Our Home: Earth, a Unique Life-Sustaining Planet
Science Class 8 — Chapter Notes
What would Earth be like if it had never known life at all?
Our planet is not just another ball of rock in space. It is the only known place in the universe that supports a vast variety of life, in places ranging from towering mountains to deep oceans and green forests. With the help of Earth-observation satellites such as those operated by ISRO, scientists study the special features that make Earth a life-sustaining planet.
In this chapter, you will explore the conditions and systems that make Earth uniquely fit for life. You will review Earth's physical structure, its place in the Solar System, the interaction between living and non-living things, and the challenges that threaten life on Earth.
About the Image
The image was captured by an ISRO Earth Observation Satellite.
It is a mosaic made by combining nearly 3,000 smaller images.
This is a false-colour image, in which special colours are used to highlight specific information.
Such images help scientists study land, water, plant growth and environmental changes more clearly.
Out of the billions of planets in the universe, Earth is the only one we know that has life in so many forms, including plants, animals, people and microorganisms.
Earth's Crust is like the thin skin of an apple
Earth's outermost solid layer is a very thin layer called the crust.
If Earth were the size of an apple, the crust where life exists would be as thin as the apple's skin.
Below the crust are the upper mantle, lower mantle, outer core and inner core. Life is found mainly on the crust and in the nearby surface regions of the atmosphere, land and water.
Earth provides many things needed for life, including air, water, soil and useful resources.
Earth supplies air to breathe, water for drinking and agriculture, and soil for growing food.
Materials such as rocks, timber and metals are available for building homes, roads and tools.
The Solar System has eight planets that orbit the Sun in nearly circular paths.
Planets in order from the Sun:
1. Mercury
2. Venus3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune
Mercury, Venus, Earth and Mars: These are smaller, rocky planets, also called terrestrial planets.
Jupiter, Saturn, Uranus and Neptune: These are much larger planets made mainly of gases and ices. Jupiter and Saturn are gas giants, while Uranus and Neptune are ice giants.
The average temperature of each planet.
The relative size, or radius, of each planet compared with Earth.
Whether each planet has an atmosphere.
Observations:
All planets receive energy from the Sun.
Planets closer to the Sun are generally hotter, while those farther away are generally colder.
Venus is not the closest planet to the Sun, but it is the hottest planet.
Its atmosphere is very thick and is made mainly of carbon dioxide (CO2).
Carbon dioxide traps heat through the greenhouse effect, so Venus retains a large amount of heat.
Earth also has a greenhouse effect. However, it is much weaker and is essential for maintaining temperatures suitable for life.
Planetary greenhouse effect: Gases in the atmosphere trap some of the infrared radiation, or heat, emitted by the planet's surface.
Plant greenhouse: Glass walls reduce air circulation and help trap warm air physically.
Both keep places warm, but the mechanisms are different.
Several physical and chemical properties of Earth combine to make it habitable. These include its position in the Solar System, its size and atmosphere, and its magnetic field.
1. Position of Earth: The Habitable Zone
Earth's distance from the Sun is just right: it is neither too close nor too far.
This distance keeps temperatures suitable for water to exist mostly as a liquid, which is essential for all known life.
If Earth were closer to the Sun, it would be too hot and much of its water could evaporate.
If Earth were farther from the Sun, it would be too cold and much of its water could freeze.
The region around a star where liquid water can exist is called the habitable zone or the Goldilocks zone.
More than 70% of Earth's surface is covered with water, giving Earth the name the Blue Planet.
Mars lies near the outer edge of the Sun's habitable zone.
Many spacecraft and rovers have studied Mars, but no proof of current life has been found yet.
Evidence suggests that Mars may once have had liquid water, but no proof of past life has been found. Future missions may provide new information.
Earth's nearly circular orbit helps keep the amount of sunlight and the temperatures fairly steady throughout the year.
If Earth were smaller, its gravity might be too weak to hold a dense atmosphere. Gases could escape into space, as has happened to a large extent on Mercury and Mars.
If Earth were much larger, its very strong gravity and other conditions could make it unsuitable for life as we know it.
The atmosphere supplies oxygen for breathing and carbon dioxide for plants.
Some oxygen in the upper atmosphere forms ozone. The ozone layer shields life from much of the harmful ultraviolet, or UV, radiation from the Sun.
The atmosphere also contributes to the greenhouse effect, which traps enough heat to keep Earth warm without making it too hot.
Mangalyaan, or the Mars Orbiter Mission, was launched by ISRO in 2013. It was an important Indian mission to study Mars.
The mission studied the atmosphere and surface of Mars.
Mangalyaan demonstrated how effective, low-cost space missions can provide valuable scientific results.
Along with Earth's suitable position, size and atmosphere, its magnetic field is another key factor that helps make Earth safe for life.
Earth behaves like a giant magnet. A freely suspended magnet, such as a compass needle, points approximately in the north-south direction.
The region around a magnet in which its influence can be felt is called the magnetic field.
Scientists believe that the movement of molten iron in Earth's outer core generates Earth's magnetic field.
Earth is constantly exposed to tiny, high-energy particles from space:
Cosmic rays come from distant regions of space.
Solar wind consists of charged particles emitted by the Sun.
These particles can damage the atmosphere, contribute to the loss of atmospheric gases, increase harmful radiation and harm living organisms.
The magnetic field acts as a shield by deflecting many of these charged particles away from Earth.
This protection helps preserve the atmosphere and makes Earth safer for life.
It is the connections between the living, or biotic, and non-living, or abiotic, parts of Earth that enable life to thrive.
Air, Water and Sunlight
The atmosphere provides oxygen for respiration and contains carbon dioxide, which plants use for photosynthesis.
Sunlight supplies energy for photosynthesis and warms Earth's surface.
Part of this heat is trapped by the atmosphere through the greenhouse effect, keeping temperatures suitable for liquid water.
Water covers nearly 70% of Earth and forms the hydrosphere, which includes oceans, seas, rivers, lakes and groundwater.
Water dissolves and transports nutrients, helps animals regulate body temperature and supports digestion. It is also essential for photosynthesis and all cellular processes.
Freshwater is required for agriculture and human consumption.
Water vapour forms clouds that produce rain and snow. These replenish rivers, lakes and groundwater.
Air movement, or wind, shapes weather and rainfall patterns. This influences farming and the availability of water.
The geosphere, including Earth's crust, contains rocks, soil and minerals that support life.
Soil provides a medium for plant growth and contains nutrients such as nitrogen and potassium. These nutrients can be released through the weathering of rocks and the decomposition of dead organisms.
Rocks and soil provide resources such as salt, iron and copper, which are useful in human life and technology.
Geodiversity, or the variety of landforms, rocks and soils, creates different habitats and supports biodiversity.
Non-living parts of nature actively shape ecosystems and biological communities.
The biosphere is the zone where life exists. It includes parts of the land, water and lower atmosphere. It contains all living organisms, including trees, grasses, herbs, animals, insects and microscopic organisms such as bacteria and fungi.
Plants perform photosynthesis, using sunlight, water and carbon dioxide to make food and release oxygen.
Animals depend on plants and other animals for food and energy.
Microorganisms, including decomposers, break down dead matter and recycle nutrients into the soil for use by plants.
These interactions form complex food chains and food webs that help maintain ecological balance. The study of these relationships is called ecology.
Earth is an interconnected system in which land, air, water and living organisms interact constantly.
Small changes, such as deforestation, can alter rainfall, soil quality, air composition and wildlife populations.
Maintaining balance in these systems is essential for a healthy and habitable planet.
Protecting clean air, water, soil and biodiversity helps secure Earth's future.
If organisms did not reproduce, species would eventually disappear. Reproduction ensures the continuity of life across generations.
What Is Reproduction?
Reproduction is the biological process by which organisms produce new individuals of the same kind.
Parents pass down instructions called genes, which are parts of genetic material that influence the form and functioning of an organism.
Genes are found in cells and guide the development of body parts and biological processes.
It keeps each species going from one generation to the next.
It can produce variation, or small differences in inherited features, which may help organisms adapt to changing environments.
The accumulation of such changes over many generations can lead to the evolution of new traits or species.
Examples of inherited adaptations include the hump of a camel, which helps it survive in desert conditions, and antibiotic resistance in some microbes.
Reproduction can produce offspring that resemble their parents but also show differences. There are two main types of reproduction:
1. Asexual reproduction: New individuals arise from a single parent and are usually genetically almost identical to that parent.
2. Sexual reproduction: Two parents contribute genetic material, producing offspring with a mixture of traits from both parents.In asexual reproduction, one parent produces new individuals that are usually genetic copies of the parent.
Example: Many plants can reproduce by vegetative propagation. In this process, a part of the plant, such as a leaf, stem or root, can grow into a new plant.
Plant stem cuttings, such as those of a money plant, potato eyes or pieces of ginger, in moist soil.
Provide water, air and sunlight.
Observe daily when roots, shoots and new leaves appear.
This demonstrates how some plants grow new individuals from a part of the original plant.
Other examples of asexual reproduction include bacteria and amoeba reproducing by binary fission, hydra reproducing by budding, planaria reproducing by regeneration, and some algae reproducing by fragmentation.
Sexual reproduction is the process in which two parents, usually a male and a female, produce offspring. This type of reproduction is common in most animals and flowering plants. Some microorganisms also have mating types that act like parents.
Special Cells for Reproduction: Gametes
Both parents produce special reproductive cells called gametes.
Male gametes: Sperm in animals; in flowering plants, male gametes are carried in pollen grains.
Female gametes: Egg cells in animals and egg cells present inside the ovules of flowering plants.
Gametes carry half the genetic information of a parent. When a male and female gamete fuse during fertilisation, they form a zygote with a complete set of genetic information.
Offspring receive a unique mixture of genes from both parents.
That is why they resemble their parents but are not exact copies.
Siblings may look different because each receives a different combination of genes.
The mixing of genes increases variation, which is useful for adaptation and evolution.
Flowering plants have male and female reproductive parts:
Anther: The part of a stamen that produces pollen grains.
Ovule: A structure inside the ovary that contains the female reproductive cell.
Pollination: The transfer of pollen from the anther to the stigma, often by wind, insects or other animals.
Fertilisation: After pollination, the pollen grain produces a pollen tube. The male gamete travels through this tube to the ovule, where the male and female gametes fuse to form a zygote.
The zygote develops into an embryo inside the seed. The ovule becomes the seed and the ovary develops into the fruit.
Seed dispersal: Fruits or seeds are carried away by animals, wind or water. Seeds that fall in suitable places germinate using their stored food and grow roots and shoots.
Animals have two types of reproductive cells: sperm, the male gamete, and egg, the female gamete.
Fertilisation occurs when a sperm and an egg unite to form a zygote.
In fish and amphibians, such as frogs, fertilisation often takes place externally in water. The parents release eggs and sperm into the water.
In birds and most mammals, fertilisation occurs inside the female's body. Sperm travel to meet the egg.
After fertilisation:
Birds lay eggs. The embryo develops using the food stored in the egg until it hatches.
Most mammals give birth to live young. The embryo develops inside the mother, who supplies it with food and oxygen.
The main difference is that egg-laying animals provide food for the embryo inside the egg, whereas most mammals provide food to the embryo inside the mother's body.
Earth's life depends on a delicate balance between living things, such as plants, animals and microorganisms, and non-living things, such as air, water, soil and sunlight. Human activities are disturbing this balance, leading to major environmental problems.
The three main global challenges today are:
1. Climate change
2. Biodiversity loss
3. Pollution
Burning fossil fuels such as coal, oil and gas releases carbon dioxide (CO2), a greenhouse gas.
These gases trap additional heat in the atmosphere, causing global warming.
Plants, trees and ocean plankton absorb some carbon dioxide. However, the excess carbon dioxide released by burning fossil fuels is added faster than natural systems can remove it.
Even a small rise in temperature can melt ice caps and raise sea levels, causing coastal flooding.
Changes in temperature can also increase extreme weather events such as heavy rainfall, storms, droughts and heat waves.
Rising temperatures can cause the extinction of plants and animals that cannot adapt quickly enough.
Long-term changes in temperature, rainfall and weather patterns are collectively known as climate change.
Destroying habitats such as forests, grasslands and wetlands causes many plants and animals to disappear.
This disturbs food chains and ecosystems. For example, if grasses disappear, herbivores lose their food; without herbivores, predators may also be unable to survive.
Each species plays a role in nature. Losing species weakens the ability of ecosystems to support life.
Air pollution comes from factories, vehicles and the burning of fuels.
It can cause respiratory illness, damage crops and produce smog and acid rain.
Water and soil pollution can be caused by industrial effluents, agricultural chemicals and plastic waste.
It harms aquatic life, makes water unsafe and reduces crop yields.
Polluted soil can introduce toxic substances into the food chain.
Small changes in global temperature, atmospheric composition or the ozone layer can endanger life.
Earth's systems, including the hydrosphere (water), biosphere (living things), atmosphere (air) and geosphere (rocks and soil), are connected. Harm to one system can affect the others.
Countries have agreed on treaties and programmes to protect Earth:
Montreal Protocol (1987): Reduced the use of chemicals such as CFCs that damaged the ozone layer and contributed to the recovery of the ozone layer.
Earth Summit (1992): Global meeting that promoted cooperation on environmental protection and sustainable development.
Kyoto Protocol (entered into force in 2005): Set binding emission-reduction targets for some countries.
Paris Agreement (2015): Countries committed themselves to limiting global warming to well below 2°C above pre-industrial levels and pursuing efforts to limit it to 1.5°C.
As of 2025, the world has not yet achieved the 1.5°C goal, so stronger action is still needed.
Cut down on pollution: Avoid burning waste and reduce vehicle emissions.
Switch to cleaner energy: Use solar, wind and other renewable sources instead of coal and oil whenever possible.
Use energy and water carefully: Turn off lights, repair leaks, and use public transport or cycle where possible.
Reduce, reuse and recycle: Repair items instead of throwing them away, and recycle paper, plastic, glass and metal.
Practise sustainable farming and waste management: Use environmentally friendly methods, reduce unnecessary chemical use and manage sewage properly.
Protect biodiversity: Conserve habitats, plant native trees and support protected areas.
Community action: Local communities that manage natural resources wisely can make a large positive difference.
Earth is uniquely suited for life because of its suitable distance from the Sun, appropriate size and gravity, atmosphere and magnetic field.
Life depends on continuous interactions among air, water, soil, rocks and living organisms.
Reproduction, both asexual and sexual, ensures the continuation and variation of life.
Human activities have introduced serious threats, including climate change, biodiversity loss and pollution.
Global treaties and local actions together can help protect Earth. Everyone can contribute through small, practical steps.
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