Once upon a time there wasn't a Solar System.

Only gases...

Only gas???

YES!

Like all families, the members of our solar system family share a common origins story. Their story started even before our solar system formed 4.56 billion years ago. Their story started when the story started for every single thing in our universe. Our universe was born from the Big Bang about 13.5 billion years ago. The first stars lived out their lives and eventually exploded, sending "star stuff" out into the cosmos. That original stellar material was recycled as another generation of stars, and many of these, too, exploded at the end of their lives. Our Sun is thought to be a third-generation star and our entire solar system is made of the recycled star stuff of previous star generations.

Our solar system began forming about 4.6 billion years ago within a concentration of interstellar dust and hydrogen gas called a molecular cloud. The cloud contracted under its own gravity and our proto-Sun formed in the hot dense center. The remainder of the cloud formed a swirling disk called the solar nebula.

Within the solar nebula, scientists believe that dust and ice particles embedded in the gas moved, occasionally colliding and clumping together. Through this process, called "accretion," these microscopic particles formed larger bodies that eventually became planetesimals with sizes up to a few kilometers across. In the inner, hotter part of the solar nebula, planetesimals were composed mostly of silicates and metals. In the outer, cooler portion of the nebula, water ice was the dominant component.

The Sun's light warmed the objects in our solar system, especially those in the inner solar system. There, it was too warm for lightweight volatiles, such as water and ammonia, to condense. In addition, particles from the Sun (the solar wind) pushed volatiles out of the inner solar system. When the volatiles reached the cold temperatures of the outer solar system -- out beyond an invisible boundary called the "frost line" -- they condensed onto the nascent giant planets. Thus, the outer planets had rocks, metals, and volatiles available to accumulate, while the relatively warm, "windy" inner region was stripped of all but the densest materials, like rock and metal. For more information about the compositions of the planets see A Family Affair.

When the nascent planets grew from a few kilometers to a few hundred kilometers across, they became massive enough that their gravity influenced each other's motions. This increased the frequency of collisions, through which the largest bodies grew most rapidly. During this "childhood" stage of growth, the bodies are referred to as planetesimals. Eventually, regions of the nebula were dominated by large protoplanets. The interiors of these more mature bodies were becoming ordered -- differentiated -- into protoplanets. The process of collision and accretion continued until only four large bodies remained in the inner solar system -- Mercury, Venus, Earth, and Mars, the terrestrial planets. In the cold outer solar nebula, much larger protoplanets formed. The largest ones swept up other protoplanets, planetesimals, and nebular gas, leading to the formation of Jupiter, Saturn, Uranus, and Neptune.

Shortly after Earth formed, the Moon did. A large object (about half as wide as Earth) collided with our world. The off-center cosmic smash-up increased Earth's spin, and its energy disintegrated the impacting object, melted Earth's outer layers, and flung debris into orbit around Earth. This material formed a ring of gas, dust and molten rock around Earth. In less than a hundred years -- an incredibly short time for the formation of an entire world -- this debris clumped (accreted), growing larger and larger to form our Moon!