A planet is defined as a celestial body that orbits a star, is massive enough for its own gravity to pull it into a nearly round shape, and has effectively cleared its orbital path of other debris. These bodies form through the slow process of gravitational accretion, drawing together dust and gas from the remnants of star formation. Planets exhibit immense diversity in composition and size, ranging from small, rocky, terrestrial worlds dominated by silicate rock and metal cores, to massive gas giants composed primarily of volatile elements like hydrogen and helium. The structure and evolution of a planetary system are governed by complex gravitational interactions and the physical processes of heat retention and planetary differentiation, which influence the geochemistry of the world over billions of years.