Ward and Brownlee challenged many widely held notions that supported the idea that complex life is out there waiting to be found. For example, while astronomer Carl Sagan often opined that our Sun is an unremarkable star, in reality, about 80 to 95 percent of stars are significantly different from our own in terms of size, mass, luminosity, lifespan, and many other factors.

Furthermore, prior researchers who had attempted to answer the question of why life on Earth was so plentiful yet so rare in the universe had not included plate tectonics in their thinking at all. Indeed, an entire chapter in Rare Earth is devoted to the topic, going to great lengths to explain the role of plate tectonics in shaping Earth into a good place for life. Earth is, to the best of our knowledge, the only body in the solar system with active plate tectonics. And there are many other features of our life-friendly planet that we haven’t seen replicated anywhere else in the universe, too.

It’s important to remember that the Rare Earth hypothesis only applies to the emergence of complex life. Ward and Brownlee believe that simple life, such as bacteria, is widespread in the universe — after all, even the harshest habitats on Earth harbor microbes. However, the pair feel that complex life, metazoans like animals and us, are exceptionally rare.

“If you find life elsewhere, it’s likely to be microbial,” said Brownlee. “You know, Earth will have a lifetime of about 12 billion years, but [compared to bacteria], metazoans have a much more restricted range of environmental criteria that they can survive in.” That means that a planet’s environment is conducive to simple life for much longer than it is conducive to complex life.