The Solar System formed about 4.6 billion years ago from a dense cloud of gas and dust. Gravity pulled the cloud together, forming a spinning disk around the young Sun. Within that disk, dust and rock gradually joined to build planets, moons, asteroids and comets.
Scientists have reconstructed this process from meteorites, observations of young stars surrounded by planet-forming disks, and models of planetary formation. The exact details of every step are still being studied, but the broad sequence is well established.
The Solar System Began as a Cloud of Gas and Dust
The material that formed the Solar System was part of a cold molecular cloud, a large region of gas and dust in space. A dense part of this cloud began collapsing under its own gravity.
Scientists think an external disturbance may have helped trigger the collapse, but the exact cause is not known with certainty.
Gravity Started the Collapse
As gravity pulled more material toward the center, the cloud became denser and its rotation became more noticeable. The collapsing material did not simply fall straight inward. Its motion helped shape the cloud into a flattened, rotating structure.
The Cloud Flattened Into a Spinning Disk
As the collapse continued, much of the material formed a protoplanetary disk—a rotating disk of gas and dust surrounding the young Sun.
Most of the material moved toward the center, where the young Sun was forming. The remaining material stayed in the disk and became the raw material for planets and smaller objects.
The Young Sun Formed at the Center
The central region became increasingly dense and hot until the young Sun developed into a star. It eventually contained more than 99% of the Solar System’s mass.
The detailed formation of the Sun is covered separately in How Was the Sun Formed?
Tiny Dust Grains Began Building Larger Objects
Inside the disk, tiny grains of dust and ice collided and sometimes stuck together. Over time, these small particles grew into larger pieces of material.
The process was not simply a smooth buildup. Collisions could make objects grow, break apart or change their paths. Still, over time, some bodies became large enough for gravity to dominate their growth.
Planetesimals Became the Building Blocks of Planets
Larger bodies called planetesimals formed from the material in the disk. They became important building blocks of the planets.
As planetesimals collided and merged, some grew into planetary embryos. These larger bodies continued collecting material until the major planets began to take shape.
Different Planets Formed in Different Parts of the Disk
The young Solar System was not equally hot everywhere. The region close to the Sun was much hotter than the outer disk.
Rocky materials could remain solid in the hotter inner region, helping form Mercury, Venus, Earth and Mars. Farther from the Sun, colder conditions allowed ice to survive, giving the growing outer planets access to additional material.
Why the Inner and Outer Planets Look Different
This difference in temperature helps explain the broad division between the inner rocky planets and the much larger outer planets.
Jupiter and Saturn became gas giants, while Uranus and Neptune became ice giants. Their final properties were also influenced by how they grew and interacted with the surrounding disk.
The Young Solar System Was Still Chaotic
The early Solar System contained many growing planets and leftover pieces of rock and ice. Their gravity affected one another, while collisions continued to reshape the system.
Some objects merged into larger bodies. Others were scattered into different orbits or remained as smaller objects. The young Solar System therefore looked very different from the relatively stable system we see today.
Leftover Material Became Asteroids and Comets
Not all of the material became planets. Some remained as asteroids, comets and other small bodies.
These leftovers are especially useful to scientists because many preserve material from the Solar System’s early history. Studying them helps researchers understand the conditions in which the planets formed.
The Solar System Eventually Became the System We See Today
Over millions of years, collisions, accretion and gravitational interactions shaped the young system. The remaining gas was eventually dispersed, leaving a system dominated by the Sun, planets and smaller solid bodies.
The basic story is simple: a cloud of gas and dust collapsed, formed a spinning disk, produced the young Sun at its center, and gradually turned the remaining material into planets and smaller worlds.
Scientists are confident about this broad picture, but some details—such as exactly how individual planets grew and how their early orbits changed—remain active areas of research.
