Edexcel GCSE Astronomy (1AS0) · Formation of Planetary Systems
Mini-Lesson
Formation of Planetary Systems
Topic 12 explains where the solar system came from — the solar nebula theory — and why the planets fall into two families. It then looks outwards, to the thousands of exoplanets now known and how they were found.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Formation · the solar nebula
The solar nebula theory
About 4.6 billion years ago a giant cloud of gas and dust — the solar nebula — began to collapse under its own gravity (perhaps triggered by a nearby supernova).
As it collapsed it spun faster (conservation of angular momentum) and flattened into a disc.
The centre grew hottest and densest, and when the core reached about 15 million °C, fusion ignited — the Sun was born.
In the disc, dust grains collided and stuck together, growing into kilometre-sized planetesimals.
Gravity took over: planetesimals swept up their neighbours by accretion and grew into protoplanets and then planets.
The young Sun's fierce radiation and solar wind blew the leftover gas away, ending planet formation.
Evidence it is right: all the planets orbit in the same direction and in nearly the same plane — exactly what a single spinning disc predicts. And astronomers can now see protoplanetary discs, with gaps carved by forming planets, around young stars.
Quick check
Why a flat disc?
?Why do all the planets orbit the Sun in nearly the same plane and the same direction?
Formation · the frost line
Why the planets come in two families
The disc was hot near the Sun and cold further out. The frost (snow) line — out around the asteroid belt, roughly 3 AU from the Sun — marks where it became cold enough for water, ammonia and methane to freeze into ice.
Inside the frost line only rock and metal could condense. There was much less of it, so the planets that formed there are small, dense and rocky: Mercury, Venus, Earth, Mars — the terrestrial planets. The Sun's heat and wind stripped away their light gases.
Outside it, ices survived, so there was far more solid material. Cores grew large fast, and their gravity captured huge envelopes of hydrogen and helium: Jupiter, Saturn, Uranus, Neptune — the giant planets, with low densities, many moons and ring systems.
What was left over: the asteroid belt between Mars and Jupiter (planetesimals stirred up by Jupiter's gravity so they never coalesced), the Kuiper belt beyond Neptune (home of Pluto and short-period comets), and, far beyond, the spherical Oort cloud — the source of long-period comets.
Saturn floats: its mean density is only about 0.7 g/cm³ — less than water. Compare the Earth at 5.5 g/cm³. Density is the quickest way to tell the two families apart.
Quick check
Where did the gas giants get their gas?
?Why did the giant planets end up with enormous hydrogen and helium atmospheres, while the terrestrial planets did not?
Formation · exoplanets
Finding planets around other stars
Thousands of exoplanets are now known. Two methods dominate:
Transit method — if the planet's orbit is edge-on to us, the planet passes in front of the star and the star's brightness dips slightly, over and over, at regular intervals. The depth of the dip gives the planet's size; the time between dips gives its orbital period. (Used by Kepler and TESS.)
Radial velocity ("Doppler wobble") — the star and planet both orbit their common centre of mass, so the star wobbles slightly towards and away from us. Its spectral lines shift blue then red. This gives the planet's mass and period.
depth of transit dip = (Rplanet ÷ Rstar)²so a 1% dip means the planet's radius is √0.01 = 0.10 of the star's
Bias to notice: both methods favour big planets orbiting close to their star (deep dips, big wobbles, short periods) — which is why the first exoplanets found were "hot Jupiters". Transits also only work for the small fraction of systems that happen to be edge-on. An Earth-like planet is much harder to spot.
Calculate
Your turn — how big is the exoplanet?
1A star's brightness dips by 1.0% (a fraction of 0.010) every time its planet transits. Use dip = (Rplanet ÷ Rstar)² to find the ratio Rplanet ÷ Rstar. (2 decimal places)
× R star
Hint: Take the square root of 0.010. √0.010 = 0.10 — the planet is a tenth of the star's radius (roughly Jupiter across a Sun-like star).
Calculate
Your turn — an exoplanet's orbit
2An exoplanet orbits a Sun-like star with a period of 8.0 years. Use Kepler's third law (T² = r³, with T in years and r in AU) to find its orbital radius r in AU. (1 decimal place)
AU
Hint: r³ = T² = 8² = 64. Then r = the cube root of 64.
Calculate
Your turn — the habitable zone
3A star is 4 times more luminous than the Sun. The middle of its habitable zone lies at a distance of √(luminosity) AU, because intensity follows an inverse square law. How far from that star, in AU, is the middle of its habitable zone?
AU
Hint: √4 = 2. At 2 AU from a star 4× as luminous, the intensity is 4 ÷ 2² = 1 — the same as the Earth receives.
Quick check
Which method?
?Astronomers watch a star's spectral lines shift steadily towards the blue and then towards the red, over and over, with a period of 12 days. What have they detected?
Sort it
Sort the solar system
Tap a statement, then tap the family of bodies it describes.
🪨 Terrestrial planets
🎈 Giant planets
☄️ Small bodies
Match it
Match the description to the term
Tap a description on the left, then its matching term on the right.
Description
Term
Quick check
Why so many hot Jupiters?
?The first exoplanets discovered were nearly all giant planets orbiting very close to their stars. Why?
Formation · dating the system
How we know it is 4.6 billion years old
You cannot date the solar system from Earth rocks — the Earth's surface is constantly recycled by plate tectonics, weathering and volcanism, so almost nothing original survives. Instead we date meteorites.
Meteorites are leftover planetesimals that never became part of a planet, so they have sat essentially unchanged since they formed.
They are dated by radioactive decay: a radioactive isotope decays into a stable daughter product at a known half-life, so the ratio of the two acts as a clock.
The oldest meteorites all come out at about 4.6 billion years — and the oldest Moon rocks agree, because the Moon has no plate tectonics either.
Consistency is the key to trusting a result. Independent methods — meteorites, lunar samples, and models of the Sun's evolution — all give the same age. That agreement is what makes 4.6 billion years a fact rather than a guess.
Quick check
Why meteorites?
?Why are meteorites, rather than Earth rocks, used to date the formation of the solar system?
Recap
The big ideas to know
Solar nebula: a cloud of gas and dust collapses under gravity, spins faster and flattens into a disc
Accretion: dust grains stick → planetesimals → protoplanets → planets; the centre becomes the Sun
Frost line: inside it only rock and metal can condense (terrestrial planets); beyond it ices survive, so cores grow big enough to capture H and He (gas giants)
Leftovers: the asteroid belt (Mars-Jupiter), the Kuiper belt (beyond Neptune) and the distant Oort cloud of comets
Age: the solar system formed about 4.6 billion years ago — dated from meteorites
Transit method: a planet crossing its star dims it slightly; dip depth = (Rplanet ÷ Rstar)²
Radial velocity: the planet tugs the star, so its spectrum shifts red then blue — a "Doppler wobble"
That is Formation of Planetary Systems covered for Edexcel GCSE Astronomy. Press Finish to see your score.
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