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How do planets form?

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Quick answer

Planets form from a disk of gas and dust that surrounds a young star. Dust grains stick together to form larger clumps, which grow into planetesimals. These collide and merge, building planetary cores. Close to the star, only rock and metal condense, forming rocky planets; farther out, ice and gas allow giant planets to grow.

Planets form from a disk of gas and dust around a young star, where tiny grains stick together into larger bodies. These bodies grow into planetesimals and then planets, with rocky planets near the star and gas giants farther out.

Step by step

  1. A cloud of gas and dust collapses under gravity, forming a spinning disk around a new star.
  2. Dust grains in the disk collide and stick together, forming pebbles and then larger clumps.
  3. These clumps grow into planetesimals, which are small bodies a few kilometres across.
  4. Planetesimals collide and merge, building up the cores of future planets.
  5. In the inner disk, rocky planets form; in the outer disk, gas giants accumulate thick atmospheres.
  6. After the disk disperses, the remaining planets continue to orbit the star.

Key things to understand

  1. A collapsing cloud of gas and dust flattens into a spinning disk around a new star.
  2. Dust grains in the disk collide and stick, forming pebbles and then larger clumps.
  3. These clumps grow into planetesimals, which are small bodies a few kilometres across.
  4. Planetesimals collide and merge, building up the cores of future planets.
  5. Close to the star, only rock and metal condense, forming small rocky planets; farther out, ice and gas allow giant planets to grow.

Labelled diagram

  1. Molecular cloud

    A cold, dense cloud of gas and dust begins to collapse under its own gravity.

  2. Protoplanetary disk

    The collapsing cloud flattens into a spinning disk around a young star.

  3. Dust clumps

    Dust grains collide and stick, forming pebbles and then larger clumps.

  4. Planetesimals

    Clumps grow into small bodies a few kilometres across.

  5. Planets

    Planetesimals merge into planets; rocky planets inner, gas giants outer.

Planet formation from a protoplanetary disk

Real-world examples

  • The rocky planets Mercury, Venus, Earth, and Mars formed close to the Sun where temperatures were high.
  • Jupiter and Saturn formed farther out where it was cold enough for ice and gas to condense.
  • The asteroid belt between Mars and Jupiter contains leftover planetesimals that never became planets.
  • Exoplanets like those in the TRAPPIST-1 system show that planet formation occurs around other stars too.

Common mistake

✗ Myth: Planets formed by a sudden collapse of a gas cloud into a solid body.

✓ Fact: Planets form gradually from a disk of gas and dust, where tiny grains stick together and grow into larger bodies over millions of years.

People also ask

How do planets form step by step?
First, a cloud of gas and dust collapses into a spinning disk around a new star. Dust grains stick together to form pebbles, then larger clumps called planetesimals. These collide and merge, building planetary cores. In the inner disk, rocky planets form; in the outer disk, gas giants grow by capturing gas. Finally, the disk disperses, leaving the planets in orbit.
What is the difference between a planet and a star?
A star is massive enough to sustain nuclear fusion in its core, producing light and heat. A planet is smaller and does not fuse elements; it shines only by reflected light. Planets form from the disk around a star, while stars form directly from collapsing gas clouds.
Why are gas giants farther from the sun?
In the early solar system, the inner region was too hot for ice and gas to condense, so only rock and metal could form planets. Farther out, temperatures were low enough for ice to condense, providing more solid material. This allowed gas giants to grow large enough to capture thick atmospheres of hydrogen and helium.
Why are some planets rocky and others made of gas?
Temperature in the disk determines what materials can condense. Near the star, it is too hot for ice and gas to stay solid, so only rock and metal form planets. Farther out, it is cold enough for ice to condense, providing more solid material and allowing gas giants to capture thick atmospheres.
How long does it take for a planet to form?
Rocky planets may take tens of millions of years to assemble from planetesimals. Gas giants must form faster, within a few million years, before the star's radiation blows away the surrounding gas. The exact timescales are still studied.
What is a protoplanetary disk?
A protoplanetary disk is a rotating disk of dense gas and dust surrounding a newly formed star. It is the birthplace of planets. The disk gradually dissipates over millions of years, leaving behind the planets and other debris.
Do all stars have planets?
Most stars are thought to have at least one planet. Observations have found thousands of exoplanets, and statistical studies suggest that planet formation is a common outcome of star formation. However, not every star may have planets.

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