[friend_and_foe]

The density of an object at hydrostatic equilibrium is a function of its gravity, which is a function of it's mass, assuming rocky and similar composition in aggregate. The likelihood that a planet would be 2x the diameter of earth and less dense is extremely low.

[autokad]

i completely disagree. Mercury is extremely dense and small, Venus is almost exactly the same size as Earth and is in fact ... less dense than Earth. There are many many things that leads to a planets density, and they call fall into a very wide range. A planet 2x the size of Earth with mars density seems very reasonable to happen - even if unlikely. Kepler-22b is very close for example