Answer: Earth

Our solar system can be divided, when talking about planet density, pretty neatly along the Mars/Jupiter dividing line. Mercury, Venus, Earth, and Mars are in a planetary class known as terrestrial. They’re composed of silicate rocks and/or metals, have a solid surface, and while their general density varies a bit as you move away from the core, it is pretty consistent overall.

Among the inner terrestrial planets, Earth is the densest planet by a somewhat narrow margin. The size of Earth combined with the mass of the iron/nickel core, the molten silicate upper and lower mantle, and the dense crust, yields an overall density of 5.514 g/cm³–our closest competitor in the density Olympics is Mercury with a density of 5.427 g/cm³ , followed by Venus (5.243 g/cm³), with Mars trailing in a distant fourth (3.933 g/cm³).

By contrast, the four outer planets, Jupiter, Saturn, Uranus, and Neptune, are classified as gas and/or ice giants and are characterized by greater size, mass, and a much lower (and wide ranging) density than the inner planets. Despite having significantly bigger volume than the inner planets, the giants have extremely low density. The densest outer planet is Neptune with a mean density of 1.638 g/cm³. Jupiter, despite its size, still manages to hold down a bit of weight and has a density of 1.326 g/cm³. Uranus is in third place at 1.27 g/cm³. So light and airy it might as well be cotton candy, the last giant, Saturn, has a density of 0.687 g/cm³. Saturn’s density, you might note, is lower than that of water–if you could find an enormous intergalactic lake to throw it into, it would float like a beach ball.