Formation of the Earth

Nebular Hypothesis

About 4.6 billion years ago, our solar system began as a giant cloud of gas and dust in space. This cloud, called a nebula, was made mostly of hydrogen, helium, and small amounts of other elements. It was from this nebula that our planet formed by the following steps:

Step 1: The Solar Nebula Collapses

Something—likely a nearby exploding star (a supernova)—disturbed the nebula, causing it to collapse under its own gravity and to begin rotating.

Step 2: Formation of the Protostar

As the nebula collapsed, it spun faster and faster and flattened into a disk. Most of the material gathered in the center of the disk to form the Protosun (early version of the Sun). A protosun (protostar) is a stage before nuclear fusion has started in the core of the protosun. It is still very hot and under a great deal of pressure, but no fusion is occurring.

Step 3: Formation of Planetesimals

In the disk around the young Sun, particles of dust and rock began to stick together in a process known as accretion due to static electricity and gravity. Over time, these clumps grew into larger bodies called planetesimals, which were a few kilometers wide.

Step 4: Formation of Protoplanets

Through collisions and gravity, planetesimals merged to form protoplanets—early versions of planets. One of these protoplanets eventually became Earth.

Step 5: Formation of Planets

Protoplanets collide with anything in their orbit and continue to accumulate material until they clear their orbit of debris. They are also big enough for their gravity to make them round in shape.

Step 6: Constant Bombardment

Leftover material from the formation of the solar system, asteroids and comets, etc., smashed into the Earth quite often between 4.1-3.8 billion years ago. The energy associated with these collisions caused the Earth to become molten.

Step 7: Chemical Differentiation

As more material crashed into early Earth, it got bigger and hotter. Radioactive elements and heat from collisions melted the inside of the planet. This caused heavy materials like iron and nickel to sink to the center, forming Earth's core, while lighter materials rose to form the mantle and crust. This process of natural separation and layering based on density is known as chemical differentiation.

Step 8: Cooling

Earth gradually cooled down, forming a solid crust and the earliest version of the planet as we know it. Before this, the Earth’s surface was molten.

Evidence for the Nebular Hypothesis

All planets orbit the Sun in the same direction and nearly the same plane

If planets formed from a swirling disk of gas and dust, we’d expect them to share a common direction and flat orbital plane.
And that’s exactly what we see today!

Planets are divided into inner rocky and outer gas/ice giants

Close to the Sun: too hot for gases and ices → only metals and rocks could form planets (Mercury, Venus, Earth, Mars).
Farther out: cold enough for gases and ices to condense → huge gas/ice planets (Jupiter, Saturn, Uranus, Neptune).
This matches the physics of a cooling solar nebula.

Moons, asteroids, and comets are leftover building blocks

Moons and asteroids resemble the same material planets are made from.
Comets are icy leftovers from the outer Solar System, consistent with the nebular model.

Young star systems show disks of gas and dust today

Telescopes (like Hubble and ALMA) have imaged protoplanetary disks around young stars.
These disks look like the “solar nebula” in action — new solar systems being born.

Meteorites provide a record of early Solar System material

Meteorites contain primitive material from ~4.6 billion years ago.
Their composition matches predictions of condensation from a cooling solar nebula.

Computer models reproduce planetary formation

Simulations show that gravity acting on a rotating disk of gas and dust naturally produces a central star and orbiting planets.

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