The smallest planet in our solar system is Mercury. It is only slightly larger than Earths Moon, with a diameter of 4,879 km and a mass of 3.3010 x 10^23 kg. Mercury is nearly 20 times less massive than Earth, and its diameter is about 2.5 times smaller. In fact, Mercury is closer in size to our Moon than to Earth. It is significantly larger than the dwarf planet Pluto, whose equatorial diameter is just 2,302 km, about half Mercurys width.