Planets are formed from the dust and particles left over from an exploding star. Our solar system was formed the same way. We can seen the same thing happen in other solar systems far away from Earth.

Right after the explosion of an old star, most of the left-over particles are much smaller than planets. These particles will "float" around in space until gravity pulls some of the biggest pieces into the center of a dust cloud. Eventually, a lot of big pieces are pulled into the center and form a star.

The other floating pieces travel around the new star as a dust cloud, but they also begin to crash into each other because of gravity. As they crash and smash into each other, they build up and create planet-sized pieces. That is how a planet is formed. Our solar system had enough pieces in its original dust cloud to form 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Exactly how and when the dust cloud pieces smashed into each other to form the 8 planets is still being studied, but it appears that there are many, many ways for dust particles to collide to form planet-sized objects.

Some pieces of the dust cloud don't get smashed into a planet, and they keep traveling around the new star in the center. For example, there are a lot of asteroids in our solar system which are not big enough to be called planets. We have an asteroid belt between Mars and Jupiter. Other objects in our solar system are bigger than asteroids, like Pluto, but they are not big enough to be called planets.

Scientists didn't always think the planets formed this way. It took many years and lots of experiments and observations to develop these ideas. We make new instruments every year as technology improves. Our ability to study space gets better each day, and sometimes we change our ideas based on scientific observation. Perhaps one day all our current ideas will change to better ideas about the formation of planets.

That depends on what you call a planet. Based on some definitions, there are at least two dozen planets in our solar system.

The number of planets in the solar system is probably an accident of the way that the solar system formed. There used to be more, but they collided with each-other.

There is no particular reason why our solar system has eight planets. When the sun formed and attracted other matter to it, gravity pushed the matter together. Depending on how much matter was around and close it was together, it formed objects of different sizes that now rotate around the sun.

To answer why eight planets, we have to define a planet. Planets are large round objects that orbit around stars in a predictable way. They don’t give off light and we need to be able to tell them apart from other objects in the sky. Our solar system has eight planets but there are many other objects around that don’t count as planets. For example, the asteroid belt contains lots of big rocks but never condensed into a single planet.

The Kuiper belt, past the orbit of Neptune, contains many large objects that might count as planets if they weren’t so close together. Pluto is one of these objects. It was first object in the Kuiper belt that we saw and it has a regular, if strange, orbit. It was considered a planet until 2006. As our telescopes and sensors advanced, we’ve found many more “planets” near Pluto and some of them are even bigger, so we no longer consider Pluto a separate planet.

There are 8 planets because of how the solar system formed. When the sun formed (billions of years ago), it was surrounded by a spinning disk of dust. That dust clumped together under its own gravity until it formed 8 planets. There are 8 because of how the dust in the original disk ended up clumping together. There are many more bodies in the solar system though: the asteroid belt, free asteroids, objects in the Kuiper belt and dwarf planets like Pluto. These objects could have formed into planets if they all clumped together, but were kept apart by the gravity of other planets. Ultimately, it's very random how many planets form in a star system.