In fact Pluto is sometimes closer to the Sun than Neptune. Asteroids are irregular lumps of rock, most of which are found in the asteroid belt between the orbits of Mars and Jupiter. Comets are icy bodies which are mainly located well outside the orbit of Pluto in the Oort Cloud . Direct evidence for this layered structure comes from the propagation of seismic waves through the Earth's interior after earthquakes. P waves are sound waves and propagate through both solids and liquids.
S waves are shear waves and propagate only through solids.
The fact that S waves do not propagate directly to the other side of the Earth from an earthquake indicates that the Earth must be liquid somewhere near the core. In fact the Earth's interior is believed to consist of a solid iron inner core, a liquid iron outer core, and a silicate mantle. Mercury has an unusually high density because it has a large iron core. Mercury may have lost most of its mantle when a large planetesimal collided with it early in the history of the solar system. The Moon has an unusually low density because it has at most a very small iron core.