There are several theories that attempt to explain how the solar system began, but the most widely accepted one is known as the nebular theory. Astronomers and physicists believe the solar system started as a large, shapeless cloud of gas, dust and ice, but something disrupted the mass and set things in motion -- perhaps the explosion of a nearby star.
If you've ever watched figure skating, you may have noticed that skaters can spin much faster if they pull their arms closer to their bodies. The more concentrated their body masses are, the faster they'll be able to rotate. The same thing happened with our solar system. The hypothetical explosion squeezed the unformed gas and dust together, which began to spin faster and faster in a circle. As the sun formed in the middle, the cloud started to flatten out into a disc, sort of like a Frisbee or a pancake, with tiny dust grains making up the rest of the disc.
Eventually, dust began to stick together and form larger bodies called planetesimals. Even more matter flying around collided with these planetesimals and stuck to them in a process called accretion. As the bodies spun themselves and gravity brought in more dust and gas, the planetesimals accreted into protoplanets, and soon into the eight planets we currently know and love -- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune (sorry, Pluto).
It's the area in between the fourth planet, Mars, and the fifth, Jupiter, that's important. An astronomical unit (AU) is the distance between the Earth and the sun, which is about 150 million kilometers -- astronomers use this distance as a ruler to measure other distances within the solar system and the Milky Way galaxy. Mars lies about 1.5 AU from the sun, or 225 million kilometers away. Jupiter, meanwhile, is about 5.2 AU from the sun, or 780 million kilometers away. If we subtract the two distances, there's about 3.7 AU between Mars and Jupiter, or 555 million kilometers. It seems like there's enough room between the two planets for yet another planet, right? What happened in between Mars and Jupiter during the formation of the solar system?
To find out what scientists think happened, read the next page.