The smallest planet in our solar system is Mercury. It is the closest planet to the Sun and has a diameter of approximately 4,879 kilometers (3,031 miles). Mercury is a rocky planet with a thin atmosphere and no moons. It has a very hot surface, with temperatures reaching up to 450 degrees Celsius (842 degrees Fahrenheit) during the day and dropping down to -180 degrees Celsius (-292 degrees Fahrenheit) at night. Mercury's rotation is very slow, taking about 59 Earth days to complete one rotation. It also has a very long orbital period, taking about 88 Earth days to orbit the Sun.