Earth in the solar system and the habitable zone
Meena sits near a bonfire on a cold evening. Too close and her face burns, too far and she shivers, and at one distance she feels just warm. Planets around the Sun are like this.
The habitable zone is the band of distances around a star that is neither too hot nor too cold, so water can stay liquid. Earth lies in the habitable zone of the Sun.
Key idea 1: Eight planets in two groups — The four inner planets (Mercury, Venus, Earth, Mars) are small and rocky. The four outer planets (Jupiter, Saturn, Uranus, Neptune) are large and made mostly of gases. Counting from the Sun, Earth is the third planet.
- Easy: Earth is the third planet, and there are 8 planets in all.
- Medium: Jupiter and Saturn are gas giants, while Mars is small and rocky.
- Exam-type: Of the 8 planets, 4 are rocky and 4 are gas giants, so Earth belongs to the rocky group.
In the figure: The planets are listed from nearest to farthest from the Sun, and Earth is third.
Key idea 2: Why Earth's distance matters — Life as we know it needs liquid water, and at Earth's distance the water in oceans and rivers stays liquid. Venus has a thick carbon dioxide atmosphere that traps heat, so it is the hottest planet. Mars sits at the cooler edge of the zone, with cold, thin air, and signs of old water make scientists ask whether it once had life.
- Easy: Earth has liquid water because of its distance from the Sun.
- Medium: Venus is hotter than Mercury because its thick air traps heat.
- Exam-type: With averages of about 170 °C and 450 °C, Venus is about 280 °C hotter than Mercury although it is farther from the Sun.
In the figure: Venus is the hottest even though Mercury is nearer the Sun, while Earth stays mild.
Earth lies in the habitable zone, where water can stay liquid.
Thinking the planet nearest the Sun is the hottest. Venus is hotter because its thick atmosphere traps heat.
Learn the planets as one sentence. Memory clue: "My Very Educated Mother Just Served Us Noodles" = Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Spot the difference
| Group | Made of | Planets |
|---|---|---|
| Inner planets | rock and metal | Mercury to Mars |
| Outer planets | mostly gases | Jupiter to Neptune |
| Habitable zone | liquid water possible | Earth |
Practice questions
- 1How many planets are there in the solar system?
- 2Which planet is third from the Sun?
- 3What is the habitable zone?
- 4Name the four rocky planets.
- 5Why is Venus hotter than Mercury?
- 6What happens to water on a planet that is too far from the Sun?
- 7One planet averages 450 °C and another 170 °C. By how much is the first hotter, and which gas helps explain it?
- 8Why would a planet only a little closer to the Sun than Earth probably have no oceans?
Show answers (try them yourself first)
- 1Eight.
- 2Earth.
- 3The band of distances around a star where water can stay liquid.
- 4Mercury, Venus, Earth and Mars.
- 5Its thick carbon dioxide atmosphere traps heat.
- 6It freezes.
- 7450 - 170 = 280 °C hotter; carbon dioxide.
- 8It would be too hot, so the water would evaporate.
Earth in the solar system and the habitable zone
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Open in the appSize, atmosphere, ozone and magnetic field
On a hot day a mother puts a hat on her sleeping child and covers the child with a thin sheet. Earth too has layers that protect it.
The atmosphere is the layer of gases around the Earth. The ozone layer is a part of it that absorbs harmful ultraviolet (UV) rays. The magnetic field is the region around Earth where its magnetism acts and pushes away harmful particles from space.
Key idea 1: The right size and an atmosphere — Earth is big enough for its gravity to hold air, but not so big that it crushes living things. The Moon and Mercury are too small to keep an atmosphere. The air gives oxygen for breathing and, through a natural greenhouse effect, keeps Earth warm instead of freezing at night.
- Easy: Gravity holds the atmosphere around Earth.
- Medium: The Moon has almost no air because its gravity is weak.
- Exam-type: Without an atmosphere, Earth would lose heat quickly and become very cold at night.
In the figure: Only the middle size can hold an atmosphere without crushing life.
Key idea 2: Three shields — The ozone layer blocks most UV rays, which can harm cells. Earth's magnetic field, believed to come from moving molten iron in the core, pushes away fast particles such as cosmic rays and the solar wind. Together with the atmosphere, they protect life on Earth.
- Easy: The ozone layer blocks harmful UV rays.
- Medium: The magnetic field pushes away harmful particles from space.
- Exam-type: A freely hanging magnet settles in a fixed direction because the Earth itself acts like a giant magnet.
In the figure: Each shield guards life from a different danger.
Gravity holds the air, ozone blocks UV rays and the magnetic field turns away particles.
Thinking ozone blocks all sunlight. It absorbs mostly the harmful UV part.
Match each shield with its danger. Memory clue: "AOM" = Air keeps warm, Ozone stops UV, Magnetic field turns particles.
Spot the difference
| Shield | What it does | Danger it handles |
|---|---|---|
| Atmosphere | holds air, traps heat | cold and no air to breathe |
| Ozone layer | absorbs UV rays | skin and cell damage |
| Magnetic field | turns away particles | solar wind, cosmic rays |
Practice questions
- 1What is the atmosphere?
- 2Which layer blocks harmful UV rays?
- 3What is believed to create Earth's magnetic field?
- 4Why does the Moon have almost no atmosphere?
- 5Why does a freely hanging magnet point in a fixed direction?
- 6Name two things that the magnetic field keeps away.
- 7A planet is so large that its gravity crushes living things. Is it suitable for life like ours? Explain.
- 8Describe one way life on the ground would be harmed if the ozone layer became much thinner.
Show answers (try them yourself first)
- 1The layer of gases around the Earth.
- 2The ozone layer.
- 3The movement of molten iron in the core.
- 4Its gravity is too weak to hold gases.
- 5The Earth acts like a giant magnet and the magnet lines up with its field.
- 6Cosmic rays and the solar wind.
- 7No, because the strong pull would crush living things.
- 8More UV rays would reach the ground and harm skin, eyes and plants.
Spheres of the Earth and the balance of life
Ravi waters a plant in a pot. The soil, the water, the air around it and the plant itself all work together in one small place.
The Earth has four connected spheres: the lithosphere (rocks and soil), the hydrosphere (all water), the atmosphere (air) and the biosphere (all living things, found within the other three).
Key idea 1: Four spheres, one planet — The lithosphere is the solid crust, the hydrosphere includes oceans, rivers, ice and groundwater, and the atmosphere is the envelope of air. Most water is in the salty oceans. The biosphere is not a separate layer; it spreads through the other three, and all four exchange matter and energy.
- Easy: Rocks and soil belong to the lithosphere, and water to the hydrosphere.
- Medium: A fish in a river lives in the hydrosphere and is part of the biosphere.
- Exam-type: A tree uses soil minerals, water and carbon dioxide, so it touches all four spheres.
In the figure: The biosphere is not drawn separately because living things are found in all three.
Key idea 2: Linked spheres and the balance of life — A change in one sphere reaches the others. Plants make food and oxygen, animals release carbon dioxide, and decomposers return nutrients to the soil. Living things depend on each other and on the spheres, which is the balance of life. If decomposers vanished, dead matter would pile up and nutrients would not return to the soil.
- Easy: Decomposers break dead matter into nutrients.
- Medium: Plants use the carbon dioxide that animals breathe out.
- Exam-type: Cutting a forest on a slope lets rain wash soil into a river, so the biosphere, lithosphere and hydrosphere are all affected.
In the figure: A change in the biosphere travels through the lithosphere to the hydrosphere.
The four spheres are linked, so a change in one affects the others.
Thinking the biosphere is a separate layer above the air. It exists inside the other spheres.
Use the word roots. Memory clue: litho = rock, hydro = water, atmo = air, bio = life.
Spot the difference
| Sphere | Contains | Example |
|---|---|---|
| Lithosphere | rocks and soil | mountain, farm soil |
| Hydrosphere | all water | river, ocean, ice |
| Biosphere | all living things | tree, fish, bird |
Practice questions
- 1Name the four spheres of the Earth.
- 2Which sphere includes all the water?
- 3What is the biosphere?
- 4A bird drinks water from a river. Which two spheres does it link?
- 5What do decomposers do?
- 6Give one link between plants and animals through gases.
- 7Rain falls on a hill and fills a stream. Which three spheres are linked?
- 8Why would removing all decomposers harm the balance of life?
Show answers (try them yourself first)
- 1Lithosphere, hydrosphere, atmosphere and biosphere.
- 2The hydrosphere.
- 3The part of the Earth where living things are found.
- 4The biosphere and the hydrosphere.
- 5They break dead matter into nutrients that return to the soil.
- 6Plants release oxygen and use carbon dioxide, which animals release.
- 7The atmosphere, lithosphere and hydrosphere.
- 8Dead matter would pile up and nutrients would not return to the soil.
Reproduction: asexual and sexual
A gardener cuts a rose stem and plants it, and a new rose grows. In the same garden a mango tree grows from a seed made after pollination.
Reproduction is the process by which living things produce new individuals of their kind. In asexual reproduction one parent produces offspring, and in sexual reproduction a male and a female gamete join.
Key idea 1: Asexual reproduction needs one parent — An amoeba splits into two (binary fission), yeast and hydra grow a bud (budding), bread mould and ferns make spores, and a rose or potato can grow from a part of the plant (vegetative propagation). The offspring are almost identical to the parent.
- Easy: An amoeba reproduces by binary fission.
- Medium: A potato bud grows into a new plant with only one parent.
- Exam-type: A farmer who wants many identical banana plants uses vegetative propagation from the good parent plant.
In the figure: Each method makes new individuals from a single parent.
Key idea 2: Sexual reproduction needs two parents — A male gamete joins a female gamete in fertilisation to form a zygote, which grows into a new individual. In flowering plants, pollination carries pollen from the anther to the stigma. Fertilisation is internal in a hen or a human and external in a frog or a fish. The offspring show variation, which helps some survive if the surroundings change.
- Easy: The joining of male and female gametes is fertilisation.
- Medium: A frog lays eggs in water, where external fertilisation takes place.
- Exam-type: A field of identical cutting-grown plants can all catch one disease, while varied plants are more likely to include survivors.
In the figure: Fertilisation makes the zygote, which develops into a new individual.
Asexual needs one parent and gives almost identical offspring; sexual needs two parents and gives variation.
Confusing pollination with fertilisation. Pollination moves pollen; fertilisation is the joining of gametes.
Count the parents. Memory clue: A = Alone (one parent), S = Share (two parents share their features).
Spot the difference
| Feature | Asexual | Sexual |
|---|---|---|
| Parents | one | two |
| Offspring | almost identical | show variation |
| Examples | amoeba, rose cutting | hen, frog, mango seed |
Practice questions
- 1What is reproduction?
- 2How many parents are needed in asexual reproduction?
- 3What is the joining of male and female gametes called?
- 4Name the method by which yeast reproduces.
- 5Give one example each of internal and external fertilisation.
- 6Why do cuttings give plants identical to the parent?
- 7A farmer wants 40 identical banana plants from one good plant. Which method should he use, and why?
- 8Why is a field of identical plants at risk from one disease?
Show answers (try them yourself first)
- 1Making new individuals of the same kind.
- 2One.
- 3Fertilisation.
- 4Budding.
- 5Internal: hen or human; external: frog or fish.
- 6Only one parent is involved, so the features are copied.
- 7Vegetative propagation, because it gives identical plants with the same good taste.
- 8All plants are alike, so the same disease can affect all of them.
Threats to life on Earth and what we can do
In Imran's school the lights stay on all day and waste is burned outside. His class forms a green club to save electricity and plant trees.
Global warming is the rise in the Earth's average temperature caused by extra greenhouse gases such as carbon dioxide and methane. It leads to climate change, a long-term change in weather patterns.
Key idea 1: Why the Earth is warming — Burning coal, petrol and diesel adds carbon dioxide, and cutting forests removes trees that absorb it. Farming and waste add methane. The extra gases trap more heat, which melts ice, raises sea level and brings heavier rain, floods, droughts and stronger cyclones. Gases called CFCs once thinned the ozone layer, and global action reduced them.
- Easy: Burning fuels releases carbon dioxide, a greenhouse gas.
- Medium: Melting glaciers and polar ice raise the sea level.
- Exam-type: More fuel burnt and fewer trees mean more carbon dioxide stays in the air and more heat is trapped.
In the figure: Each step leads to the next, ending with a rising sea.
Key idea 2: What we can do — We can reduce, reuse and recycle, use clean energy such as solar power, plant trees, save water and electricity, walk or cycle, and avoid burning waste. Mission LiFE (Lifestyle for Environment) asks everyone to adopt small habits that protect the environment. Many small actions together make a big difference.
- Easy: Carry a cloth bag instead of taking a new plastic bag.
- Medium: Walking, cycling or using a bus burns less fuel than a private car.
- Exam-type: A school that saves electricity, plants trees and cuts waste lowers its carbon footprint more than one that does only a single thing.
In the figure: The three Rs together cut waste and save resources.
Fewer greenhouse gases and more trees help slow global warming.
Thinking one person's small actions do not matter. Many small actions add up.
Follow the three Rs. Memory clue: Reduce, Reuse, Recycle and plant a Tree.
Spot the difference
| Term | Meaning | Example |
|---|---|---|
| Deforestation | cutting forests | clearing a hill slope |
| Afforestation | planting trees | a new school grove |
| Greenhouse gas | traps heat | carbon dioxide, methane |
Practice questions
- 1What is global warming?
- 2Name one greenhouse gas.
- 3What are the three Rs?
- 4Why does melting ice from glaciers raise the sea level?
- 5How do trees help against global warming?
- 6Give two things you can do every day to protect the environment.
- 7A school saves 30 units of electricity every week. How many units does it save in 12 weeks?
- 840 people travel in one bus instead of 40 separate cars. How many fewer vehicles burn fuel, and why does this help?
Show answers (try them yourself first)
- 1The rise in the Earth's average temperature caused by extra greenhouse gases.
- 2Carbon dioxide or methane.
- 3Reduce, reuse and recycle.
- 4The extra water from land ice flows into the oceans.
- 5They absorb carbon dioxide from the air.
- 6For example: switch off lights and carry a cloth bag.
- 730 x 12 = 360 units.
- 839 fewer vehicles; less fuel burns, so less carbon dioxide enters the air.
Threats to life on Earth and what we can do
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