there are eight planets in the Solar System,and each one is very different.Some planets,like Jupiter and Saturn are very large.Others,like Mercury and Mars are smaller.Jupiter has moons that are larger than Mercury.The planets also have different atmospheres.Uranus,Jupiter and Saturn have atmospheres of hydrogen and helium. The atmosphere on Venus is made up of carbon dioxide.Earth has a nitrogen and oxygen atmosphere.Neptune's atmosphere is mostly hydrogen.The planets also have different temperatures.Uranus is the coldest and Venus is the hottest.