Mercury is the smallest planet in our solar system; Jupiter is the largest.Pluto was once considered to be the smallest planet, but since Pluto is no longer thought to be a planet, Mercury now takes the title.
Mercury is roughly 38 percent of the diameter of Earth. Earth is 12,742 kilometers across, while Mercury is just 4,879 miles across. Mercury's volume is even smaller in comparison — just 0.05 percent of the Earth's volume. Twenty planets the size of Mercury could fit inside the earth if the earth were hollowed out. Nonetheless, it is a dense planet composed mainly of rock and iron.