How the Solar System formed
The Sun and planets formed from a cloud of gas and dust about 4.6 billion years ago. Chondrite meteorites contain solid grains from that time, so they are direct evidence for the story.
- Describe how the Sun and planets formed from a nebula under gravity.
- Explain why chondrites are evidence for the early Solar System.
- Use standard form to compare ages and timescales.
Links to: GCSE physics (separate), space physics: the formation of the Solar System from a cloud of dust and gas pulled together by gravity; maths: standard form.
From nebula to Sun
The Solar System began as part of a cloud of gas and dust, a nebula, made mostly of hydrogen and helium with small amounts of heavier elements made in earlier stars. Gravity pulled part of the cloud together. As it collapsed, it heated up and began to spin faster, flattening into a disc with a dense, hot centre, a protostar.
When the core became hot and dense enough, hydrogen nuclei began to fuse into helium, releasing energy. The outward pressure from this energy balanced the inward pull of gravity, and the Sun became a stable main-sequence star, as it still is today.
Building planets
In the disc around the young Sun, dust grains stuck together into larger and larger bodies. Bodies a few kilometres across, called planetesimals, then grew by gravity and collisions into the planets. Close to the Sun it was too hot for ice, so the inner planets are rocky. Further out, ice was available and the giant planets grew large enough to collect huge amounts of gas.
Between Mars and Jupiter, Jupiter's gravity stirred the planetesimals so much that they never built a planet. Many survive as asteroids.
The evidence in chondrites
Chondrites come from asteroids that never melted. They contain calcium-aluminium-rich inclusions (CAIs), the first solids to form in the disc, dated to about 4,567 million years ago, the age used for the Solar System. They also contain chondrules, once-molten droplets that formed in the disc over the next few million years.
The proportions of most elements in the rarest chondrites, the CI chondrites, match those in the Sun's outer layers, measured from the Sun's spectrum, apart from the lightest gases. That match supports the idea that the Sun and the asteroids formed from the same cloud.
Compressing 4.6 billion years
If the 4.6 × 10⁹ years since the Solar System formed were squeezed into one year of 365 days, how many years would each day represent? On this scale, how long ago did the dinosaurs die out, 6.6 × 10⁷ years ago?
- Years per day = (4.6 × 10⁹) ÷ 365 ≈ 1.26 × 10⁷ years, about 12.6 million years per day.
- Days for the dinosaurs = (6.6 × 10⁷) ÷ (1.26 × 10⁷) ≈ 5.2 days.
Answer: Each day stands for about 12.6 million years, and the dinosaurs died out only about 5 days before the end of the year, around 26 December.
Draw a labelled flow diagram of the formation of the Solar System in five stages: nebula, collapse and disc, protostar, fusion begins, planets and asteroids. Under each stage, write one piece of evidence from meteorites or observations that supports it.
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1What force caused the nebula to collapse?
Gravity pulled the gas and dust together, starting the formation of the Sun and planets.
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Question 2What balances gravity in a stable star like the Sun?
In a main-sequence star, fusion releases energy that creates outward pressure equal to the inward pull of gravity.
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Question 3What are CAIs?
Calcium-aluminium-rich inclusions are about 4,567 million years old, the oldest dated solids.
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Question 4Why did no planet form in the asteroid belt?
Jupiter's strong gravity disturbed the region, so planetesimals collided too violently to merge into a planet.
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Question 5Which is 4,567 million years in standard form?
4,567 million = 4,567,000,000 = 4.567 × 10⁹.