Around 1960 a group of scientists got together for a serious discussion on the possibility of intelligent life elsewhere in the galaxy. A lot of people were convinced ETs were out there but up to that point there hadn’t been a concerted effort to do anything about it.
One result of this meeting was the Drake equation, which, assuming I get it all right, looks like this:
N = R * fp * ne * fl * fi * fc * L
Where the variables means the following (and initial values given by Drake according to Wikipedia)
N = The number of civilizations able to broadcast a signal (i.e. return our call) – about 10 currently
R = The average rate of formation of suitable stars in our galaxy – about 10 per year
fp = The fraction of stars that are able to form planets – about half
ne = The average number of habitable planets per star – about 2 per star
fl = The fraction of habitable planets where life emerges – nearly 100% of habitable planets develop life
fi = The fraction of habitable planets with life where intelligent life emerges – 1% of those planets develop intelligent life
fc = The fraction of planets with intelligent life that develop interstellar communication – 1% of those planets develop intelligent life
L = Years a civilization remains detectable
We have come a long way in our understanding of the cosmos since 1960 and not surprising, some of Drakes numbers appear to be off. For instance, there appear to many more planets than we expected orbiting stable stars (like ours or red dwarfs). In fact I wouldn’t be surprised to find out there are as many habitable planets as there are stars in the galaxy. In our own solar system we have currently 1 we know of and several candidates (Mars, Enceladus, Europa, heck, maybe even Titan).
But what are the odds? Drake seemed to think (based upon his project of 10 civilizations for 100 billion stars) about 1 in 10 billion. I have heard other claims of numbers as high as 1 in 10 thousand.
Outside of Earth, while I think there may be life in a lot of places, I think the chances of a civilization like ours is about 0 for the entire universe. Yes, I believe we are a miracle and all alone in the cosmos. Drake and his fellows, in my humble opinion, have widely overestimated the ability of microbial life to evolve to us – hence the miracle. For starters:
Life like ours requires stability. Gammy-ray bursts, supernovas, magnetars and other cosmic phenomena do nasty things to a planet, like strip off its atmosphere, scorch its surface, etc. We need a very stable galactic neighborhood and home star that lasts for hundreds of millions of years at least. From the Cambrian explosion to now is almost 550 million years of evolution. While there were several mass extinctions, life was always able to recover and progress. I have heard that only somewhere in the neighborhood of about 25% of the milky way galaxy is stable enough to support life over an extended period. So 1 in 4.
We need just the right amount of water. Too much and you have a water-world. No fire, no technology, no civilization. Too little and the world becomes a dry desert. This time you have no fuel for your fire, no technology, etc. Just a few percentage points more or less water than what we have here on Earth and you have a planet that might sustain microbial life, but little else. Making matters worse, the planet in question has to originally start in the solar system’s dry zone so there is little water to work with. In the end, you need a nice balance in which you have a roughly 70/30 ratio of dry land to water. Since the surface water is such a small sliver of total planetary mass, the margin of error is terribly thin. Let’s say the odds of Earth having just the right amount of water on its surface (however it was delivered to us) is probably less than 1 in 10,000.
We need just the right amount of gravity. Most rocky planets seem to be super-Earths, about 10 times the size of our Earth. That’s too much gravity for things like trees or us. Anything more than twice Earth’s gravity makes it almost impossible to have range of motion. There are theories as to why the rocky planets in our solar system are so comparatively small (like they are second generation), but they appear to be an outlier. Let’s take a swag and say 1 in 10.
We need just the right amount of atmosphere, for much the same reason. Another 1 in 10.
We need a planet like Jupiter, sitting at the edge of the ice zone, huge and gobbling up planetesimals and comets but not wandering in to disrupt the inner planets. How about 1 in 2?
We need a stable sun, not prone to throwing off planet-killing solar flares like some (which almost happened in 2014 – miracle?), but also able to provide enough light for photosynthesis. How about 1 in 2 again to be generous.?
Our planet can’t be too close to the sun (even if it is cool enough) that it would get tidally locked, with one side perpetually roasted while the other is in perpetual darkness (like Mercury). Takes out most of planets around red dwarf stars so 1 in 5 maybe.
We need a circular orbit so the seasons are stable. Out of the 8 planets in our solar system, Earth ranks #3 in stability. So about 1 in 2.
This is getting long so let me just touch on a few more, such as a stable, hot iron core, an oversized moon to stabilize the axis, planetary events to steer evolution in just the right direction such as Snowball Earth, the Cretaceous extinction and most recent warming period. Easily a million to 1 shot.
Plus there’s plenty more. I have read claims of as many as 300 different occurrences, any one of which would have likely prevented intelligent life from occurring. Just looking at the few I listed here creates about a 1 in 160 trillion possibility. Added to Drake’s equation, there aren’t enough galaxies and stars in the known (about 4 trillion billion stars) universe to even make one other civilization as advanced as ours. And remember, this is a short list.
Now if you want to throw in the multiverse, sure, you got me there. Pretty much anything you want can happen if you have infinite ways to make it. But if you are going that route, why not God?
I’ll take that question up in another post.
