Within that short time, no-one knows for sure which of them formed first. The solar system is said to be over 5 billion years old. There are no physical boundaries in space. Pluto, the farthest planet from the Sun orbits approximately as far as 40 astronomical units. An astronomical unit is a unit of length used by astronomers.
Planets are different in sizes and colors.
The four planets closer to the Sun are called 'rocky' planets. They are small in size and similar to Earth in composition. The four outer planets, also called 'gas giants', are much larger than the 'rocky' planets. They all have rings and have many moons. The 'gas giants' are made up mostly of hydrogen, helium, frozen water, ammonia, methane, and carbon monoxide.