Question

Are all planets layered?

Answer 1

Yes. It's inherent to formation of a planet.

Explanation:

Planets are massive enough so that their own gravity causes different phase materials to settle, a process called planetary differentiation. The denser phases, like metal on Earth, sink into a core underneath the less dense ones. The lighter phases, like rock on Earth, surround the core to form a mantle and either a crust (solid planets like Earth) or a thick cloud of gas (gaseous planets like Jupiter).

Some things are not massive enough to emerge as full-fledged planets but still they show differentiation. Our Moon and large moons in the Solar System generally do this. Dwarf planets like Pluto and Ceres are also differentiated (or expected to be so) because of their sufficient mass.