In 1961, Drake used his equation to estimate there could be twenty civilizations in our galaxy. Even today, the equation can produce a result from a low of 2 to a high of 280,000,000, depending what values you use for the variables.
One solution called the “Rare Earth Theory” takes the position that Earth is such a rare combination of conditions that it is unique. If you accept this theory there are no extraterrestrials.
The Rare Earth theory has its own equation. This has more variables than the Drake equation. These additional variables impose new requirements for a planet to develop a technological civilization. This theory argues that in addition to a rocky planet in a goldilocks orbit, the planet also needs the following:
- It must have the right arrangement of planets with the gas giants on the outside.
- Plate tectonics is essential for the emergence and sustenance of complex life.
- A large moon is needed for its tides.
- Few mass extinction events.
Again, as with the Drake equation the results depend on what variables you consider important, and what values you give them.
In an almost humorous turn of events, one advocate of the Rare Earth Theory points to the lack of extraterrestrials as proof.
The detractors of the Rare Earth theory build their positions in different ways. They feel that the stated preconditions for technological civilizations are too stringent. According to David Darling, the Rare Earth hypothesis is neither hypothesis nor prediction, but merely a description of how life arose on Earth. In his view Ward and Brownlee have done nothing more than select the factors that best suit their case.
At the end of the day, the Rare Earth Theory and the Drake equation before it require too many estimates to be useful. If the Earth is a one in a billion long shot, there are probably forty in this galaxy.
Personally, I remain open to the possibility there is intelligent life out there, and that brings me back to Fermi’s question, “Where is everybody?”