Big Bang, Planet Formation and the Evolution of the Earth's Layers, Atmosphere and Life (Class 11 Geography, Fundamentals of Physical Geography, Chapter 2)
Updated 17 October 2026 · Fact-checked
The modern theory of the universe's origin is the Big Bang, about 13.7 billion years ago. Stars formed from nebula clumps and planets by accretion of planetesimals. The earth separated into layers by differentiation; its atmosphere evolved in three stages, oceans formed within 500 million years and life began about 3,800 million years ago.
Understand Big Bang, Formation of Planets and Evolution of the Earth's Layers, Atmosphere and Life
Why this topic matters. Questions ask for stages in the right order, with the right keywords.
1. Early theories. The nebular hypothesis of Immanuel Kant, revised by Laplace in 1796, said planets formed from a cloud of material around a young, slowly rotating sun. In 1950 Otto Schmidt (Russia) and Carl Weizsacker (Germany) revised it: the sun was surrounded by a solar nebula of mostly hydrogen, helium and dust; friction and collision produced a disc-shaped cloud, and the planets formed by accretion.
2. Big Bang. Also called the expanding universe hypothesis. Edwin Hubble gave evidence in 1920 that galaxies move further apart. All matter existed as a "tiny ball" (singular atom) of infinite temperature and density. It exploded about 13.7 billion years ago; the first atom formed within three minutes; within 300,000 years temperature fell to 4,500 K, atomic matter formed and the universe became transparent. Hoyle's steady state concept was the alternative. The balloon analogy is only partly correct: space between galaxies increases, not the galaxies themselves.
3. Stars. Uneven distribution of matter led to density differences and gravitational differences. A galaxy starts as a huge cloud of hydrogen called a nebula, which develops clumps that become stars about 5-6 billion years ago. Galaxies are 80,000-150,000 light years across. A light year is a distance: light travels 300,000 km per second, so one light year is 9.461 x 10^12 km. Sunlight takes about 8.311 minutes to cover 149,598,000 km.
4. Planets. Three stages: (i) a core and a rotating disc of gas and dust form inside a nebula lump; (ii) the gas cloud condenses into small rounded objects that cohere into planetesimals; (iii) planetesimals accrete into a few large bodies, the planets.
5. Differentiation. The early earth was hot, barren and rocky, with a thin hydrogen-helium atmosphere. As density increased, temperature inside rose; heavier material such as iron sank to the centre and lighter material rose. This separation into layers is differentiation, giving crust, mantle, outer core and inner core. A giant impact during the moon's formation heated the earth further.
6. Atmosphere in three stages. (i) Loss of the primordial atmosphere to solar winds; (ii) degassing: gases and water vapour from the interior and volcanoes, giving water vapour, nitrogen, carbon dioxide, methane, ammonia and little free oxygen; (iii) modification by photosynthesis.
7. Oceans and life. Water vapour condensed, carbon dioxide dissolved in rain, and rainwater collected in depressions within 500 million years, so oceans are about 4,000 million years old. Life began about 3,800 million years ago; photosynthesis evolved 2,500-3,000 million years ago; about 2,000 million years ago oxygen began to flood the atmosphere. Origin of life is seen as a chemical reaction in which complex organic molecules assembled and duplicated themselves.
Note. The solar system, the moon and the geological time scale are outside the syllabus after rationalisation.
Big Bang, Formation of Planets and Evolution of the Earth's Layers, Atmosphere and Life: key facts
- Big Bang timeline
13.7 billion years ago explosion 3 minutes first atom 300,000 years 4,500 K, atomic matter, transparent universe 5-6 billion years ago stars - Order of events.
- Planet formation
Stage 1 core and rotating disc Stage 2 condensation into planetesimals Stage 3 accretion into planets - Three stages.
- Atmosphere stages
Stage 1 loss of primordial atmosphere by solar winds Stage 2 degassing and volcanoes Stage 3 photosynthesis changes composition - Three stages.
- Key distances and dates
- Numbers.
How to answer Big Bang, Formation of Planets and Evolution of the Earth's Layers, Atmosphere and Life questions
Origin questions are sequencing questions.
- 1Name the theory and its proponent.
- 2Write stages in order with keywords.
- 3Attach a number to each stage.
- 4Link differentiation to density.
- 5Close with the oxygen timeline.
Big Bang, Formation of Planets and Evolution of the Earth's Layers, Atmosphere and Life 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.
Layers from density
Suppose a jar of muddy water with oil, stones and sand is shaken and then left to settle.
Which process of the earth's evolution does this resemble?
Try it first, then show the solutionSolution
- Heavy material sinks and light material rises.
- The earth separated into layers by density.
Answer: It resembles differentiation, where heavier material like iron sank to the centre.
Memory hook: Heavy sinks, light floats.
Expanding dots
Suppose small dots are drawn on a balloon and the balloon is inflated.
What does it show about the universe and where does the analogy fail?
Try it first, then show the solutionSolution
- The dots move apart, like galaxies.
- But the dots also grow, whereas galaxies do not grow; space between them increases.
Answer: It shows that galaxies move apart, but it is only partly correct because the space between galaxies increases, not the galaxies.
Memory hook: Space stretches.
Tricks to remember Big Bang, Formation of Planets and Evolution of the Earth's Layers, Atmosphere and Life
Short memory aids. Say each one aloud once, then use it on a question the same day.
13.7
Big Bang billions of years ago
Use it when: Age of the universe.
C-P-A
Core, Planetesimals, Accretion
Use it when: Planet formation.
L-D-P
Loss, Degassing, Photosynthesis
Use it when: Atmosphere stages.
Big Bang, Formation of Planets and Evolution of the Earth's Layers, Atmosphere and Life: solved examples
Ordered from board level to HOTS. Try each one before you open the solution.
Example 1: Who revised the nebular hypothesis · 1 mark · MCQBoard level
The nebular hypothesis was proposed by Immanuel Kant and revised by:
(A) Hubble
(B) Laplace
(C) Hoyle
(D) Darwin
Try it first, then show the solutionSolution
- Laplace revised it in 1796 (1 mark).
Answer: (B)
Example 2: Light year · 2 marks · Very short answerBoard level
Explain why a light year is a unit of distance and not of time. (2 marks)
Try it first, then show the solutionSolution
- It is the distance light travels in one year (1 mark).
- Light travels at 300,000 km per second, so a light year is 9.461 x 10^12 km (1 mark).
Answer: A light year is the distance light covers in one year, about 9.461 x 10^12 km at 300,000 km per second, so it measures distance.
Example 3: Stages of planet formation · 3 marks · Short answerHarder board level
Describe the three stages of the formation of planets. (3 marks)
Try it first, then show the solutionSolution
- Core and rotating disc of gas and dust form (1 mark).
- Condensation into small rounded objects that cohere into planetesimals (1 mark).
- Accretion of planetesimals into a few large planets (1 mark).
Answer: First, gravity inside a nebula lump forms a core with a rotating disc of gas and dust. Second, the gas cloud condenses into small rounded objects that stick together by cohesion into planetesimals. Third, collisions and gravitational attraction make planetesimals accrete into a few large bodies, the planets.
Example 4: Assertion and reason: Big Bang · 1 mark · Assertion-reasonHarder board level
Assertion (A): The expansion of the universe is shown by space increasing between galaxies.
Reason (R): The galaxies themselves grow in size as the universe expands.
(A) Both true, R explains A
(B) Both true, R does not explain A
(C) A true, R false
(D) A false, R true
Try it first, then show the solutionSolution
- The space between galaxies increases, not the galaxies (1 mark).
Answer: (C)
Example 5: HOTS: sequence of the early earth · 5 marks · Long answerHOTS / competency
Arrange the following in order and justify the sequence: formation of oceans, differentiation, photosynthesis, degassing, loss of the primordial atmosphere. (5 marks)
Try it first, then show the solutionSolution
- Differentiation formed layers as the earth heated and denser material sank (1 mark).
- Loss of primordial hydrogen-helium atmosphere to solar winds (1 mark).
- Degassing from the interior added water vapour and gases (1 mark).
- Cooling led to condensation and oceans within 500 million years (1 mark).
- Photosynthesis later changed the atmosphere and oxygen flooded it about 2,000 million years ago (1 mark).
Answer: The order is: differentiation and loss of the primordial atmosphere by solar winds, then degassing, then formation of oceans, then photosynthesis. Differentiation separated the earth into layers by density. The thin hydrogen-helium atmosphere was lost to solar winds. Degassing and volcanoes added water vapour and gases; as the earth cooled the vapour condensed and rainwater formed oceans within 500 million years. Photosynthesis evolved later, 2,500-3,000 million years ago, and oxygen began to flood the atmosphere about 2,000 million years ago. (The exact order of the first two is not dated in the text; the order given follows its account.)
Where students lose marks in Big Bang, Formation of Planets and Evolution of the Earth's Layers, Atmosphere and Life
The slips examiners see most often, and what to write instead.
Mistake: Treating light year as time
The word year suggests time.
Wrong working
A light year is about 365 days.
Right working
A light year is a distance, 9.461 x 10^12 km.
Do this: Write "distance".
Usually costs: 1 mark.
Mistake: Reversing density order in layers
Heavy sinks is forgotten.
Wrong working
Lighter material sank to the centre.
Right working
Heavier material such as iron sank; lighter rose.
Do this: Link density with position.
Usually costs: 1 mark.
Mistake: Mixing steady state and Big Bang
Both are on the page.
Wrong working
Hoyle proposed the Big Bang.
Right working
Hoyle proposed steady state; Big Bang is the expanding universe.
Do this: Name the alternative.
Usually costs: 1 mark.
Exam tips
- Always give keywords: accretion, planetesimals, differentiation, degassing.
- Write numbers with units: billion years, million years, km.
- Draw a small timeline for events.
Practice questions on Big Bang, Formation of Planets and Evolution of the Earth's Layers, Atmosphere and Life
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
The age of the universe according to the Big Bang theory is about:
(A) 4.6 billion years
(B) 13.7 billion years
(C) 3.8 billion years
(D) 5-6 billion yearsHint
Age of the explosion.
Answer
(B) 13.7 billion years.
Q2 · 2 marks · Very short answerHarder board level
Light from the sun takes about 8.311 minutes to cover the mean sun-earth distance of 149,598,000 km. Find the approximate speed of light in km per second. (2 marks)
Hint
Convert minutes to seconds.
Answer
8.311 x 60 = 498.66 s; 149,598,000 / 498.66 = about 300,000 km per second.
Q3 · 3 marks · Short answerHarder board level
Describe the three stages in the evolution of the earth's atmosphere. (3 marks)
Hint
Loss, degassing, photosynthesis.
Answer
(i) The early hydrogen-helium atmosphere was lost through solar winds. (ii) Degassing from the hot interior and volcanoes released water vapour, nitrogen, carbon dioxide, methane and ammonia. (iii) Photosynthesis modified the composition by adding free oxygen.
Q4 · 3 marks · Short answerBoard level
How did oceans form, and how old are they? (3 marks)
Hint
Condensation.
Answer
As the earth cooled, water vapour condensed and carbon dioxide dissolved in rainwater; more cooling led to more rain. Rainwater collected in depressions to form oceans within 500 million years of the earth's formation, so oceans are about 4,000 million years old.
Q5 · 4 marks · Short answerHOTS / competency
A student says life began as soon as the earth formed. Evaluate with evidence from the chapter. (4 marks)
Hint
Early earth conditions; blue algae.
Answer
The earth began hot, barren and rocky, with a thin atmosphere of hydrogen and helium, not conducive to life. Life is assumed to have begun about 3,800 million years ago, supported by fossils of microscopic structures related to blue algae in rocks older than about 3,000 million years. Origin of life is seen as a chemical process, so the statement is wrong.
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.Who gave evidence in 1920 that the universe is expanding?Show answer
Edwin Hubble.
2.What is a nebula?Show answer
A huge cloud of hydrogen gas from which stars form.
3.What is degassing?Show answer
The release of gases and water vapour from the earth's interior.
4.How old is life according to the chapter?Show answer
About 3,800 million years.
Big Bang, Formation of Planets and Evolution of the Earth's Layers, Atmosphere and Life: frequently asked questions
Is the solar system in the syllabus?
No, it was removed in NCERT's rationalisation.
Who proposed the steady state idea?
Hoyle.
What did photosynthesis change?
It changed the composition of the atmosphere by adding oxygen.