The smallest planet in our solar system is Mercury. It has a diameter of about 4,880 kilometers (3,032 miles), making it smaller than several of the moons in the solar system. Mercury orbits the Sun at an average distance of about 57.9 million kilometers (36 million miles) and is known for its extremely high temperatures during the day and very low temperatures at night. It has a rocky terrain and lacks a substantial atmosphere, which contributes to its temperature extremes and cratered surface.