Big Bang, Nebular and Planetesimal Theories (ISC Class 11 Geography)
Updated 17 October 2026 · Fact-checked
The Big Bang theory says the universe expanded from one hot, dense point about 13.7 billion years ago. For the solar system, the nebular hypothesis (Kant, Laplace) forms planets from a rotating gas cloud, while the planetesimal hypothesis (Chamberlin, Moulton) forms them from small solid bodies. Modern accounts keep accretion of planetesimals inside a solar nebula.
Follow the arrows: the universe expands from one point, and the solar nebula later spins into a sun with rings that form planets.
Illustration drawn for QuestionBuddy.
Understand Big Bang, Nebular and Planetesimal Theories
The Big Bang theory (the universe). All matter was once packed into a single point of enormous temperature and density. About 13.7 billion years ago it began to expand rapidly, and some energy turned into matter. In the first few minutes the first atomic nuclei formed; after about 300,000 years the temperature fell to about 4,500 K, atoms formed and the universe became transparent. Uneven matter was pulled by gravity into clumps that became galaxies, and nebulae within galaxies condensed into stars. The solar system formed from such a nebula about 4.6 billion years ago.
- Evidence: Hubble found in the 1920s that galaxies are moving away from one another; faint background radiation from the early universe has been detected; the abundance of hydrogen and helium fits the theory.
- Rival idea: Fred Hoyle's steady state theory held the universe was always roughly the same; evidence for expansion made scientists prefer the Big Bang.
The nebular hypothesis (the planets). Kant proposed it in 1755 and Laplace developed it in 1796. A hot, slowly rotating cloud of gas cooled and contracted, spinning faster; in Laplace's version rings separated from its edge and each ring condensed into a planet while the centre became the sun.
- Strength: it explains why planets revolve in the same direction and in nearly one plane.
- Weakness: the sun holds most of the mass yet rotates slowly, while the planets carry most of the angular momentum; gas rings are unlikely to condense into planets.
- Revision: about 1950 Otto Schmidt and Carl Weizsacker described a solar nebula of hydrogen, helium and dust forming a disc in which planets grew by accretion.
The planetesimal hypothesis. Chamberlin and Moulton, early in the twentieth century, suggested that a passing star pulled streams of matter from the sun. The matter cooled into small solid planetesimals that collided and stuck together to form planets and moons.
- Strength: accretion of small solid bodies is still accepted.
- Weakness: a star passing so close is extremely unlikely, and hot gas pulled from the sun would probably disperse.
Modern view. A rotating disc of gas and dust formed around a gas core, matter condensed into small rounded planetesimals, and these collided and accreted into a few large planets.
Big Bang, Nebular and Planetesimal Theories: key facts
- Big Bang timeline
13.7 billion yr ago expansion begins About 300,000 yr about 4,500 K, atoms form About 4.6 billion yr ago solar system forms - Proposers
- Nebular: Kant 1755, Laplace 1796, revised by Schmidt and Weizsacker about 1950 | Planetesimal: Chamberlin and Moulton
- Big Bang evidence: Hubble.
- Modern view
- Disc of gas and dust
- Planetesimals
- Accretion into planets
How to answer Big Bang, Nebular and Planetesimal Theories questions
Theory questions are 4 to 7 marks. Answer in a fixed order.
- 1Name the proposers and approximate dates.
- 2State the central idea in numbered steps.
- 3Give one strength and one weakness for each hypothesis.
- 4Close with the modern view or the evidence, for example Hubble for the Big Bang.
Big Bang, Nebular and Planetesimal Theories in real life
The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.
Expanding universe
Suppose dots are drawn on a balloon and the balloon is inflated slowly.
What does the movement of the dots illustrate?
Try it first, then show the solutionSolution
- Every dot moves away from every other dot as the balloon grows.
- This is the picture used for galaxies moving apart in an expanding universe.
Answer: It illustrates the expanding universe, as Hubble's observations of receding galaxies show.
Memory hook: Dots on a balloon drift apart.
Snowball growth
Suppose a child rolls a small snowball that picks up smaller flakes and bits of ice until it becomes large.
Which hypothesis does this resemble and why?
Try it first, then show the solutionSolution
- Small bodies sticking together to form a large one resembles accretion of planetesimals.
- The modern view of planet formation depends on this accretion.
Answer: Planetesimal accretion.
Memory hook: Small pieces stick, a planet grows.
Tricks to remember Big Bang, Nebular and Planetesimal Theories
Short memory aids. Say each one aloud once, then use it on a question the same day.
Who said what
Nebular = Kant, Laplace; Planetesimal = Chamberlin, Moulton; Expansion proof = Hubble.
Use it when: Names.
Gas versus solid
Nebular begins with gas; planetesimal begins with solid lumps.
Use it when: Distinguish the two ideas.
Big Bang, Nebular and Planetesimal Theories: solved examples
Ordered from board level to HOTS. Try each one before you open the solution.
Example 1: Expansion evidence · 1 mark · MCQBoard level
The observation that galaxies are moving away from one another was made by:
(a) Kant
(b) Laplace
(c) Hubble
(d) Hoyle
Try it first, then show the solutionSolution
- The expansion evidence is by Hubble in the 1920s.
Answer: (c) Hubble.
Example 2: Proposers and years · 3 marks · Short answerBoard level
Name the proposers of the nebular hypothesis with the years given for them, and the two scientists who revised it about 1950.
Try it first, then show the solutionSolution
- Immanuel Kant, 1755.
- Pierre-Simon Laplace, 1796.
- Revised by Otto Schmidt and Carl Weizsacker about 1950.
Answer: Kant (1755), Laplace (1796); revised by Schmidt and Weizsacker (about 1950).
Example 3: Time gap · 4 marks · Long answerHarder board level
Taking the universe to be 13.7 billion years old and the solar system 4.6 billion years old, find how long before the solar system the Big Bang occurred, and describe the stages in between.
Try it first, then show the solutionSolution
- Subtract: 13.7 - 4.6 = 9.1 billion years.
- Within minutes: first atomic nuclei; about 300,000 years: about 4,500 K, atoms form.
- Gravity clumped matter into galaxies; nebulae condensed into stars.
- Our solar system formed from a nebula 9.1 billion years after the Big Bang.
Answer: 9.1 billion years; the stages were nuclei, atoms, galaxies and stars, then the solar nebula.
Example 4: Evaluate planetesimal hypothesis · 4 marks · Long answerHarder board level
Evaluate the planetesimal hypothesis by giving one point in its favour and two against, and state what idea survives in modern thinking.
Try it first, then show the solutionSolution
- For: planets can grow by accretion of small solid bodies.
- Against 1: a star passing close enough to the sun is extremely unlikely.
- Against 2: hot gas pulled out of the sun would probably disperse instead of condensing.
- Survives: accretion of planetesimals within a solar nebula.
Answer: Accretion is its strength; the unlikely close encounter and dispersing gas are its weaknesses; accretion survives in the modern view.
Example 5: Judge a claim · 5 marks · Case-basedHOTS / competency
Case (invented): A student argues that because the sun rotates slowly while the planets hold most of the angular momentum, the idea that planets formed from the solar nebula must be discarded altogether. (a) Name the hypothesis whose weakness this is. (b) Say whether the revised 1950 version is a complete rejection. (c) Give the modern view that combines the two ideas.
Try it first, then show the solutionSolution
- (a) The nebular hypothesis of Kant and Laplace.
- (b) No. Schmidt and Weizsacker kept a solar nebula but had a disc of hydrogen, helium and dust in which planets grew by accretion.
- (c) A rotating disc around a gas core; matter condenses into planetesimals; they collide and accrete into planets.
Answer: (a) Nebular (b) the revision keeps the nebula with accretion (c) disc, planetesimals, accretion.
Where students lose marks in Big Bang, Nebular and Planetesimal Theories
The slips examiners see most often, and what to write instead.
Mistake: Mixing the proposers
Names of the two hypotheses are swapped.
Wrong working
Planetesimal: Kant and Laplace
Right working
Planetesimal: Chamberlin and Moulton
Do this: Nebular: Kant and Laplace; planetesimal: Chamberlin and Moulton.
Usually costs: 1 mark.
Mistake: Giving only the theory, no evidence
Hubble and background radiation are missed.
Do this: Add at least two lines of evidence for the Big Bang.
Usually costs: 1 to 2 marks.
Mistake: No weakness stated
Only the idea is described.
Do this: Add one strength and one weakness each.
Usually costs: 1 mark.
Exam tips
- Always attach a name and approximate date to each hypothesis.
- Use numbered stages for the Big Bang.
- Mention accretion in the modern view.
- Do not claim the Big Bang explains the origin of the planets only; it concerns the universe.
Practice questions on Big Bang, Nebular and Planetesimal Theories
Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.
Q1 · 1 mark · MCQBoard level
Fred Hoyle is associated with which theory?
(a) Big Bang
(b) Steady state
(c) Planetesimal
(d) NebularHint
Universe always roughly the same.
Answer
(b) Steady state.
Q2 · 3 marks · Short answerHarder board level
Explain how the Schmidt and Weizsacker revision improved the nebular hypothesis.
Hint
Disc and accretion.
Answer
They described a solar nebula of hydrogen, helium and dust around the sun; friction and collisions formed a disc and the planets grew by accretion, which became the basis of modern views.
Q3 · 4 marks · Long answerHarder board level
Give three lines of evidence supporting the Big Bang theory and explain one.
Hint
Hubble, radiation, light elements.
Answer
Galaxies are moving apart (Hubble), faint background radiation from the early universe has been detected, and the abundance of hydrogen and helium matches the theory. Hubble's result shows the universe is expanding, which implies it was once much smaller and denser.
Q4 · 3 marks · Short answerHarder board level
Arrange in order and justify: planetesimals collide; disc of gas and dust forms; planets emerge.
Hint
Order of modern view.
Answer
Disc of gas and dust forms; matter condenses into planetesimals; planetesimals collide and accrete into planets.
Q5 · 4 marks · Case-basedHOTS / competency
Case: Two textbooks give different origins for planets, one from a gas cloud and one from a star passing the sun. (a) Name each hypothesis. (b) Say which one needs an unlikely event. (c) Explain which idea the modern view keeps from each.
Hint
One needs a near encounter.
Answer
(a) Nebular and planetesimal. (b) The planetesimal hypothesis, which needs a close star. (c) From the nebular, a rotating solar nebula; from the planetesimal, accretion of small solid bodies.
Test your recall
Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.
1.Age of the universe?Show answer
About 13.7 billion years.
2.Who developed the nebular idea in 1796?Show answer
Laplace.
3.What are planetesimals?Show answer
Small solid bodies that accreted into planets.
4.Temperature when atoms formed?Show answer
About 4,500 K.
5.Who showed galaxies are moving apart?Show answer
Hubble.
Big Bang, Nebular and Planetesimal Theories: frequently asked questions
Is the Big Bang an explosion in space?
The theory describes the expansion of space and matter from a dense start, not an explosion into existing space.
Which theory has no unlikely event?
The nebular hypothesis, though it has other weaknesses.
Do both hypotheses survive?
Their shared idea of accretion survives in the modern view.