Step by Step
1
The solar nebula hypothesis
A cloud of gas and dust collapsed under its own gravity roughly 4.6 billion years ago. Due to conservation of angular momentum, this collapsing cloud flattened into a spinning disk, with the Sun igniting at its center.
2
From planetesimals to planets
Dust grains within the disk stuck together, forming pebbles, then kilometer-sized objects, and eventually full planets through a process called runaway accretion.
3
Differentiation
As rocky planets formed, they melted from the heat of ongoing collisions and radioactive decay, allowing dense material like iron to sink toward the core while lighter material rose to the surface.
4
The Grand Tack and Late Heavy Bombardment
The Grand Tack hypothesis proposes that Jupiter migrated inward and then back outward early in the solar system's history, helping explain why the asteroid belt has so much less mass than expected. The Late Heavy Bombardment, around 4 billion years ago, describes a period when Jupiter's migration sent many asteroids and comets careening into the inner solar system. Isotopic dating of meteorites confirms the solar system's age at 4.568 billion years.
Applied Walkthrough
1
About 4.6 billion years ago, a cloud of gas and dust began collapsing under its own gravity — conservation of angular momentum caused it to flatten into a spinning disk, with the Sun forming at the center as pressure and temperature rose.
2
Within this protoplanetary disk, dust grains gradually stuck together, building up first into pebbles, then kilometer-sized planetesimals, and finally into full-sized planets through runaway accretion.
3
As the rocky planets formed, the intense heat from ongoing impacts and radioactive decay melted them enough for dense material like iron to sink toward their cores — a process called differentiation, explaining why Earth (and similar planets) have distinct layered structures today.
4
Early in this process, Jupiter is thought to have migrated inward and then back outward (the Grand Tack), an event that helps explain the asteroid belt's surprisingly low mass and also triggered the Late Heavy Bombardment, a period when many asteroids and comets were flung into the inner solar system.
Exam Application
Exams test whether you can trace the full sequence of solar system formation (nebula collapse → disk → planetesimals → planets → differentiation), and whether you know the Grand Tack and Late Heavy Bombardment as key events shaping the early solar system's structure.
⚠ Common Trap
The most common trap is thinking planets formed instantly or in their current orbits — the Grand Tack hypothesis specifically shows that Jupiter's position changed significantly over time, reshaping the distribution of material throughout the solar system.
✓ Quick Self-Check
1. What is the solar nebula hypothesis?
The idea that the Sun and planets formed from a collapsing cloud of gas and dust roughly 4.6 billion years ago.
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2. What is runaway accretion?
The process by which dust grains build up into pebbles, then kilometer-sized planetesimals, and finally full planets.
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3. What is differentiation, in the context of planet formation?
The process where a molten planet's dense material (like iron) sinks to the core while lighter material rises to the surface.
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4. What does the Grand Tack hypothesis propose?
That Jupiter migrated inward and then back outward early in the solar system's history, helping explain the asteroid belt's low mass.
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5. How old is the solar system, based on isotopic dating of meteorites?
4.568 billion years.
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