This chapter elucidates the physical and chemical mechanisms involved in the formation of planets, the conventional abodes of life. The first part is devoted to protoplanetary discs, wherein planet formation unfolds. The topics covered include the minimum mass required for assembling the solar system (minimum mass solar nebula), the thermal and density structure of protoplanetary discs, and the rich chemistry that occurs in these settings. The second delves into the many stages of planet formation starting from the coagulation of dust to the hurdles encountered (e.g., metre barrier) in forming kilometre-sized planetesimals and subsequently to collisions between planetesimals engendering planetary cores and eventually terrestrial planets; a brief description of how giant planets are assembled is also delineated. The final part outlines how interactions between a given planet and its neighbouring gas or planetesimals can contribute to the migration of the former, as well as influence the delivery of water and other volatiles to the planet.

Earth’s Moon is quite distinct from other moons in the solar system, in being a comparable size to Earth. We explore the theory that a giant impact in the chaotic early solar system led to the Moon’s formation, and bombardment by ice-laden asteroids provided the abundant water we find on our planet. Further, we find that Earth’s magnetic field shields us from solar wind protons, that protect our atmosphere from being stripped away. The icy moons of Jupiter and Saturn are the best targets for exploring if life exists elsewhere in the solar system.