Mercury, Venus, Earth, and Mars are the only solid planets in our solar system. Because they are solid planets they are also the densest planets, and because of how dense theta are they're also the smallest planets. Jupiter and Saturn are gas giants. Since they are gas planets they are not very dense, in fact the least dense planets in our solar system and the largest planets. Uranus and Neptune are our 2 ice giants. They are in the middle of the other planets when it comes to size. The difference between ice giants and gas giants is that gas giants are mostly hydrogen and helium while ice giants are more oxygen, carbon, nitrogen, and sulfur.