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About 4.6 billion years ago, our Solar System began forming from a huge cloud of gas and dust called the solar nebula. At the center of this cloud, the Sun began to form. As the Sun grew, the remaining gas, dust, and rocks began coming together to form planets. The first giant planets to form were Jupiter and Saturn. Jupiter became the largest planet because it formed early and had access to enormous amounts of gas and other material. Saturn formed farther from the Sun, followed by Uranus and Neptune.

Closer to the Sun, smaller rocky planets began to form. These included Mercury, Venus, Earth, Mars, and several other early planetary bodies. One of these bodies was Theia, a Mars-sized object that is believed to have formed in the same general region as the young Earth.

However, the early Solar System was a chaotic place. The orbits of the young planets and other objects were constantly changing because of their gravitational interactions.

One important event may have been Jupiter's Grand Tack. According to this hypothesis, Jupiter may have migrated inward toward the Sun and later moved outward again because of its interactions with Saturn and the surrounding material. This movement could have affected the distribution of material in the inner Solar System and helped shape the formation of the rocky planets.

Jupiter's strong gravity also influenced the region between Mars and Jupiter. Instead of forming a full-sized planet, the material there remained scattered and eventually became the asteroid belt. One of the largest objects in this region is Ceres, which is classified as a dwarf planet. Meanwhile, the young rocky planets continued to grow by colliding with smaller bodies and clearing material from their orbital regions.

Eventually, about 4.5 billion years ago, Theia is thought to have collided with the young Earth. The enormous impact changed both bodies. Much of the material combined to form the Earth, while debris thrown into orbit around Earth eventually came together to form the Moon.

And so, from a chaotic cloud of gas, dust, and rocks, the Solar System gradually became the planetary system we know today—with the Sun at its center and eight planets orbiting around it.