Within that short time, no-one knows for sure which of them formed first. The solar system is about 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 come in different sizes and colors.
They are small in size and similar to Earth in composition. They all have rings and have many moons. Pluto, the most remote planet, might be little more than a giant comet. Its composition is similar to that of comets, and its orbit is quite different from that of the other comets and planets. The asteroid belt is a zone between the orbits of Mars and Jupiter.