Astronomy & Space · Unit 14
How the Solar System Formed
The nebular hypothesis, planet formation and the system's age
The Solar System formed from a collapsing cloud of gas and dust, and the nebular hypothesis explains a striking amount: why the planets orbit in one plane, in one direction, and why the inner ones are rocky.
The unit covers the collapse and disc formation, how planets accrete, why rocky and gas giants ended up where they did, the early bombardment, and how the system's age is measured.
This unit breaks down into 21 short steps and 120 questions, starting at difficulty 3 and building to 4. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.
- Steps
- 21
- Questions
- 120
- Difficulty
- 3-4
What this unit covers
- The Nebular Hypothesis
- How Planets Form
- Early Bombardment
- Rocky vs Gas Giants
- The Age of the Solar System
Where this fits
Best after The Planets in Depth and Moons, Asteroids and Comets.
Where people slip
The planets did not necessarily form where they are now. Migration models are needed to explain the giant planets' positions, and the tidy account of everything forming in place is out of date.
How the unit is structured
How the Solar System Formed runs as 21 short steps that unlock in order. 15 are practice rounds and 6 are challenge rounds that pull together everything before them. Questions start at difficulty 3 and climb to 4 as you progress.
Challenge rounds
Example questions
30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 120 in the unit altogether.
Early Bombardment
- Build the sentenceLevel 3
1. Build a true sentence about the Moon's origin.
Answer: A giant impact likely formed the Moon
A giant impact likely formed the Moon from debris blasted off the early Earth.
- Fact or fibLevel 3
2. The Late Heavy Bombardment was a time with almost no impacts.
Answer: False
It was the opposite: a time of many violent impacts that cratered young worlds.
- Fill the blankLevel 3
3. The many craters on the ____ record the heavy bombardment of the early solar system.
- Mooncorrect
- Sun
- Milky Way
- ocean floor
With no weather to erase them, the Moon's craters preserve that ancient history.
- Choose all that applyLevel 4
4. Which statements about the giant-impact hypothesis are true? Select all that apply.
- A Mars-sized body struck the early Earthcorrect
- The debris came together as the Mooncorrect
- Earth and the Moon have similar compositionscorrect
- The impact created the Sun
A Mars-sized body hit Earth, its debris formed the Moon, and that is why Earth and Moon are so alike.
- Guess the numberLevel 4
5. About how long ago, in billions of years, did the Late Heavy Bombardment peak?
Answer: 3.9 billion years
The impact spike peaked around 3.9 billion years ago.
- Match the pairsLevel 4
6. Match each term to its meaning.
Answer: Theia = Mars-sized impactor; Late Heavy Bombardment = Impact spike near 3.9 billion years ago; Lunar maria = Ancient lava plains
Theia was the impactor, the Late Heavy Bombardment was an impact spike, and maria are lava plains.
How Planets Form
- Choose all that applyLevel 3
7. Which statements about planetesimals are true? Select all that apply.
- They are building blocks of planetscorrect
- They form from smaller grains sticking togethercorrect
- They are larger than the Sun
- They are a kind of galaxy
Planetesimals are km-sized building blocks made from smaller grains sticking together.
- Fact or fibLevel 3
8. The gas giants captured huge amounts of hydrogen and helium gas.
Answer: True
Jupiter and Saturn pulled in thick envelopes of hydrogen and helium from the disk.
- Fill the blankLevel 3
9. Small dust grains grew into pebbles, then into ____ a few km across.
- planetesimalscorrect
- galaxies
- stars
- moons of Jupiter
Planetesimals are the km-sized stepping stones between dust and planets.
- Guess the numberLevel 3
10. How many planets formed in our solar system?
Answer: 8 planets
Eight planets formed and remain today, from Mercury out to Neptune.
- Match the pairsLevel 3
11. Match each formation stage to what it is.
Answer: Dust grains = Stick together; Planetesimals = Km-sized bodies; Protoplanets = Nearly planet-sized
Grains stick together, then build planetesimals, which merge into protoplanets.
- Multiple choiceLevel 3
12. In planet formation, what do tiny dust grains do first?
- They stick together into larger clumpscorrect
- They instantly become full-size planets
- They turn into stars
- They collapse into black holes
Dust grains bump into each other and stick together into larger clumps and pebbles.
Rocky vs Gas Giants
- Build the sentenceLevel 3
13. Build a true sentence about where gas giants formed.
Answer: Gas giants formed beyond the frost line
Gas giants formed beyond the frost line, where ice added extra building material.
- Choose all that applyLevel 3
14. Which of these are gas giants? Select all that apply.
- Jupitercorrect
- Saturncorrect
- Mars
- Mercury
Jupiter and Saturn are the two gas giants; Mercury and Mars are small rocky planets.
- Fill the blankLevel 3
15. Inside the frost line, mostly rock and ____ could turn solid.
- metalcorrect
- ice
- gas
- hydrogen
Near the Sun only rock and metal condensed, so the inner planets are small and rocky.
- Multiple choiceLevel 3
16. Why are the four inner planets rocky?
- Near the Sun it was too warm for ice, so mostly rock condensed
- The Sun pulled all the gas to the inner planets
- They formed long after the outer planets
- They are made of leftover comets
Close to the Sun it was too warm for ice, so mainly rock and metal could turn solid there.
- Odd one outLevel 3
17. Which one is NOT a rocky inner planet?
- Jupitercorrect
- Mercury
- Venus
- Mars
Jupiter is a giant gas planet, not one of the small rocky inner worlds.
- Fact or fibLevel 4
18. Beyond the frost line there was more solid material because ices could also freeze there.
Answer: True
Adding ice to rock beyond the frost line gave the outer planets more mass to grow with.
The Age of the Solar System
- Build the sentenceLevel 3
19. Build a true sentence about the oldest rocks.
Answer: Meteorites are the oldest rocks in the solar system
Meteorites are the oldest rocks in the solar system, so we use them to find its age.
- Fact or fibLevel 3
20. The Sun will shine for a total of only about 100 million years.
Answer: False
The Sun's total lifetime is roughly 10 billion years, not 100 million.
- Fill the blankLevel 3
21. Moon rocks brought back by Apollo astronauts are about 4.5 ____ years old.
- billioncorrect
- million
- thousand
- hundred
Lunar samples date to roughly 4.5 billion years, close to the age of the whole system.
- Match the pairsLevel 3
22. Match each idea to the right fact.
Answer: Solar system age = About 4.6 billion years; Origin of the Moon = A giant impact; Late Heavy Bombardment = Many craters
The solar system is about 4.6 billion years old, the Moon formed by a giant impact, and craters record heavy bombardment.
- Choose all that applyLevel 4
23. Which statements about dating the solar system's age are true? Select all that apply.
- Meteorites give about 4.57 billion yearscorrect
- It relies on radioactive decaycorrect
- Apollo Moon rocks agree at about 4.5 billion yearscorrect
- Carbon-14 is the main method used
Meteorites give about 4.57 billion years using radioactive decay, and Apollo Moon rocks agree.
- Guess the numberLevel 4
24. About how many billion years will the Sun live in total?
Answer: 10 billion years
The Sun's total main-sequence life is roughly 10 billion years.
The Nebular Hypothesis
- Build the sentenceLevel 3
25. Build a true sentence about where the Sun formed.
Answer: The Sun formed at the center of the nebula
The Sun formed at the center of the nebula, where most of the mass gathered.
- Fact or fibLevel 3
26. The solar nebula was made mostly of hydrogen and helium.
Answer: True
Like the Sun today, the nebula was mostly hydrogen and helium with a little of everything else.
- Fill the blankLevel 3
27. Most of the mass of the nebula fell to the center to form the ____.
- Suncorrect
- Moon
- asteroid belt
- Earth
The Sun holds almost all of the solar system's mass at its center.
- Multiple choiceLevel 3
28. What does the nebular hypothesis say the solar system formed from?
- A giant cloud of gas and dustcorrect
- A single huge planet that broke apart
- Two stars crashing together
- A cloud made only of iron
The nebular hypothesis says the Sun and planets condensed from a giant cloud of gas and dust.
- Choose all that applyLevel 4
29. Which statements about the solar nebula are true? Select all that apply.
- It was mostly hydrogen and heliumcorrect
- It collapsed under gravitycorrect
- It was made mostly of iron
- It was completely solid
The nebula was mostly hydrogen and helium and collapsed under its own gravity.
- Match the pairsLevel 4
30. Match each part of the forming solar system to its description.
Answer: Solar nebula = Cloud of gas and dust; Proto-Sun = Forms at the center; Protoplanetary disk = Where planets form
The nebula is the cloud, the proto-Sun forms in the middle, and the disk is where planets form.
Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.
How you practise
This unit mixes 14 different question formats, so you are recalling and applying rather than recognising the same layout every time.
- Build the sentence
- Choose all that apply
- Fact or fib
- Fill the blank
- Guess the number
- Match the pairs
- Multiple choice
- Odd one out
- Picture question
- Put in order
- Sort into groups
- Spell it
- True or false
- Type the answer
Practise How the Solar System Formed
120 questions across 21 steps. Start with step one and crawl at your own pace.
Play this unitRead about How the Solar System Formed
Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.
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- The Sun and StarsWhat stars are, star types, brightness, colour and distance
- Moons, Asteroids and CometsMoons, asteroids, comets, meteors and the outer belts
- Gravity and OrbitsGravity, orbits, Kepler's laws, tides and weightlessness