Abstract

In their 2000 book, Rare Earth, Peter Ward and Don Brownlee argue that complex life (i.e., animal life) is rare, for a variety of reasons, some of which are based on the idea that habitable planets are themselves rare. Possible reasons for this include: 1) Plate tectonics (possibly necessary to stabilize planetary climates) is rare; 2) large moons (possibly necessary to stabilize planetary obliquities) are rare; 3) magnetic fields (possibly necessary to retain atmospheres) are rare; 4) the Sun is anomalously metal-rich; 5) Jupiter-sized outer planets (possibly necessary to protect the Earth from frequent large impacts) are rare. In my talk, I will review these Rare Earth arguments and show that most, or all, of them are less troubling than Ward and Brownlee supposed. That said, there could be other factors not discussed by these authors that could make habitable planets scarce. But this should not discourage us from building the types of large space telescopes required to actually answer this question.