Small, rocky solid bodies with cores of metals. Planets like Jupiter. Jupiter, Saturn, Uranus, and Neptune. Much larger than the terrestrial planets and further out from the Sun. No solid surface but only an atmosphere of hydrogen, helium, methane, and ammonia that gets thicker as you go deeper into the planet.
Cores are very small and rocky.
Lots of moons made of rock and ice, and rings made of smaller bits. Pluto is a misfit and there is still on-going argument about whether to classify it as a terrestrial planet or as something more similar to a comet. Any theory of the formation of the solar system has to explain why the inner and outer planets can be divided into two distinct classes. They contain original material from the formation of the solar system. Our theory has to explain why we have so many comets and asteroids and explain their properties.