Rare Earth

"This book looks at a classic Carl Sagan idea that if there are zillions of stars and bazillions of planets in the universe, then there must be at least millions of habitable planets with complex life. But the book looks beyond statistics and considers in detail the series of ‘coincidences’ that occurred to make complex life on Earth. To do so, they must go through many of the important events that occurred in Earth history, so, first and foremost, this book represents a useful introduction to the history of Earth. They conclude that Sagan might have even underestimated the amount of bacteria in the universe, which can survive crazy conditions. But complex life on Earth is the consequence of so many freak coincidences (and not just habitability – like size of planet and distance from sun) and is so precarious that we might actually be alone in the universe. Well, the idea about bacteria is that wherever you look, even at the bottom of the sea or in the middle of a rock kilometres underground, there is almost always some crazy bacteria surviving by strange and wonderful chemoautotrophic metabolisms. This observation let scientists realise that almost any planet, no matter how inhospitable, might have some form of life in the form of bacteria because bacteria are so adaptable and resilient. But the most interesting part of this book is when they discuss complex life. Instead of just using the statistics arguments and saying there are zillions of planets and stars therefore there must be x number of Earth-like planets, which may well be true, but the question is, would they have complex life or just a bunch of bacteria? And Ward and Brownlee make the argument that it takes more than habitability to make complex life, especially humans. It takes a special set of unique events and their consequences that result in the life that we have on Earth today. For example, had Earth not been hit by large meteorites at just the right time, had Earth not received a major delivery of water from a stray comet, had Earth not endured multiple global glaciations, would animals have evolved the way they did? I think this book is a useful introduction to some of the major events that shaped Earth history, and at the same time gives us a provocative new way to think about the origin of complex life."

"Rare Earth was one of the prime motivations for me to write my book . I know both the authors and they are fine scientists, but they are very pessimistic about the chances of complex life outside Earth, by which they mean animal life, and, of course, that includes intelligent life. They do. They have a whole list of arguments about things that they think the Earth was lucky about, which makes Earth so rare. And some of those things, I think, are not so rare. For example, plate tectonics – the crust and the lithospheric plates lying around on top of the asthenosphere. It actually is very important not just for geology but for climate legislation because it helps to modulate the carbon cycle that controls CO2 levels over long time scales. I agree with them that plate tectonics is important for a planet. But what Ward and Brownlee say is that in our solar system there are 20 large solid bodies – planets and moons – and of those Earth is the only one that has plate tectonics, so that makes it rare. But you have to then ask why the other ones don’t have plate tectonics. For Mars the answer is simple: it is much smaller than the Earth. It is only about a tenth of the Earth’s mass so it cooled off too quickly. Small objects cool off faster than larger ones. And for Venus, the problem is probably lack of water. Water lubricates plate movement just like oil lubricates an engine. So Venus’s lack of plate tectonics is most likely caused by the fact that it lost its water. After reading Rare Earth I decided to write a book showing that some of their arguments are not so strong."