Merkur 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. Merkur has a very thin atmosphere, which means it cannot retain heat, leading to extreme temperature variations between day and night. The surface of Merkur is covered with craters, similar to the Moon, due to its lack of atmosphere to protect it from impacts. It has a slow rotation period, taking about 59 Earth days to complete one rotation, while it orbits the Sun in about 88 Earth days.