If there are about 100 billion stars in the galaxy, with 10% or 10 billion in the disk, and 10% of those of the right spectral type, or 1 billion, and 1% of those have a rocky planet in the Liquid Water Zone, with the right gravity, composition, magnetic field, axial tilt, eccentricity, and rotation, that leaves about 10 million stars. If 0.1% of them have a moon that came from a Lagrangian point and made a glancing strike on the proto-planet, resulting in masses of organics necessary for life to originate, that is 10 thousand stars, and if 10% of them evolve intelligent creatures, that is 1 thousand alien civilizations.
The disk area of the galaxy is about 100 thousand light years around, 10 thousand light years wide and a thousand light years deep, meaning it has a volume of a trillion cubic light years. Each alien civilization would be in a volume about a billion cubic light years, which is a cube a thousand light years on a side. So, if these calculations are the right order of magnitude, alien civilizations should be about a thousand light years apart.
Let’s throw a little time into the mix. The galaxy is about ten billion years old, so that means about 10 stars are born every year. If an alien civilization is bent on preserving itself, and it recycles to the extreme, uses resources wisely, and does everything else right, it might last a million years on its home planet before resource exhaustion forces it to find a new solar system. According to the calculation above, there is about a one in ten million chance that a new solar will grow life and have an easy to colonize planet. That means in the million years that the alien civilization survives on its original solar system, or a colony world, there is about one planet that arises somewhere in the disk that is suitable for them to move to. It could be up to a hundred thousand light years away. Finding it is going to be hard and getting there also hard. They would have to live in their ships for a million years, as long as they inhabited the planet they used to call home. Obviously, only a small number would make the trip, or it would be fully automated and ready to do the colonization work itself when it arrived.
These simple calculations indicate that alien civilizations are likely to simply die in place. A lucky one might find itself near a colonizable planet, but few would get lucky twice. On the other hand, there are plenty of planets and satellites with mineral resources, so a type of nomad existence, where some aliens travel from solar system to solar system harvesting resources so they can continue their voyage, is possible. Possible is hardly a mandate, because life aboard such a ship might not be sufficiently appealing to make the choice of building one popular. The alien civilization might simply decide that there is no good reason to build such a ship, and create modified aliens to live on it. Easier just to calmly go extinct.
Another way of looking at the numbers is to say that every million years a new alien civilization arises, on the average. They last a million years and disappear. This means that at any time in the galaxy, there is only about one alien civilization. Maybe there are two or none, but not a hundred. This means SETI has no change of finding anyone. It means the valiant astronomers who search for Earth-like planets are going to find some, only none with life. It means that no alien ship is going to appear over a capital city on Earth and demand to speak to our leader. It means that no aliens are going to try and take over our planet.
It means the Milky Way is a lonely, lonely place.
The weakness in this calculation is in that part about the origination of life. The theory of life origination espoused in this blog was used, and it requires some unusual events to occur. If some other life origination theory were used, which had a ten times greater probability of origination of life, all the resulting numbers would go up by ten. This means that there might be ten alien civilizations running around the galaxy at any one time, going from solar system to solar system every million years, and continuing to exist for long periods, except for one thing. The distance from a home solar system to the nearest colonizable one only changes by about two, meaning it would be fifty thousand light years away. This is almost as bad. If this serves as an insurmountable barrier, it means that at any time in the galaxy, there might be about ten alien civilizations, all living and then going extinct. Still SETI is not going to work, astronomers are not going to find Earth-like worlds with life, and no one is going to come by to visit us.
Even if the probability of life origination goes up by a hundred from the original numbers, the distance only drops to about twenty thousand light years. If a ship gets up to 0.1 c, this is a two hundred thousand year voyage, and if the ship can only get up to 0.01 c, it is a two million year voyage. There is no time for an alien civilization to send out a probe ship to a prospective planet, even if one could be found.
Despite the huge number of stars in our galaxy, there are too many obstacles here to have life popping up in a nearby star. Nothing so far has spoken about the length of time that life itself would last on a planet. Life is naturally recyclable, and continues to evolve to match conditions, so only a peril of some nature would extinguish it. One of course is the growth of the star to a size that either takes the planet out of the LWZ, or does something worse to it. These factors reduce an already small number.
It does seem sad that we aren’t likely to find any new friends.
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