Class 8 Science · Chapter 13 NotesOur Home: Earth, a Unique Life Sustaining Planet

Earth is a planet orbiting the Sun, but it is not just any planet — it is the only one we know that supports life. All the mountains, rivers, forests, animals and people live in a very thin layer on the surface called the crust. If the Earth were the size of an apple, this life-supporting crust would be as thin as the apple's skin. Earth provides us with the air we breathe, the water we drink, the soil that grows our crops, and materials like rock and timber for building. Satellites, including those of ISRO, take images of the Earth that help scientists study plants, ocean organisms, ocean temperature, oil spills and wind direction. These images show how beautiful and varied our planet is, and how important it is to understand what makes it special.

• Crustthe thin outer layer where all life exists
• Earth provides air, water, soil, rocks and timber
• Satellite images help study plants, oceans and weather

Our solar system has eight planets. In order of increasing distance from the Sun they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Mercury, Venus, Earth and Mars are small rocky planets, while Jupiter, Saturn, Uranus and Neptune are large planets mostly made of gases. Generally, planets closer to the Sun are hotter and those farther away are colder. But Venus, the second planet from the Sun, is the hottest planet. This is because its thick atmosphere is almost entirely carbon dioxide, which traps heat and does not let it escape. This trapping of heat by gases in the atmosphere is called the greenhouse effect. On Earth too, gases like carbon dioxide trap some heat and keep the temperature just right for life. A plant greenhouse works differently — it traps warm air simply because it is a closed space with glass walls.

• Order from the SunMercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
• Venus is the hottest planet because its carbon dioxide atmosphere traps heat
• Greenhouse effectgases in the atmosphere absorb radiation given off by the Earth and trap heat

The most important reason Earth supports life is its distance from the Sun. It lies in the habitable zone, also called the Goldilocks zone — the range of distances from a star where water remains liquid. If Earth were closer to the Sun, water would evaporate; if it were farther, water would freeze. Liquid water is essential for life to evolve. Earth's orbit is nearly circular, so sunlight and heat stay nearly steady through the year. The size of the Earth also matters. If Earth were much smaller, its gravity would be too weak to hold the atmosphere, and gases would escape into space — as on Mars, whose atmosphere is 100 times thinner than Earth's, and Mercury, which has no atmosphere. If Earth were much larger, gravity would be so strong that our bones could be crushed. Oxygen in the atmosphere is used for breathing, and some of it forms ozone, which blocks harmful ultraviolet rays. Earth's magnetic field, believed to come from molten iron moving in the core, acts as a shield against cosmic rays and the solar wind.

• Habitable zone (Goldilocks zone)the range of distances from a star where water stays liquid
• Right size gives right gravity to hold the atmosphere
• Ozone layer blocks harmful ultraviolet rays
• Magnetic field shields Earth from cosmic rays and solar wind

Earth can be thought of as made of four connected spheres. The atmosphere is the layer of gases around the Earth; it contains oxygen for respiration and carbon dioxide for photosynthesis, and its greenhouse effect keeps the Earth warm. The hydrosphere is all the water on Earth — ponds, lakes, rivers, springs, seas, oceans and groundwater. Water covers about 70 per cent of Earth's surface, which is why Earth looks blue from space and is called the Blue Planet. Water is a good solvent, carries nutrients from soil to leaves in plants, and in animals regulates body temperature and aids digestion. The geosphere is the solid part of the Earth — rocks, soil and minerals. Soil is rich in nutrients like nitrogen and potassium that plants need. The variety of landforms, rocks and soils is called geodiversity. The biosphere is all living beings together with the places where they live.

• Atmospherelayer of gases around the Earth
• Hydrosphereall water on Earth, about 70 per cent of the surface
• Geospheresolid part — rocks, soil and minerals
• Biosphereall living beings and the places they live
• Geodiversityvariety of landforms, rocks and soils

Life on Earth is sustained by the connections between living and non-living things. In the presence of sunlight, plants take carbon dioxide from the air and water from the soil to make food by photosynthesis, releasing oxygen needed for respiration. Water vapour in the air forms clouds and brings rain or snow, which refills rivers, lakes and underground water; rainfall affects which plants and animals can live in a place. Moving air shapes weather and rainfall, influencing farming and water supply. In the biosphere, plants make food, animals eat plants or other animals, and decomposers break down dead matter and return nutrients to the soil. Earth works as one vast system, and even a small change in one part — such as cutting down a forest — can affect rainfall, soil, air quality and the animals living there. It is this balance that keeps our planet habitable.

• Photosynthesis links air, water, sunlight and soil
• Water cycle refills rivers, lakes and groundwater
• Decomposers return nutrients to the soil
• A change in one part of the system affects the others

If living things did not reproduce, life would eventually disappear from Earth. Parents pass on instructions called genetic material or genes, stored inside every cell, which guide how offspring develop. In asexual reproduction, a single parent produces new individuals that are exact copies of itself. Many plants reproduce this way when a leaf, stem or root is planted in soil — this is called vegetative propagation, as in ginger and potato. Single-celled organisms like bacteria and amoebae divide into two identical individuals, Hydra grows buds, and Planaria can regrow from a fragment of its body. In sexual reproduction, two parents are involved. Each makes specialised reproductive cells called gametes, which carry only half of the parent's genetic material. When male and female gametes join, the new cell has a complete set of instructions — half from each parent — so the offspring is not exactly like either parent.

• Genesinstructions inside every cell that guide development
• Asexual reproductionone parent, offspring are exact copies
• Vegetative propagationreproduction from leaf, stem or root
• Gametesreproductive cells carrying half the parent's genetic material

In flowering plants, pollen grains inside the anther are the male gametes, and ovules deep inside the flower are the female gametes. Pollen is carried to another flower by wind, insects or animals — this is pollination. When the male and female gametes combine, it is called fertilisation, forming a zygote that becomes the seed. The fleshy part of the flower around the ovule develops into a fruit, and animals that eat the fruit help scatter the seeds. In animals, the male gamete is the sperm and the female gamete is the egg. In fish and frogs, fertilisation takes place in water and the zygote develops into an embryo there too. In birds and mammals, sperm are deposited inside the female's body. In birds, the fertilised egg is laid and the embryo develops during hatching; in most mammals, the embryo develops inside the mother's body, which provides food and oxygen until birth.

• Pollinationtransfer of pollen from anther to another flower
• Fertilisationjoining of male and female gametes to form a zygote
• In birds the egg is laid; in most mammals the embryo develops inside the mother

Human actions are disturbing the delicate balance that supports life. The three biggest environmental challenges — climate change, biodiversity loss and pollution — are together called the triple planetary crisis. Burning fossil fuels like coal and oil releases greenhouse gases such as carbon dioxide and methane, which trap more heat and cause global warming. Trees, plants and ocean planktons absorb carbon dioxide, but they cannot absorb the extra carbon released fast enough. Even a small rise in temperature can melt ice caps, raise sea levels, cause extreme weather and make many plants and animals disappear. Long-term changes in temperature, rainfall and weather patterns are called climate change. Destroying habitats makes plants and animals disappear and upsets ecosystems. Air, water and soil pollution harm people and nature. Countries have made agreements such as the Montreal Protocol (1987), the Earth Summit (1992), the Kyoto Protocol (2005) and the Paris Agreement (2015) to protect the environment. Everyone can help by reusing, repairing and recycling, saving energy and water, and using cleaner energy like solar and wind.

• Triple planetary crisisclimate change, biodiversity loss and pollution
• Greenhouse gasescarbon dioxide and methane
• Global warmingrise in Earth's temperature due to trapped heat
• Climate changelong-term changes in temperature, rainfall and weather patterns