Twenty years ago a couple of scientists wrote a book that argued intelligent life is extremely rare in the universe. Since then we've discovered many exoplanets orbiting other star systems, and the authors have released a new book to reflect that, although nothing has moved them from their Rare Earth standpoint.
I'm always cautious about taking an anthropocentric point of view on these things. Absent any other information, the odds-on assumption is that we're on an average planet in an average star system. Yet the authors make some cogent observations regarding Earth. The ratio of water to land, the geological diversity, the particular constituents of the atmosphere, the happy accident of having a moon that's the right size to give us a beneficial tidal system; all these things are necessary for intelligent life as far as we know.
The authors are not arguing there is no intelligent life elsewhere, merely that it's rare. Then again, what is 'rare' in a galaxy of at least 100 billion stars and in an observable universe of at least 200 billion galaxies?
It's important for one of the numbers we'd plug into the Drake Equation:
N = R x p x e x l x i x c x L
The parts of that equation are as follows:
- R: the average rate of star formation in our galaxy.
- p: the fraction of formed stars that have planets.
- e: for stars that have planets, the average number of planets that can potentially support life.
- l: the faction of those planets that actually develop life.
- i: the fraction of those planets that develop intelligent life.
- c: the fraction of those planets with intelligent life that actually develop communications that can stretch out into space.
- L: the length of time over which such civilisations transmit said communication. In other words, the length of time such intelligent civilisations exist before something wipes them out.
The Rare Earth authors are arguing that i, above, is very small indeed.