Mercury. Cutaway illustration showing the interior structure of the planet Mercury. The large central core consists of iron, and accounts for about 75% of the planet's diameter. Surrounding this is a rocky mantle and then a thin crust of sun-baked rock. Mercury is the second smallest planet in the Solar System and the closest to the Sun. A day on Mercury lasts almost twice as long as a mercurian year: sunrise to sunrise takes 156 Earth-days while the planet takes only 88 Earth- days to orbit the Sun. Mercury is so close to the Sun that its surface temperature at midday is 420 degrees C. At night, with no atmosphere to insulate it, this plunges to -180 degrees C.