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Why is Earth the only planet known to support life?

Class 8 ScienceNCERT chapter: Our Home: Earth, a Unique Life-Sustaining Planetइसे हिंदी में पढ़ें

Exam-ready answer

Earth is at just the right distance from the Sun, so its temperature allows water to stay liquid. Its air contains oxygen for breathing and carbon dioxide for plants. The ozone layer blocks most harmful ultraviolet rays, and Earth's magnetic field shields it from charged particles from the Sun. No other known planet has all these together.

Explanation

Earth lies in the region around the Sun where it is neither too hot nor too cold, often called the habitable zone. Venus, closer to the Sun, is extremely hot, and Mars, farther away, is very cold. On Earth the temperature range allows water to exist as a liquid on the surface. Liquid water is essential for life because the chemical processes inside living cells take place in water.

Earth's atmosphere supplies oxygen that most living things need for respiration, and carbon dioxide that plants use to make food. Gases such as carbon dioxide and water vapour trap some of the Sun's heat, keeping the average surface temperature at about 15 °C instead of freezing. The ozone layer high in the atmosphere absorbs most of the harmful ultraviolet radiation from the Sun.

Earth also has a magnetic field that deflects most of the charged particles streaming from the Sun, protecting the atmosphere and living things. Its rotation gives regular day and night, so temperatures do not swing to extremes. Scientists are searching other planets and moons for signs of life, but so far Earth is the only place where life is known to exist, so protecting its air, water and soil matters.

Points to remember

  1. Earth is at a suitable distance from the Sun (the habitable zone), so its temperature is moderate.
  2. Liquid water on the surface is essential for all known life.
  3. The atmosphere provides oxygen and carbon dioxide and keeps Earth warm, with an average temperature of about 15 °C.
  4. The ozone layer absorbs most of the Sun's harmful ultraviolet rays.
  5. Earth's magnetic field protects it from charged particles coming from the Sun.

Exam tip

List the conditions as separate points with a one-line reason each: suitable distance and temperature, liquid water, atmosphere with oxygen, ozone layer, and magnetic field. Linking each condition to how it helps life earns full marks.

Questions students also ask

Why can't life as we know it survive on Venus or Mars?
Venus is extremely hot because of its thick carbon dioxide atmosphere, so no liquid water can exist on its surface. Mars has a very thin atmosphere and is very cold, and liquid water does not stay on its surface. Neither has a breathable atmosphere.
What is the ozone layer and why is it important?
The ozone layer is a region high in the atmosphere that contains more ozone gas than other levels. It absorbs most of the harmful ultraviolet rays from the Sun, which could otherwise damage the skin, eyes and living cells of plants and animals.
What is meant by the habitable zone?
The habitable zone is the region around a star where a planet receives just enough heat for water to stay liquid on its surface. It is not too close, where water would boil away, and not too far, where it would freeze.

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