Our Home Earth Class 8: Why Earth Sustains Life, Reproduction and the Triple Planetary Crisis

What Our Home: Earth, a Unique Life Sustaining Planet is about

Key concepts

Earth's thin life layer and the planets

NCERT section 13.2 What Do the Planets of Our Solar System Look Like?

All mountains, rivers, forests and people sit on a very thin outer layer. From the highest peak to the deepest trench, the crust is tiny compared with Earth's size: if Earth were an apple, the crust would be as thin as its skin.

Activity 13.1: list features we take for granted, such as air that does not escape to space (though gas particles move freely) and a heart that pumps blood up to the head against gravity. Earth gives us air, water, soil, rock and timber.

The solar system (13.2): eight planets in nearly circular orbits: Mercury, Venus, Earth, Mars (small and rocky) and Jupiter, Saturn, Uranus, Neptune (large, mostly gas).

Activity 13.2: collect temperature, size and atmosphere data. Planets generally get colder farther from the Sun, but Venus (about 450 °C) is hotter than Mercury (about 170 °C). Venus's thick atmosphere is almost all carbon dioxide, which traps heat: the greenhouse effect.

On Earth, a mild greenhouse effect (carbon dioxide absorbing heat radiated by the warmed ground) keeps the average near 15 °C. A plant greenhouse works differently: it traps warm air in a closed glass space.

Worked example

Mercury is closer to the Sun than Venus, yet Venus is hotter. Explain.

Try it first, then show the solutionSolution

1. Mercury has no atmosphere, so its heat escapes into space.
2. Venus has a very thick atmosphere of carbon dioxide, which absorbs and traps heat (greenhouse effect).
3. So Venus (about 450 °C) stays hotter on average than Mercury (about 170 °C).

Answer: Venus's thick carbon dioxide atmosphere traps heat.

Position: the habitable zone

NCERT section 13.3.1 Position of the Earth

The most important reason Earth supports life is its distance from the Sun.

• Closer: too hot, water would evaporate. Farther: too cold, water would freeze.
• At Earth's distance, water is mostly liquid, which (as far as we know) life needs to evolve, even though some microbes survive freezing.
• The range of distances from a star where water stays liquid is the habitable zone, or Goldilocks zone (not too hot, not too cold).
• Water covers most of the surface, so from space Earth looks blue: the Blue Planet.

Mars lies at the edge of the Sun's habitable zone. Rovers have found no proof of life, but Mars may once have had liquid water and lakes. India's Mangalyaan (Mars Orbiter Mission, 2013) studied Mars's atmosphere, surface and signs of past water using low-cost technology. Science stays open to new evidence.

Worked example

Why is Earth's habitable zone also called the 'Goldilocks zone'?

Try it first, then show the solutionSolution

1. In the Goldilocks story, the porridge chosen was neither too hot nor too cold.
2. Earth's distance from the Sun is likewise neither too close (too hot) nor too far (too cold).
3. At this 'just right' distance, water stays liquid.

Answer: It is 'just right' for liquid water.

Size, orbit, atmosphere and ozone

NCERT section 13.3.2 Size of the Earth

Orbit: Earth's nearly circular orbit keeps sunlight and heat steady through the year, avoiding extreme summers and winters at most places.

Size and gravity: Earth is the right size to hold an atmosphere.

• Much smaller (same average density): gravity too weak, gases would escape into space. Mars's atmosphere is about 100 times thinner than Earth's; Mercury has none.
• Much larger: gravity so strong that our bones might be crushed.

Oxygen in the atmosphere lets almost all life breathe. Some oxygen becomes ozone (a molecule of three oxygen atoms), forming the ozone layer, which acts as a shield by blocking harmful ultraviolet (UV) rays that can damage living cells.

Without an atmosphere, Earth would lose its heat to space and become too cold, there would be no air to breathe, no ozone shield, and water would not stay liquid.

Worked example

Mars is smaller than Earth and its atmosphere is about 100 times thinner. Use the idea of gravity to link these two facts, and give one consequence for water on Mars.

Try it first, then show the solutionSolution

1. A smaller planet has less mass, so its gravity is weaker.
2. Weak gravity cannot hold on to gas particles well, so much of Mars's atmosphere has escaped, leaving it very thin.
3. A thin atmosphere traps little heat and gives little pressure, so water cannot stay liquid on the surface: it freezes or escapes.

Answer: Weaker gravity let Mars lose most of its air; with so thin an atmosphere, liquid water cannot last on its surface.

Earth's magnetic field

NCERT section 13.3.3 Magnetic field of the Earth

A freely hanging magnet settles in a fixed direction because Earth behaves like a giant magnet. Its magnetic field is believed to come from the movement of molten iron in the core.

Earth is constantly hit by tiny high-energy particles:

• cosmic rays from far across the universe;
• the solar wind from the Sun.

These can damage the atmosphere, thin the ozone layer and let in more UV rays. The magnetic field acts as a protective shield, pushing many of these particles away and keeping the atmosphere and life safe.

Summary: position (liquid water), size (atmosphere), atmosphere (oxygen, ozone, warmth) and magnetic field together make Earth a planet where life can arise and thrive.

Worked example

Explain two different ways in which living things depend on Earth's magnetic field: one from the Electricity chapter and one from this chapter.

Try it first, then show the solutionSolution

1. Migratory birds, fish and some other animals sense Earth's magnetic field and use it to find their way on long journeys.
2. The magnetic field also pushes away cosmic rays and the solar wind, which would otherwise damage the atmosphere and thin the ozone layer.
3. Without that shield, more harmful UV rays would reach the surface and damage living cells.

Answer: It guides animal navigation, and it shields the atmosphere and ozone layer from high-energy particles.

The four spheres and balance

NCERT section 13.4 What Allows Life to Be Sustained on Earth?

Life is sustained by connections between living and non-living parts:

• Atmosphere (air): oxygen for respiration; carbon dioxide for photosynthesis, which releases oxygen; a mild greenhouse effect keeps water liquid; moving air shapes weather and rainfall.
• Hydrosphere (water): covers about 70% of the surface (ponds, lakes, rivers, springs, seas, oceans, groundwater). Water is a good solvent, carries nutrients in plants, regulates body temperature and aids digestion in animals; oceans are home to millions of life forms, many undiscovered. Water vapour forms clouds and rain, refilling rivers and groundwater.
• Geosphere (solid Earth: rocks, soil, minerals): soil holds nutrients such as nitrogen and potassium, from broken-down rocks and dead remains; minerals give salt, coal, oil, iron and copper. Geodiversity is the variety of landforms, rocks and soils and the processes shaping them; it creates varied habitats.
• Biosphere (all living beings and where they live, on land, in water and air): producers, consumers and decomposers.

Balance (13.4.4): Earth is a vast linked system. Cutting a forest can change rainfall, soil, air quality and wildlife. Protecting clean air, water, soil and all life keeps Earth habitable.

Worked example

Trace one raindrop falling on a forest until it reaches the sea, naming each of Earth's four spheres it passes through or affects.

Try it first, then show the solutionSolution

1. It forms from water vapour in a cloud and falls as rain: atmosphere.
2. It lands on a leaf and drips to the ground, where plant roots may take it up: biosphere.
3. It soaks into the soil, dissolving nutrients from soil and rocks: geosphere.
4. It flows as groundwater into a stream, a river and finally the sea: hydrosphere.

Answer: Atmosphere (cloud and rain), biosphere (leaf and roots), geosphere (soil), hydrosphere (groundwater, river, sea).

Reproduction: genes, similarity and variation

NCERT section 13.5 What Keeps Life from Disappearing?

Without reproduction, each kind of life would disappear. Reproduction maintains the continuity of life.

Offspring resemble parents because parents pass on genetic material (genes), a detailed instruction manual in every cell that guides how a single cell develops into, say, a cow (making blood, bone, muscle, skin).

Reproduction also allows small changes in the instructions. Some help survival in new environments: over time camels developed humps storing fat for desert life; some bacteria became resistant to antibiotics. Over many generations, changes can add up to new features or even new kinds of organisms.

Two kinds of reproduction:

• Asexual: one parent; offspring are exact copies (in their instructions).
• Sexual: instructions from two parents combine; offspring share traits with both but are not exact copies. This mixing keeps useful features and lets new ones appear.

Worked example

Why do brothers and sisters with the same parents look different from each other?

Try it first, then show the solutionSolution

1. Each child forms from one gamete from each parent.
2. Each gamete carries a different selection of the parent's instructions (eye colour, hair type and so on).
3. These combine differently each time, so each child gets a unique mix.

Answer: Different gametes give each child a different mix of genes.

Asexual and sexual reproduction in plants and animals

NCERT section 13.5.2 Sexual reproduction

Asexual reproduction:

• Vegetative propagation: a new plant grows from a leaf, stem or root (Activity 13.3: money plant cutting in water, potato 'eyes', ginger pieces in moist soil; note when roots and the first leaf appear). Bamboo and sugarcane are grown from stem pieces.
• Bacteria and amoeba divide into two identical individuals; algae regrow from fragments; Hydra forms buds that break off; Planaria regenerates from a small piece.

Sexual reproduction: two parents (male and female; even bacteria and yeast have two 'mating types').

• Each parent makes gametes with half its genetic material, so when two join, the new cell has a complete set, not double.
• Plants: pollen in the anther gives male gametes; ovules deep in the flower hold female gametes. Pollination carries pollen (wind, insects, animals); fertilisation forms a zygote, which becomes the seed; the part around the ovule becomes the fruit. Animals eating fruit spread seeds, as when a banyan seed sprouts in a wall crack.
• Animals: gametes are sperm and eggs. Fish and frogs: external fertilisation in water, and the embryo develops in water. Birds and mammals: internal fertilisation. Birds lay the fertilised egg, packed with food, and the embryo develops until hatching. Most mammals: the embryo develops inside the mother, who supplies food and oxygen until birth.

Worked example

Suppose a body cell of an imaginary animal holds 20 sets of instructions. How many sets does each gamete carry, and what would happen over three generations if gametes did not carry half?

Try it first, then show the solutionSolution

1. Gametes carry half: $\frac{20}{2}=10$ sets; two gametes join to give $10+10=20$ sets again.
2. Without halving, each child would get $20+20=40$ sets, then 80, then 160.
3. So halving keeps the amount of genetic material the same in every generation.

Answer: 10 sets per gamete; without halving it would double to 40, 80, 160.

Threats to life: the triple planetary crisis

NCERT section 13.6 What Are the Threats to Life on Earth?

Small changes in global temperature, oxygen or ozone can endanger life. The three biggest challenges, the triple planetary crisis, are:

• Climate change: burning coal and oil releases carbon dioxide and methane, which trap extra heat (global warming). Trees, plants and ocean plankton absorb some carbon dioxide, but fossil fuels release carbon locked underground for millions of years faster than Earth can absorb it. Results: melting ice caps, rising seas flooding coastal cities, extreme weather and loss of species; long-term changes in temperature, rainfall and weather patterns are climate change.
• Biodiversity loss: destroying habitats removes species; if grasses vanish, deer and grasshoppers struggle, and so do tigers or foxes that eat them.
• Pollution: air pollution (factories, vehicles, burning fuels) causes breathing problems, crop damage, smog and acid rain; water and soil pollution from factory, farm and plastic waste spread harm through food chains.

Global agreements: Montreal Protocol (1987) cut CFCs so the ozone layer is slowly recovering; Earth Summit (1992); Kyoto Protocol (2005); Paris Agreement (2015), aiming to limit warming to 1.5 °C, which the world is not yet on track to meet (as of 2025).

Actions: renewable energy (solar, wind), saving energy and water, greener travel, protecting biodiversity, community-led sustainable use, and reusing, repairing and recycling.

Worked example

Classify each as mainly climate change, biodiversity loss or pollution: (a) a river foaming with factory waste; (b) a frog species vanishing after its wetland is drained; (c) glaciers shrinking as average temperatures rise; (d) thick smog over a city in winter.

Try it first, then show the solutionSolution

1. (a) Factory waste in water: pollution.
2. (b) Loss of a species after habitat destruction: biodiversity loss.
3. (c) Shrinking glaciers from rising temperatures: climate change.
4. (d) Smog from vehicles, factories and burning fuels: (air) pollution.

Answer: (a) pollution (b) biodiversity loss (c) climate change (d) pollution.

Science in real life: Our Home: Earth, a Unique Life Sustaining Planet

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Reproduction and the Continuity of Life
• Why Earth Supports Life: Habitable Zone, Atmosphere, Spheres and Threats

Our Home: Earth, a Unique Life Sustaining Planet: formulas and key facts

Habitable zone

$\text{too hot}\mid \text{just right (liquid water)}\mid \text{too cold}$

Ozone

${\text{O}}_3:\text{three oxygen atoms; blocks UV rays}$

Four spheres

$\text{atmosphere}+\text{hydrosphere}+\text{geosphere}+\text{biosphere}$

Gametes

$\frac{1}{2}+\frac{1}{2}=1\text{ complete set of instructions}$

Triple planetary crisis

$\text{climate change}+\text{biodiversity loss}+\text{pollution}$

Agreements timeline

$1987\text{ Montreal}\to 1992\text{ Earth Summit}\to 2005\text{ Kyoto}\to 2015\text{ Paris}$

Our Home: Earth, a Unique Life Sustaining Planet: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Our Home: Earth, a Unique Life Sustaining Planet

The question patterns to expect, and the method that works for each.

Tricks to remember Our Home: Earth, a Unique Life Sustaining Planet

Short memory aids. Say each one aloud once, then use it on a question the same day.

• PSOM keeps us alive

  Position, Size, Ozone, Magnetic field: four shields of life.

  Use it when: Why Earth is habitable.

• Goldilocks porridge

  Not too hot, not too cold: just right for liquid water.

  Use it when: Habitable zone.

• Four spheres: A-H-G-B

  Air, H₂O, Ground, Bugs (life).

  Use it when: Atmosphere, hydrosphere, geosphere, biosphere.

• Half and half make whole

  Each gamete brings half the book; together they make the full book.

  Use it when: Why genetic material does not double.

• CBP crisis

  Climate, Biodiversity, Pollution: the three-headed crisis.

  Use it when: Triple planetary crisis.

Our Home: Earth, a Unique Life Sustaining Planet: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Our Home: Earth, a Unique Life Sustaining Planet

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying Venus is hottest because it is nearest the Sun

  Distance alone is assumed to decide temperature.

  Wrong working

  Venus is closest to the Sun, so it is hottest.

  Right working

  Mercury is closest; Venus is hottest because its thick carbon dioxide atmosphere traps heat.

  ✓Do this: Mention the greenhouse effect.

  Usually costs: 1 mark.

• ✗Mistake: Confusing the planetary greenhouse effect with a glass greenhouse

  Same name.

  Wrong working

  The atmosphere warms Earth like a glass roof trapping air.

  Right working

  Greenhouse gases absorb heat radiated by the warmed Earth; a glass greenhouse simply traps warm air in a closed space.

  ✓Do this: Absorb radiation versus trap air.

  Usually costs: 1 mark.

• ✗Mistake: Saying the ozone layer gives us oxygen to breathe

  Ozone is a form of oxygen.

  Wrong working

  The ozone layer supplies oxygen.

  Right working

  The ozone layer blocks harmful UV rays; we breathe ordinary oxygen.

  ✓Do this: Ozone = UV shield.

  Usually costs: 1 mark.

• ✗Mistake: Saying offspring of sexual reproduction are exact copies

  Children resemble parents.

  Wrong working

  Sexual reproduction makes copies of the parents.

  Right working

  Only asexual reproduction makes exact copies; sexual reproduction mixes instructions from two parents, so offspring vary.

  ✓Do this: One parent = copy; two parents = mix.

  Usually costs: 1 mark.

• ✗Mistake: Forgetting that gametes carry half the genetic material

  The doubling puzzle is skipped.

  Wrong working

  The child gets both parents' full instructions.

  Right working

  Each gamete carries half, so the zygote has one complete set.

  ✓Do this: Half + half = whole.

  Usually costs: 1 mark.

NCERT exercise guide: Our Home: Earth, a Unique Life Sustaining Planet

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Our Home: Earth, a Unique Life Sustaining Planet

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The ozone layer protects life by
   (a) supplying oxygen
   (b) blocking ultraviolet rays
   (c) trapping heat
   (d) producing rain

   Hint

   Shield.

   Answer

   (b) blocking ultraviolet rays

2. Q2 · 1 mark · MCQExam level

   Which of these is an example of sexual reproduction?
   (a) Potato eyes sprouting
   (b) Hydra forming a bud
   (c) A mango tree growing from a seed
   (d) A money plant cutting rooting in water

   Hint

   Seeds come from fertilisation.

   Answer

   (c): the seed forms after pollination and fertilisation

3. Q3 · 1 mark · Very short answerExam level

   What is the solar wind?

   Hint

   From the Sun.

   Answer

   A stream of high-energy particles coming from the Sun.

4. Q4 · 2 marks · Very short answerExam level

   Name the four spheres of Earth and what each consists of.

   Hint

   Air, water, solid, life.

   Answer

   Atmosphere (air), hydrosphere (water), geosphere (rocks, soil, minerals), biosphere (living beings and where they live).

5. Q5 · 2 marks · Very short answerHarder exam level

   Why does Earth look blue from space?

   Hint

   70%.

   Answer

   Water covers about 70% of Earth's surface, and the vast oceans make the planet appear blue: the Blue Planet.

6. Q6 · 2 marks · Very short answerHarder exam level

   Why don't the amounts of genetic material double each generation?

   Hint

   Gametes.

   Answer

   Each parent makes gametes with only half its genetic material; when two gametes join, the zygote gets one complete set.

7. Q7 · 3 marks · Short answerHarder exam level

   Describe how a flower forms seeds and fruit.

   Hint

   Pollination, fertilisation.

   Answer

   Pollen (male gametes) from the anther is carried by wind, insects or animals to another flower (pollination). Male and female (ovule) gametes fuse (fertilisation) to form a zygote, which develops into the seed. The fleshy part around the ovule becomes the fruit.

8. Q8 · 3 marks · Short answerHarder exam level

   Explain how burning fossil fuels causes global warming, and give two effects.

   Hint

   Locked carbon.

   Answer

   Burning coal and oil releases carbon dioxide and methane, including carbon locked underground for millions of years. Plants and oceans cannot absorb it fast enough, so more heat is trapped. Effects: melting ice caps and rising seas, and more extreme weather (also species loss).

9. Q9 · 3 marks · Short answerHarder exam level

   List the four global agreements in the chapter with years and the main aim of each.

   Hint

   1987 to 2015.

   Answer

   Montreal Protocol (1987): reduce CFCs to protect the ozone layer. Earth Summit (1992): international action on climate and biodiversity. Kyoto Protocol (2005) and Paris Agreement (2015): cut greenhouse gas emissions; Paris aims to keep warming below 1.5 °C.

10. Q10 · 5 marks · Long answerHarder exam level

    Describe the activity of growing new plants from a money plant, potato and ginger, and explain what type of reproduction it shows and why the new plants are like the parent.

    Hint

    Activity 13.3.

    Answer

    Put a money plant cutting in a glass of water, and plant potato eyes and a piece of ginger separately in moist soil, not too deep. Give water, air and sunlight and note daily when roots, shoots and the first new leaf appear. This is vegetative propagation, a form of asexual reproduction: one parent, no gametes. The new plants carry the same instructions as the parent, so they are copies of it.

11. Q11 · 3 marks · Short answerHOTS / competency

    Average temperatures: Venus about 450 °C, Mercury about 170 °C, Earth about 15 °C. How much hotter is Venus than Mercury? Explain why, if Earth had a Venus-like atmosphere, life would be impossible.

    Hint

    Greenhouse.

    Answer

    $450-170=280$ °C hotter. A thick carbon dioxide atmosphere would trap so much heat that water would boil away and no liquid water could exist, so life as we know it could not survive.

12. Q12 · 2 marks · Very short answerHOTS / competency

    Bright colours and scents of flowers: how do they help plants reproduce?

    Hint

    Pollinators.

    Answer

    They attract insects and birds, which visit for nectar and carry pollen from one flower to another, helping pollination and so seed formation.

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. 1.If Earth were an apple, what would the crust be like?Show answer

   As thin as the apple's skin.

2. 2.Which is the hottest planet?Show answer

   Venus.

3. 3.What is the Goldilocks zone?Show answer

   The habitable zone, where water can stay liquid.

4. 4.How much thinner is Mars's atmosphere than Earth's?Show answer

   About 100 times thinner.

5. 5.What does the ozone layer block?Show answer

   Harmful ultraviolet (UV) rays.

6. 6.What is believed to cause Earth's magnetic field?Show answer

   Movement of molten iron in Earth's core.

7. 7.What is the hydrosphere?Show answer

   All the water on Earth: oceans, seas, rivers, lakes, ponds, springs and groundwater.

8. 8.What is vegetative propagation?Show answer

   Growing a new plant from a leaf, stem or root.

9. 9.What is formed when male and female gametes fuse?Show answer

   A zygote.

10. 10.What are the three parts of the triple planetary crisis?Show answer

    Climate change, biodiversity loss and pollution.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you explain why life exists only in a thin layer?
• ☐Can you explain why Venus is hotter than Mercury?
• ☐Can you define the habitable zone and place Earth and Mars?
• ☐Can you say what happens if Earth were much smaller or larger?
• ☐Can you explain the roles of ozone and the magnetic field?
• ☐Can you name and describe the four spheres?
• ☐Can you give five examples of vegetative propagation?
• ☐Can you explain why gametes carry half the genetic material?
• ☐Can you compare egg-laying with live birth?
• ☐Can you name the triple planetary crisis and four agreements with years?

Our Home: Earth, a Unique Life Sustaining Planet: frequently asked questions