickly, or both.
The most optimistic calculations say there are currently more than 10 million civilized planets in our galaxy.
If there are a lot of civilized alien planets, that also means those planets have existed for a long time. And if civilizations don’t last very long, there probably aren’t many alien planets.
In a strange sense, believing in aliens also makes you an optimist about the future, and about civilization surviving a long time.
How far away will those other planets be?
If you pick one other star at random, in the milky way, it’s maybe 37,000 light years from the Earth.
So if there’s only one other civilized planet in the galaxy, it’s still going to be hard to reach, and too far away to have a conversation with. We need there to be more than one other in the galaxy to have one that’s nearby.
How close is the closest one, if there are N planets?
I tried running some simulations where I just distributed N planets randomly across the galaxy:
For 1 planets, the closest is 36,800 light years away.
For 10 planets, the closest is 9,700 light years away.
For 100 planets, the closest is 3,800 light years away.
For 1,000 planets, the closest is 1,500 light years away.
For 10,000 planets, the closest is 460 light years away.
For 100,000 planets, the closest is 210 light years away.
For 1 million planets, the closest is 99 light years away.
For 10 million planets, the closest is 52 light years away.
So, you need a lot of inhabited worlds before there’s one nearby.
In the pessimistic case where civilization is rare and doesn’t last very long, our civilization will be over long before we can send a message to some other planet, 10,000 light-years away.
In the optimistic case where civilization are common and last a very long time, the closest one might still be 50 light-years away.
For the lower limit, we already know of another planet 4 light years away from the sun, so if all 10 billion habitable planets in the galaxy are civilized, then the closest one would be 4 light years away.
Another way to think about this would be, “how many alien worlds know about us?”
Suppose that there are many advanced civilizations in the universe, and that every one of them is listening with radio telescopes.
People on Earth have been emitting radio signals for about 100 years. Those radio waves keep on spreading out, further and further away. How many alien worlds have noticed them, already?
For one alien world to have noticed those transmissions already, there must be more than a million civilized planets in the galaxy.
But, remember, if there are a million civilized planets, then the average civilization lasts a long time, like 100 million years.
The odds are overwhelming that the other civilization has been around for longer than our civilization has. They have been emitting radio waves for longer than we have. That gives us an advantage: we should be able to detect them before they detect us.
One thing the drake equation ignores: colonization
Life doesn’t have to evolve on every planet in the galaxy, it only has to evolve on one. Suppose one civilization develops, becomes much more advanced than humans, and they learned how to colonize other planets.
If they could travel at only one tenth the speed of light, then within a million years they could colonize the whole galaxy.
Even in the optimistic case above, only 1 out of 1,000 planets has an active civilization on it. The other 99.9% of habitable worlds were being wasted, either with no life or with no civilization. So an advanced civilization might have an incentive to go grab the unused ones and take them over.
In the pessimistic case, life is rare, every planet except for one is unused, and all the rest can be converted for use. The first civilization that figures out interstellar travel gets to take all the other planets.
So the drake equation needs a second term for “civilization evolves” and “one civilization colonizes all the planets”.
SETI is for figuring out which of these galaxies we live in
We could live in a lot of possible universes. Theory alone can’t explain which, but it can give some ranges.
We could live in a universe where there’s no other intelligent life except on Earth. My math says that’s unlikely — there are probably aliens somewhere.
We could live in one where intelligent civilization arises often, but usually snuffs itself out quickly. If that’s the case, we might be the only active civilization in our galaxy, at this moment in time.
We could live in a galaxy with 10 million advanced civilizations, most of which last a very long time, and we’re the newcomer in that club.
And we could live in a galaxy where there’s only one other civilization, but it aggressively scouts and colonizes every other planet.
To sort that out, we need to look for evidence to figure out what’s true. SETI, the Search for Extraterrestrial Intelligence, has been looking for decades now. So far, it hasn’t found signals from aliens.
But, it’s important to consider what the limits of our technology are — How sensitive are our telescopes? How far away can we search?
I’ll answer that in my next article:
