685 The Evolution of Habitability Characteristics of Habitable Planets Charles H. Lineweaver, Aditya Chopra, and Sarah R. N. McIntyre Habitability is a commonly used word. Its usage is usually vague… (Cockell et al. 2016) The number one goal of NASA’s astrobiology roadmap is to “understand the nature and distribution of habitable environments in the universe” (Des Marais et al. 2008, NASA 2011). The search for habitable planets simultane- ously addresses two distinct questions: Which planets are habitable for extraterrestrial life of any kind? Which planets could we inhabit? These are existential questions because our survival may depend on our ability (1) to fnd and understand aliens (Stevenson and Large 2017) and (2) to fnd a second habitable Earth in order to become a multi- planet species (Musk 2017) (Figure 10.1.1). 10.1.1 EARLIER VIEWS OF HABITABLE PLANETS Our notions of habitability can be traced back to pre-scientifc analogies: to consider the Earth the only populated world in infnite space is as absurd as to assert that in an entire feld sown with millet only one grain will grow. (Metrodorus of Chios, pupil of Democritus, ffth century BC) Ancient fanciful speculation (e.g., Lucian of Samosata second century AD) developed into modern speculation (e.g., Nicolas of Cusa 1440, Bruno 1584, Kepler 1634, de Fontenelle 1686, Huygens 1698, see also Duhem 1987), like Metrodorus 10.1 CONTENTS 10.1.1 Earlier Views of Habitable Planets ............................................................................................................................. 685 10.1.2 The Physics of Habitability ......................................................................................................................................... 686 10.1.2.1 Circumstellar Habitable Zones, Atmospheres, and Water ......................................................................... 686 10.1.2.2 Insolation, Escape Velocity, and Atmospheres ................................................................................................ 686 10.1.2.3 The M-Star Habitability Problem (Early Devolatilization) ....................................................................... 689 10.1.3 Using the Earth as a Habitability Calibrator .............................................................................................................. 690 10.1.4 The Biology of Habitability ........................................................................................................................................ 691 10.1.4.1 The Case for a Gaian Bottleneck ............................................................................................................... 691 10.1.4.2 The Abiogenesis Habitable Zone: Conditions Required to Start Life ....................................................... 693 10.1.4.3 The Evolution of Habitability..................................................................................................................... 693 10.1.4.4 Defnitions of Life and Defnitions of Habitability Are Not Helpful ......................................................... 694 10.1.5 Outstanding Issues ...................................................................................................................................................... 695 References ................................................................................................................................................................................. 695 FIGURE 10.1.1 Martians circa 1908. “There are certain features in which they are likely to resemble us. And as likely as not they will be covered with feathers or fur. It is no less reasonable to suppose instead of a hand, a group of tentacles or proboscis like organs.” (Drawing by William R. Leigh, “The Things That Live on Mars” by H.G. Wells, Cosmopolitan Magazine, March 1908.)