To get an idea of just how long intelligent life could take to develop on other planets, we need to go through some of the major events in the history of life on Earth: events that were necessary for the evolution of human-level intelligence to be possible.

In this part, I’ll get into the core of the Rare Earth hypothesis, and look at the specific planetary characteristics of Earth that allows it to have complex life.

The idea that habitable planets are extremely rare has been a popular Fermi paradox solution for a very long time. Before the first extrasolar planets were discovered in the 1990s, it was an open question in astronomy whether planets of any type were common.

The most recent estimates put the total number of stars in the Milky Way galaxy at somewhere between 200 and 300 billion, and we now have evidence that most of those stars have a system of planets around them.

Are we alone? It’s one of those big questions that humans have pondered for centuries.