What triggered the Solar System's formation?

The most widely accepted trigger is a supernova shockwave. The Solar System formed from a giant molecular cloud — a cold, dark gas cloud with a mass of thousands of solar masses. These clouds are generally stable, but the passage of a shockwave from a nearby supernova explosion can compress part of the cloud enough to trigger gravitational collapse. Evidence for this comes from meteorites: they contain short-lived radioactive isotopes (such as aluminium-26) that decay rapidly and could only have been produced by a recent supernova, incorporated into the solar nebula while still "fresh". The collapse was not instantaneous — it took ~100,000 years for the cloud to flatten into the protoplanetary disc. The disc itself evolved over several million years before the Sun and planets reached their final forms.

Why are the inner planets rocky and the outer planets gaseous?

The answer lies in the frost line (also called the snow line) — the distance from the forming Sun at which water ice could remain solid (~2.7 AU in the early Solar System). Inside the frost line, only refractory materials (silicates, metals) could condense into solid particles; water, ammonia, and methane remained gaseous and were blown away by the young Sun's intense solar wind. So only rocky, metallic planetesimals formed in the inner Solar System. Beyond the frost line, ices could condense, dramatically increasing the amount of solid material available. This allowed the outer protoplanets to grow massive enough (above ~10 Earth masses) to gravitationally capture hydrogen and helium directly from the solar nebula — creating the gas giants Jupiter and Saturn, and the ice giants Uranus and Neptune.

How did Earth get its Moon?

The leading hypothesis is the giant impact hypothesis: approximately 4.5 billion years ago, a Mars-sized protoplanet (named Theia) struck the young Earth in a glancing collision. The impact vaporised both bodies' outer layers; the debris was flung into Earth's orbit and quickly (within ~1,000 years) coalesced under gravity into the Moon. Evidence: the Moon's composition closely matches Earth's mantle; it has a very small iron core (consistent with originating from the outer layers of both bodies); and oxygen isotope ratios on Earth and the Moon are nearly identical, suggesting a common origin. The Moon is unusually large for its parent planet — roughly 1/81 of Earth's mass — making this type of origin plausible. See our guide to how orbits work for how the Moon stabilises Earth's axial tilt.

Did Jupiter's migration affect the inner Solar System?

Yes — significantly. The Grand Tack hypothesis proposes that Jupiter migrated inward to about 1.5 AU (inside Mars's current orbit) early in the Solar System's history, before Saturn's gravity pulled it back out to its current orbit. This inward-then-outward migration would have swept the asteroid belt, explaining why it has so little mass, and may have scattered volatile-rich material (including water) into the inner Solar System. The Nice model proposes a later rearrangement (~600 million years after formation) where Jupiter, Saturn, Uranus, and Neptune reached an orbital resonance that drove the Late Heavy Bombardment — the period of intense cratering visible in the lunar record. Both models are still debated but explain features of the Solar System that simple accretion models cannot.

How the Solar System Formed — The Nebular Hypothesis

The nebular hypothesis — five stages

The nebular hypothesis, first proposed by Kant and Laplace in the 18th century and refined by modern astrophysics, explains the Solar System's formation from a rotating cloud of gas and dust. The key stages, spanning roughly 800 million years, are:

Molecular cloud collapse

4.6 Gyr ago

A giant molecular cloud — possibly triggered by a nearby supernova shockwave — began collapsing under gravity. Angular momentum caused it to flatten into a rotating disc.

Protostellar disc

4.59 Gyr ago

The central concentration heated up, forming a protostar. A protoplanetary disc of gas and dust (the solar nebula) surrounded it. Dust grains stuck together — accretion began.

Planetesimals

4.57 Gyr ago

Dust grains grew to centimetre-, then metre-, then kilometre-sized bodies called planetesimals through collisions and gravitational attraction. Millions of them filled the disc.

Protoplanets

4.55 Gyr ago

Planetesimals collided and merged into Moon-to-Mars-sized protoplanets. Rocky planets formed in the hot inner disc; gas giants formed rapidly in the cold outer disc beyond the "frost line".

Late heavy bombardment

~4.1–3.8 Gyr ago

A period of intense asteroid and comet impacts — recorded in lunar craters — as orbital resonances with Jupiter shifted debris belts inward. Possibly delivered water to Earth.

The raw material left over from formation — the asteroid belt and Kuiper Belt — tells us a great deal about the young Solar System's chemistry. See our asteroids, comets and meteors guide for what these remnants reveal. How the resulting planets orbit is covered in how orbits work.

How the Solar System formed

The nebular hypothesis — five stages

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