This is a drawing that shows the history of the inside of a giant planet.
Click on image for full size
Image from: The New Solar System
How the Interior of Neptune Formed
The drawing shows the possible history of the inside of giant planets.
As the planets drew material from the solar cloud, bits of heavy rock came to where the planet was forming, as shown in figure A.
Once the planet finished forming, these heavy bits of rock fell into the middle of the planet, as illustrated in figure B.
As shown in the picture, the gas part of the planet is much bigger than the rocky part. That is because the amount of gas and ice which came to Uranus in the beginning, depended upon where Neptune was in the original solar cloud.
Eventually, the heavy, rocky material at the center became a core, as shown in figure C.
You might also be interested in:

As shown in this picture, while they were forming in the solar nebula, the core of the planets-to-be drew material to themselves from the cloud of gas and dust around them. The bigger planets-to-be were
...more
The position of the planets in the solar nebula affected how big they became and what they were made of. The blue line in the picture shows where it became so cold that ice began to form. Planets that
...more
Minerals are the building blocks of rocks. There are many different types of minerals. All of them are solid and all are made of atoms of elements. Minerals can grow even though they are not alive. Most
...more
There is no surface to the giant planets. There is only a gradual change from the atmosphere, as shown in this drawing. Therefore the giant planets do not have layers, as the earth-like planets do. Ice
...more
One of the many cloud patterns of Neptune is the "scooter" which was discovered by Voyager in 1989. In this picture, the Great Dark Spot is in the upper left of the image, and just below the Spot is a
...more
Neptune has three small plasmaspheres which are greatly affected by Neptune's rings. Particles enter the plasmasphere from Neptune's atmosphere as well as the magnetotail. Particles might leave the plasmasphere
...more
Triton is the largest moon of Neptune. It isn't quite as big as Earth's Moon. The surface of Triton is very, very cold. It is colder than the surface of any other planet or moon in our Solar System. It
...more