The smallest planet in the Solar System is Mercury (the biggest planet is Jupiter). For the longest time, the smallest planet was considered to be Pluto, but now Pluto isn’t a planet any more, so we’re back to Mercury.
Mercury measures 4879 km along its equator. Just for comparison, Earth is 12,742 km across. So Mercury is only 38% the diameter of Earth. In terms of volume, it’s even less. Mercury has only 0.05 the volume of the Earth. In other words, if the Earth was a hollow shell, you could fit 20 Mercurys inside with room to spare.
Even though it’s very small, Mercury is extremely dense. It’s composed mostly of iron and rock, and so it has a density of 5.4 grams per cubic centimeter. Only Earth has a higher density, and that’s partly due to our larger size compressing down the interior. If Mercury were the same size as Earth, it would be much denser.
If you could stand on the surface of Mercury, you would experience 38% of Earth’s gravity. Even thought Mars is a larger planet, you would experience more gravity on Mercury because it’s so dense.
We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.