Dust to planets: accretion builds the worlds
What happened
Inside the disk, grains of dust collided and stuck, and the clumps that grew biggest pulled in more material with their stronger gravity. This runaway sticking, called accretion, built pebbles into boulders, boulders into kilometre-wide planetesimals, and planetesimals into Mars-sized planetary embryos. Close to the Sun the disk was too hot for ice or gas to survive, so only rock and metal could condense; that inner zone produced the small rocky planets Mercury, Venus, Earth, and Mars. Past the frost line, where it was cold enough for ice and gas to pile onto growing cores, the giant planets Jupiter and Saturn took shape.
Why it matters
Accretion explains why the Solar System is sorted the way it is, rocky worlds near the Sun and gas giants beyond. Earth sits in the inner zone because that is the only place its iron-and-silicate recipe could come together.
How we know
The rocky-inner, gas-giant-outer pattern is exactly what the disk's temperature gradient predicts, and it matches the Solar System we observe. The leftover planetesimals that never joined a planet still orbit today as asteroids and comets.
Sources
- NASA (Astrobiology Program). How did our Solar System form? (2024) · Reputable sourcescience.nasa.gov · The domain "science.nasa.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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Part of a timelineThe Formation of the Solar System11 events · The 800 million years that turned a collapsing cloud of gas into the Sun, the planets, and a habitable Earth, from the oldest solids we can date to the first oceans.View all →