I don’t recall the exact Drake formula, but the basics of it are simple, why I remember it. Sticking to our Milky Way galaxy for now, as that’s all we can usefully search for ETI:
It goes (something) like this.
- N = The number of Human-like societies out there in the galaxy, EQUALS
- S = the number of stars in the galaxy, TIMES
- Ss = the proportion of those “S” that are like the Sun, TIMES
- Ps = the proportion of “Ss” that have planetary systems, TIMES
- E = the proportion of “Ps” that have planets like the Earth, TIMES
- G = the proportion of “E” that are in the Goldilocks zone, just close enough to the Ss to have liquid water, TIMES
- T = the proportion of “G” that have a civilisation that has reached Technical level same as ours
And it goes on a bit further, but you get the idea. Also, I’m sure not to have got the formula exactly right, as I’m going on what I recall, but the concept of it is so simple, that I’ve got it more or less right. And the main idea, the main principle, is surely clear.
What I’ve always thought, and most folks seem to have thought, is that “S”, the number of stars in the universe is so huge that the likely number of “N”, Earth like planets with an advanced civilisation, is also likely to be high.
HOWEVER, what we’re learning recently is this: that the number of “contingencies” that needed to happen to create an Earth is so great, that the likelihood of another Earth with beings like Homo Sapiens, is so small as to approach zero.
The contingencies are like: Planet systems like those of our Sol are very rare. An Earth has to be protected by a gas giant like Jupiter, to protect against the bombardment by asteroids. An Earth needs to have a Moon, to create life-making tides. And just the right size and right distance from the Earth. That it turn needs a planet like Theya, to hit the other Earth, at just the right time and in the right way to create a perfect Moon.
These likelihood of these contingencies have to be multiplied together to get the likelihood of another Earth. Some have calculated each one at a maximum of one in a thousand chance. And they may be much rarer. Say you have six such contingencies and each is one in a thousand chance, you get the likelihood of Another Earth, with Intelligent Life, as one in 10 with 18 zeros or 1x10^18. Less than one in a Qudrillion. That makes it less than the number of stars we know of in the Galaxy, max ~400 billion or 4x10^14. So the chance of Human-like life -- based on the likelihood of contingencies -- is less the number of stars in the Milky Way.
Therefore, goes the current thinking, at least some of the current thinking: the likelihood of another Human-like civilisation in our Galaxy is very unlikely.
I hope not. But there it is.
