Mercury is the closest planet to the Sun in our solar system. It is a small, rocky planet with a diameter of about 4,880 kilometers, making it the smallest planet in the solar system. Mercury has a very thin atmosphere, which means it cannot retain heat, leading to extreme temperature variations between day and night. The surface of Mercury is covered with craters, similar to the Moon, due to impacts from asteroids and comets. It has a very short orbital period, taking just 88 Earth days to complete one orbit around the Sun. Despite its proximity to the Sun, Mercury is not the hottest planet; that title belongs to Venus due to its thick, heat-trapping atmosphere.