How did the solar system form? As with many questions about astronomy, there's no one who was there that we can ask. Instead, we must rely on scientific theories and our knowledge of physics to try and answer this question.
Forming the Sun and Planets
About five billion years ago, there was a cloud of gas and dust that was several light years across. A light year is about six trillion miles. Most astronomers believe that this rotating cloud collapsed, causing it to spin faster. The rotation from the spinning allowed the gas and dust close to the center to collapse faster than the gas and dust at the edges of the core. The core then became a spinning disk. As more gas and dust accumulated, it formed the sun.
Over a long period of time, some of the dust particles stayed and settled mid-plane of the disk. The dust created clumps, and over time, more dust collected on the existing particles to make bigger clumps, then rocks. As these rocks collided with each other, they fused and their gravity increased, drawing in other nearby material. These clumps eventually made the planets.
The planets Jupiter, Saturn, Uranus and Neptune are gas planets. Their cores, which are made of rock, attracted gas particles. Jupiter and Saturn are made primarily of hydrogen, while Uranus and Neptune were formed from ice chunks. Astronomers still aren't sure if the planets formed in their current locations, or if they've moved around from the places where they first formed.
Forming the Solar System
There are more things in the solar system than planets. There are comets, asteroids and smal lumps of ice chunks or dust. These things stayed in orbit rather than smashing into the sun or becoming part of planets. Many are still in our orbit, floating around with the planets. There is even an asteroid belt in our solar system, located between Mars and Jupiter. This collection of rocks, including the dwarf planet Ceres, is believed to be held in place by the gravitational pull from both the Sun and from Jupiter.
The planets all formed within a very short period, probably a few million years, about five billion years ago. The other objects in our solar system are remnants of the material that formed our sun and planets. Occasionally asteroids will land on the Earth, and we can determine their origin by looking at their chemical and mineral composition.
There are no boundaries to our solar system. Astronomers consider the eight planets and everything in between them part of our solar system. Neptune is the planet that is farthest from the sun, and it is about 30 astronomical units from the sun. One astronomical unit is equal to about 93 million miles.
Our solar system extends past Neptune. The Kuiper Belt begins near Neptune's orbit and stretches to about 50 astronomical units from the sun. Pluto's elliptical orbit extends past the end of the Kuiper Belt. After Pluto's orbit, the Oort Cloud can be found; this extends to 50,000 astronomical units from the sun.
It's important to remember that all of these statements about the solar system are theories, and not facts. We don't know for sure how the solar system formed, and there are some other explanations for how the sun and planets came into being. One of the great things about astronomy is that we're constantly learning about how our solar system works. So while we'll never be able to actually see the formation of the solar system, we'll someday have a solid understanding of exactly how it happened.
When making solar system projects for kids, you can always order a solar system mobile kit. But you can make the project even more fun if you're willing to go the extra mile, literally.
Check out these fun astronomy facts for kids when you're looking for a science project. Your kids can also build up their online research skills as they learn more about astronomy.