ScienceCBSE Class 8 · Our Home: Earth, a Unique Life Sustaining Planet

Why Earth Supports Life: Position, Atmosphere, Spheres and Threats (Class 8)

Updated 17 October 2026 · Fact-checked

Earth supports life because it lies in the habitable zone, where water stays liquid, is the right size to hold an atmosphere, has an ozone layer against ultraviolet rays and a magnetic field against harmful particles, and has interacting atmosphere, hydrosphere, geosphere and biosphere. Climate change, biodiversity loss and pollution, the triple planetary crisis, threaten this balance.

Understand Why Earth Supports Life: Habitable Zone, Atmosphere, Spheres and Threats

A thin layer of life. All life lives on the thin crust; if Earth were an apple, the crust would be as thin as its skin. Earth gives air, water, soil, rock and timber. The solar system has eight planets, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Mercury, Venus, Earth and Mars are small rocky planets; Jupiter, Saturn, Uranus and Neptune are large and mostly gas.

Temperatures and the greenhouse effect (Table 13.2). Mercury averages about 170 degrees Celsius with no atmosphere; Venus averages about 450 degrees Celsius with a radius of 0.95 of the Earth's and an atmosphere; Earth averages 15 degrees Celsius. Venus is the hottest planet though Mercury is closer to the Sun, because its thick atmosphere, almost entirely carbon dioxide, traps heat: the greenhouse effect. On Earth too, gases like carbon dioxide absorb radiation given off by the warmed Earth, keeping the temperature right for liquid water; without an atmosphere Earth would lose heat to space and be too cold. A plant greenhouse works differently, simply as a closed glass space that keeps warm air in.

Position: the habitable zone. Earth is at the right distance from the Sun so that water remains liquid. Closer, the water would evaporate; farther, it would freeze. The range of distances in which water stays liquid is the habitable zone or 'Goldilocks zone'. Most of the Earth's surface is water, so it is the Blue Planet. Mars lies at the edge of the habitable zone; no proof of life has been found, though scientists think it may once have had liquid water, and India's Mangalyaan (launched in 2013 by ISRO) studied its atmosphere and surface. Earth's nearly circular orbit also keeps heat from the Sun nearly steady through the year.

Size. Earth is the right size to hold an atmosphere: if it were much smaller (with the same density), gravity would be too weak to hold the gases; if much larger, gravity would be so strong that our bones could be crushed. Oxygen supports respiration and some of it forms ozone (a three-atom oxygen molecule), the ozone layer that blocks harmful ultraviolet (UV) rays.

Magnetic field. Earth behaves like a giant magnet, probably because of the movement of molten iron in its core. Cosmic rays and the solar wind can damage the atmosphere and the ozone layer; the magnetic field pushes many of these particles away like a shield.

Spheres. The atmosphere is the air; the hydrosphere is all the water (about 70 per cent of the surface: oceans, lakes, rivers, springs, groundwater); the geosphere is the solid parts, rocks, soil and minerals; the biosphere is living beings with their places of living. Geodiversity is the variety of landforms, rocks and soils and the processes that shape them. These spheres are connected, so damage to one affects the others. Plants take in carbon dioxide and water and release oxygen; water cycles through clouds and rain; soil is rich in nutrients like nitrogen and potassium from the breakdown of rocks and remains of living things. Even a small change, such as cutting a forest, can affect rainfall, soil, air quality and animals.

Threats (the triple planetary crisis). *Climate change:* burning coal and oil releases carbon dioxide and methane, which trap more heat (global warming), melt ice caps, raise sea levels, and cause extreme weather. *Biodiversity loss:* when habitats are destroyed, species vanish and food chains weaken. *Pollution:* air pollution causes breathing problems, smog and acid rain; factory, farm and plastic waste harm water; excess fertilisers harm soil. Agreements: the Montreal Protocol (1987) reduced CFCs so that the ozone layer can recover; the Earth Summit (1992); the Kyoto Protocol (2005); the Paris Agreement (2015) aims to keep global warming well below 2 degrees Celsius and to pursue efforts to limit it to 1.5 degrees Celsius, but as of 2025 the world is not on track. Actions include renewable energy, saving energy and water, reusing, repairing and recycling.

How life continues through reproduction is in the next topic.

Why Earth Supports Life: Habitable Zone, Atmosphere, Spheres and Threats: formulas and key facts

Order of planets from the Sun: $M er c u r y ∣ V e n u s ∣ E a r t h ∣ M a r s ∣ J u p i t er ∣ S a t u r n ∣ U r an u s ∣ N e pt u n e$; First four are rocky; the last four are mostly gas.
Why Earth is habitable: Habitable zone
Nearly circular orbit
Right size
Atmosphere with ozone
Magnetic field; Water stays liquid.
Greenhouse effect: CO2 in atmosphere absorbs radiation from warmed surface > heat trapped; Venus about 450 C; Earth about 15 C; Mercury about 170 C with no atmosphere.
Spheres of Earth: $A t m os p h er e (ai r) ∣ H y d r os p h er e (w a t er) ∣ G eos p h er e (so l i d) ∣ B i os p h er e (l i f e)$; Interconnected.
Triple planetary crisis: Climate change
Biodiversity loss
Pollution; Agreements: Montreal 1987, Earth Summit 1992, Kyoto 2005, Paris 2015.

How to solve Why Earth Supports Life: Habitable Zone, Atmosphere, Spheres and Threats questions

Use this method for habitability and threat questions.

1List the condition asked about: position, size, atmosphere, ozone, magnetic field, water.
2State the 'too much' and 'too little' cases (too close: water evaporates; too far: frozen).
3Link to life: liquid water, atmosphere, protection from UV and particles.
4For threats, give the cause (burning fuels), the effect (warming, sea level rise) and an action.
5For comparing planets, use temperature, atmosphere and distance.
6End with the idea of interconnected spheres and balance.

Why Earth Supports Life: Habitable Zone, Atmosphere, Spheres and Threats in real life

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.

Reading planet data

Using the chapter's Table 13.2 values, Mercury averages 170 degrees Celsius and Venus 450 degrees Celsius, while Earth averages 15 degrees Celsius.

(a) How much hotter is Venus than Mercury on average? (b) How much hotter than the Earth? (c) Why is Venus hotter than Mercury though farther from the Sun? (d) How many times larger is Earth's radius than Venus's if Venus is 0.95 of Earth's?

Try it first, then show the solutionSolution

450 − 170 = 280 degrees.
450 − 15 = 435 degrees.
Venus has a thick carbon dioxide atmosphere that traps heat; Mercury has no atmosphere.
1 ÷ 0.95 ≈ 1.05 times.

Answer: (a) 280 degrees; (b) 435 degrees; (c) greenhouse effect; (d) about 1.05 times.

Memory hook: Distance gives energy; atmosphere keeps it.

Apple-skin comparison

Imagine a model Earth the size of an apple of radius 4 cm whose skin is 0.1 cm thick (invented dimensions for the model, as the chapter compares the crust with an apple skin).

(a) What fraction of the radius is the skin? (b) Express it as a percentage. (c) If real Earth's radius were represented by 6400 units, how thick would the proportional skin be in the same units? (d) What does this show about life?

Try it first, then show the solutionSolution

0.1 ÷ 4 = 0.025.
0.025 × 100 = 2.5%.
2.5% of 6400 = 160 units.
Life exists in a thin layer, so it is delicate and needs protecting.

Answer: (a) 1/40; (b) 2.5%; (c) 160 units (model proportion only, not the real crust thickness); (d) life depends on a thin layer.

Memory hook: A thin skin carries all of life.

Tricks to remember Why Earth Supports Life: Habitable Zone, Atmosphere, Spheres and Threats

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

My Very Educated Mother Just Served Us Noodles
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Use it when: Order of planets.
Goldilocks
Not too hot, not too cold, just right: the habitable zone.
Use it when: Habitable zone.
AHGB
Atmosphere, Hydrosphere, Geosphere, Biosphere.
Use it when: Earth's spheres.
Three threats
Climate, Creatures (biodiversity), Contamination (pollution).
Use it when: Triple planetary crisis.

Why Earth Supports Life: Habitable Zone, Atmosphere, Spheres and Threats: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Habitable zone · 3 marks · Short answerBoard level

What is the habitable zone? What would happen if Earth were (a) closer to the Sun, (b) farther away?

Try it first, then show the solutionSolution

The range of distances from a star over which water remains liquid. (1)
(a) Closer: too hot, water would evaporate. (1)
(b) Farther: too cold, water would freeze. (1)

Answer: Zone where water stays liquid; closer: water evaporates; farther: water freezes.

Example 2: Hottest planet · 1 mark · MCQBoard level

Venus is hotter than Mercury because
(a) it is closer to the Sun
(b) its thick carbon dioxide atmosphere traps heat
(c) it has no atmosphere
(d) it is larger than Jupiter

Try it first, then show the solutionSolution

Greenhouse effect on Venus.

Answer: (b)

Example 3: Protective shields · 5 marks · Long answerHarder board level

(a) Name two protective features of the Earth against radiation and particles from space. (2) (b) Explain how each works. (2) (c) What would happen without an atmosphere? (1)

Try it first, then show the solutionSolution

(a) The ozone layer and the magnetic field. (2)
(b) Ozone, a three-atom oxygen molecule, blocks harmful UV rays; the magnetic field pushes many harmful cosmic rays and solar wind particles away. (2)
(c) The Earth would lose heat to space and become too cold, and water, temperature and life would be affected. (1)

Answer: Ozone layer and magnetic field; UV blocked, particles pushed away; no atmosphere means too cold and no protection.

Example 4: Greenhouse comparison · 4 marks · Case-basedHarder board level

Read the passage and answer.

A student says: 'The greenhouse effect on Earth works like the glass greenhouse for plants, trapping warm air.' Another says the greenhouse effect is bad.
(a) Correct the first statement. (1)
(b) Explain the role of the greenhouse effect on Earth. (1)
(c) How do human activities change it? (1)
(d) Name two consequences of enhanced warming. (1)

Try it first, then show the solutionSolution

(a) On Earth gases like carbon dioxide absorb the radiation given off by the warmed Earth; the plant greenhouse just keeps warm air in a closed space. (1)
(b) It keeps the Earth warm enough for liquid water, which is good in its mild form. (1)
(c) Burning fossil fuels releases extra carbon dioxide and methane that trap more heat. (1)
(d) Melting ice caps and sea level rise, extreme weather (any two). (1)

Answer: (a) Different mechanisms; (b) keeps Earth warm; (c) extra greenhouse gases; (d) sea level rise, extreme weather.

Example 5: Forest removed for a road · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

A city cuts a large patch of forest for roads and buildings. Within a few years the local temperature rises, rainfall becomes irregular and some animals vanish.
(a) Name the spheres affected. (1)
(b) Suggest how rainfall could be affected. (1)
(c) Link the loss of plants to the food chain. (1)
(d) Suggest two actions to reduce harm. (1)

Try it first, then show the solutionSolution

(a) Biosphere (living things), atmosphere (air quality, temperature), hydrosphere (water) and geosphere (soil). (1)
(b) Forests influence rainfall; cutting them can change it, as the chapter says a small change like cutting a forest can impact rainfall. (1)
(c) Without plants, herbivores lose food and then their predators lose prey. (1)
(d) Plant trees, protect remaining habitats, use cleaner energy and save water (any two). (1)

Answer: (a) All four spheres; (b) rainfall can change; (c) food chain weakens; (d) tree planting and protection.

Where students lose marks in Why Earth Supports Life: Habitable Zone, Atmosphere, Spheres and Threats

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

Mistake: Saying Venus is hottest because it is closest to the Sun.
Hot equals near is assumed.
Wrong working
Venus is hottest because it is closest to the Sun.
Right working
Venus is hottest because its thick carbon dioxide atmosphere traps heat (Mercury is closer).
Do this: Mention greenhouse effect.
Usually costs: 1 mark.
Mistake: Confusing ozone with oxygen.
Both are forms of oxygen.
Wrong working
Ozone is the gas we breathe.
Right working
Ozone is a three-atom form of oxygen that blocks UV rays; we breathe oxygen.
Do this: Ozone = O3.
Usually costs: 1 mark.
Mistake: Writing that the greenhouse effect is entirely harmful.
News reports.
Wrong working
The greenhouse effect should not exist.
Right working
A mild greenhouse effect keeps Earth warm; excess greenhouse gases cause global warming.
Do this: Say 'extra'.
Usually costs: 1 mark.
Mistake: Confusing geosphere with biosphere.
Soil has living organisms.
Wrong working
The biosphere is rocks and soil.
Right working
Geosphere: solid parts (rocks, soils, minerals); biosphere: living beings and where they live.
Do this: Geo = Earth; bio = life.
Usually costs: 1 mark.

Exam tips

Give the cause and the effect for each habitability condition.
Quote Table 13.2 values only as the chapter prints them.
Separate the natural greenhouse effect from the enhanced effect.
Name the four spheres in any 'interconnected' answer and give one link.
For threats, name climate change, biodiversity loss and pollution.

Practice questions on Why Earth Supports Life: Habitable Zone, Atmosphere, Spheres and Threats

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

Q1 · 2 marks · Very short answerBoard level
What are the four spheres of the Earth system? Define any two.
Hint
Air, water, solid, life.
Answer
Atmosphere, hydrosphere, geosphere and biosphere (1). E.g. hydrosphere is all the water; biosphere is living beings with their places of living (1).
Q2 · 1 mark · MCQBoard level
Which gas forms the ozone layer that shields us from UV rays?
(a) Nitrogen
(b) Oxygen in a three-atom form
(c) Carbon dioxide
(d) Methane
Hint
O3.
Answer
(b)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): If Earth were much smaller with the same density, it might not hold its atmosphere.
Reason (R): A smaller planet would have weaker gravity.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Gravity holds gases.
Answer
(a) R explains A.
Q4 · 4 marks · Short answerHarder board level
A planet's average temperature is 450 degrees Celsius with a carbon dioxide atmosphere, while a closer planet with no atmosphere averages 170 degrees Celsius. (a) Find the difference. (b) Which planet shows the greenhouse effect? (c) The Earth is at 15 degrees Celsius; find how many times higher the 170 degree figure is than the Earth's (to one decimal). (d) Why is Earth's average temperature not 170 degrees Celsius?
Hint
170 ÷ 15.
Answer
(a) 450 − 170 = 280 degrees. (b) The planet with the thick carbon dioxide atmosphere (Venus). (c) 170 ÷ 15 = 11.3 times. (d) Earth is in a different position from Mercury, has an atmosphere and a mild greenhouse effect, and is in the habitable zone (1 mark each).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
A friend says: 'The Earth has always had climate changes, so today's global warming is nothing new.' Another suggests the answer lies in the extra carbon locked underground for millions of years.
(a) Respond to the friend using the chapter. (b) Explain why the extra carbon matters. (c) Name two effects of warming. (d) Name two actions a village can take.
Hint
Burning fossil fuels releases it quickly.
Answer
(a) Present warming is linked to human activities such as burning fossil fuels, which release greenhouse gases faster than the Earth can absorb. (b) The Earth cannot absorb this extra carbon fast enough, so heat builds up. (c) Melting ice caps, higher sea levels, extreme weather. (d) Plant trees, save water and energy, use renewable energy (1 mark each).

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.What is the Goldilocks zone?Show answer
The habitable zone, the range of distances from a star where water stays liquid.
2.What does the ozone layer do?Show answer
It blocks harmful ultraviolet rays from the Sun.
3.What is the hydrosphere?Show answer
All the water on the Earth.
4.What does the Montreal Protocol (1987) address?Show answer
Reducing chemicals such as CFCs so that the ozone layer can recover.
5.What is the triple planetary crisis?Show answer
Climate change, biodiversity loss and pollution.

Why Earth Supports Life: Habitable Zone, Atmosphere, Spheres and Threats: frequently asked questions

Did Mars ever support life?

No proof has been found; scientists think Mars may once have had liquid water and conditions for simple life.

What is Mangalyaan?

India's Mars Orbiter Mission launched by ISRO in 2013 to study the Martian atmosphere, surface and signs of past water.

What is geodiversity?

The variety of landforms, rocks and soils on the Earth and the processes that shape them.

Is Earth's magnetic field important to life?

Yes. It acts as a shield that pushes harmful cosmic rays and solar wind particles away.