Even though Jupiter is made of hydrogen and helium, the lightest gasses, it is the heaviest planet in the Solar System.

Jupiter is a giant ball of gas and liquid, with barely any solid surface to stand on. Astronomers believe that Jupiter is primarily made up of hydrogen and helium, the two lightest elements. However, Jupiter has the most mass of any planet in the solar system. It weighs as much as 318 Earths! It also would take 1,000 Earths to fill up the volume of Jupiter. Jupiter's characteristic features are the Great Red Spot, and a complex system of red belts that look like stripes on the surface of the planet. Jupiter recently lost one of its stripes, and NO ONE KNOWS WHY. (sources: 1, 2) - OMG Facts - Your Mind. Blown.: