This table lists the average density of all the planets in our solar system. We could also consider the Sun (average density 1.41 g/cm³) and Pluto (average density 1.88 g/cm³). The densest body (Earth) and the least dense (Saturn) only differ by a factor of less than 8. And if we neglect Saturn, the average density of the next-least-dense planets (Jupiter and Uranus) only differ from the Earth's by about a factor of 4. By contrast, many other solar and planetary properties range over many orders of magnitude. Is there a reason that the density is so uniform (by astronomical standards), considering that the planets' elemental compositions are so different?
All the planets and the Sun formed from the same protoplanetary disk, which explains why most of them have similar angular momentum (i.e. orbital and rotational) directions. But I don't see why this would lead to a uniform density. The rocky planets are made of elements that are fairly incompressible (both under pressure and temperature changes), so I can understand why their densities are quite uniform, but I would have expected the gas giants' densities to be highly sensitive to the planet's size and temperature, since the density of an ideal gas is proportional to its pressure (determined by the planet's total mass) divided by its temperature. Moreover, I would have thought that by far the most important factor determining a planet's properties is its distance from the Sun, but the density seems to depend quite non-monotonically on that distance - e.g. Saturn has only half the density of both its closer and its farther neighbor relative to the Sun.