How the Solar System Formed

Everything in the solar system, from the Sun to the dust on the Moon to the atoms in your body, shares a common origin.

About 4.6 billion years ago, none of it existed in its present form.

There was only a cold, dark cloud of gas and dust drifting through our galaxy.

How that cloud became a star with eight planets, hundreds of moons and countless smaller bodies is one of the great detective stories in science.

Here is the process step by step,

why the planets ended up where they are and the evidence that lets scientists reconstruct events that happened long before Earth existed.

A Cloud of Gas and Dust

The story begins in a giant cloud of gas and dust, the kind of stellar nursery astronomers see scattered throughout the Milky Way today.

It was made mostly of hydrogen and helium, with small amounts of heavier elements such as carbon, oxygen, silicon and iron.

Those heavier elements had been forged inside earlier generations of stars and scattered into space when those stars died.

At some point, a region of this cloud began to collapse under its own gravity.

What set it off is uncertain.

One popular idea is that the shock wave from a nearby exploding star gave it a push,

and certain short-lived radioactive elements found in ancient meteorites fit that idea, though other explanations are possible.

The Sun was probably not born alone. Most stars form in groups from the same cloud,

so the Sun likely had many sibling stars that have long since drifted apart and scattered across the galaxy.

From Cloud to Spinning Disk

As the collapsing cloud shrank, it began to spin faster, in the same way a figure skater spins faster by pulling in their arms.

The spinning material flattened into a broad, rotating disk around a dense, hot center. Astronomers call this a protoplanetary disk.

This step explains one of the most striking features of the solar system.

All eight planets orbit the Sun in the same direction and in nearly the same flat plane,

and the Sun itself spins in that direction too. They inherited that motion from the spinning disk they formed in.

The Birth of the Sun

Most of the collapsing material fell into the center of the disk, where it grew denser and hotter.

Eventually, the temperature and pressure in the core became high enough for hydrogen to fuse into helium,

and the Sun began to shine as a true star.

It ended up with about 99.8 percent of the mass in the solar system.

The young Sun was restless, blowing out strong winds and flares.

Over the first few million years, its radiation and winds,

along with material falling into the Sun and being built into planets, helped clear the leftover gas from the disk.

That set a deadline: any planet that wanted to collect gas had to grow quickly.

Building Planets From Dust

Planets were built from the bottom up, in a sequence of stages that took tens of millions of years in all:

1. Dust sticks together. Microscopic grains in the disk collided and clung to each other, forming fluffy clumps and then pebbles.
2. Planetesimals form. Pebbles gathered into bodies roughly a kilometer to hundreds of kilometers across. Many scientists think this happened when dense swarms of pebbles collapsed under their own gravity.
3. Protoplanets grow. The biggest planetesimals pulled in their neighbors, sweeping up material and growing into protoplanets the size of the Moon or Mars.
4. Giant collisions. In the inner solar system, protoplanets smashed into one another over tens of millions of years, merging into the rocky planets. One of those late collisions is thought to have formed Earth's Moon.

Why Rocky Planets Inside and Giants Outside

The key to the solar system's layout is temperature.

Close to the young Sun, the disk was too warm for water to freeze, so only rock and metal could condense into solid grains.

Those materials were relatively scarce, so the inner planets stayed small and rocky.

Farther out, beyond a boundary known as the frost line or snow line,

which lay somewhere in the region of today's asteroid belt, water and other substances froze into ice.

With far more solid material to work with, planetary cores out there grew much larger.

Once a core reached roughly ten times Earth's mass, its gravity could pull in huge amounts of the surrounding hydrogen and helium gas.

Jupiter and Saturn grew fast enough to gather enormous gas envelopes before the disk cleared, becoming gas giants.

Uranus and Neptune, farther out where material was more spread out, grew more slowly and captured less gas, leaving them as ice giants.

A Restless Young Solar System

The planets did not simply form in place and stay put.

Computer models suggest that the giant planets moved significantly early on, as their gravity interacted with the gas and with leftover planetesimals.

Some models propose that Jupiter drifted inward and then back out, which might explain why Mars is so small and the asteroid belt so sparse.

Others suggest the giant planets' orbits rearranged themselves later, scattering icy bodies far and wide.

These models are still debated, but they help explain the Kuiper Belt,

the distant Oort Cloud and the heavy cratering seen on old surfaces like the Moon's.

The asteroids, comets and icy bodies of today are the leftovers: building material that never became part of a planet.

How Scientists Know

No one was there to watch, so how can scientists be so confident about this story?

Several independent lines of evidence point the same way:

• Meteorites. The oldest solid grains inside certain meteorites can be dated precisely using radioactive elements, giving an age of about 4.6 billion years for the start of the solar system.
• Rocks from the Moon. Samples returned by Apollo astronauts preserve an early record that Earth's restless surface has erased, including evidence of a hot, molten youth and heavy early bombardment.
• The planets' orbits. The shared direction and flat arrangement of the orbits are exactly what a spinning disk would produce.
• The pattern of composition. Rocky worlds near the Sun and icy, gas-rich worlds farther out match the temperature pattern expected in the disk.
• Other young stars. Modern telescopes have captured images of dusty disks around young stars, some with rings and gaps where planets may be forming. We can see the process happening elsewhere.
• Computer simulations. Models of collapsing clouds and growing planets reproduce many features of the real solar system, though not yet every detail.

Discoveries of planets around other stars have added a twist.

Many planetary systems look quite different from ours, with giant planets orbiting very close to their stars.

That suggests planet formation can follow many paths, and our solar system is only one outcome.

Still Writing the Story

The broad outline of the solar system's birth is well established: a collapsing cloud, a spinning disk, a new star and planets built from dust.

Many details, from what triggered the collapse to how far the giant planets wandered, are still being worked out.

Every new meteorite, spacecraft mission and image of a young star system adds another page to that story.