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Earth is the only place we know where life exists and thrives. But why here, and not on Mars or Venus? The final chapter of the Class 8 science book pulls the whole year together to answer that question. This guide is for students and parents: what the chapter covers, a worked example comparing planets with real NASA data, the common mix-ups and a revision routine.
Where is this chapter in the Class 8 book?
Our Home: Earth, a Unique Life Sustaining Planet is Chapter 13, the last chapter of Curiosity, the NCERT science textbook for Grade 8 (reprint 2026-27). The chapter PDF calls itself the concluding chapter of the middle stage. It recalls the Class 6 chapter Beyond Earth, then covers:
what makes Earth unique, and how the other planets compare
the conditions that make Earth habitable
how air, water, sunlight, soil and living things support life
how reproduction keeps life going
the threats to life: climate change, biodiversity loss and pollution
What makes Earth suitable for life?
The chapter gives four main reasons.
Position. Earth is at a distance from the Sun where water can stay liquid. This range of distances is the habitable zone, sometimes called the Goldilocks zone. Mars lies at its edge.
Size. Earth's gravity is strong enough to hold an atmosphere. If Earth were much smaller with the same density, the gases would escape into space.
Oxygen and ozone. Oxygen lets us breathe, and the ozone layer blocks harmful ultraviolet rays from the Sun.
Magnetic field. Earth's magnetic field pushes away many harmful particles from the Sun (the solar wind) and from space (cosmic rays).
Water covers about 70 per cent of Earth's surface. The chapter also points out that Earth's crust, where all life exists, is as thin compared with the planet as an apple's skin is to an apple.
Why is Venus hotter than Mercury?
You might expect the planet nearest the Sun to be hottest. It is not. Venus's atmosphere is almost all carbon dioxide, which traps heat. This is the greenhouse effect. On Earth a milder greenhouse effect keeps the temperature high enough for liquid water. The chapter notes that this is not how a glass greenhouse for plants works: that traps warm air simply because it is a closed space.
Worked example: filling the planet table with NASA data
Activity 13.2 asks you to collect each planet's temperature and size. Here are the inner planets from NASA's planetary fact sheet, giving distance from the Sun, diameter, mean temperature and surface pressure:
Mercury: 57.9 million km; 4879 km; 167 °C; no atmosphere to speak of.
Venus: 108.2 million km; 12,104 km; 464 °C; 92 bars.
Earth: 149.6 million km; 12,756 km; 15 °C; 1 bar.
Mars: 228.0 million km; 6792 km; -65 °C; 0.01 bar.
Now work with the numbers.
How much farther is Venus than Mercury? 108.2 ÷ 57.9 ≈ 1.87 times. Yet Venus is 464 - 167 = 297 °C hotter. Distance alone cannot explain that; the atmosphere does.
How thick is the air on Venus and Mars? Venus: 92 ÷ 1 = 92 times Earth's surface pressure. Mars: 1 ÷ 0.01 = 100, so Earth's surface pressure is about 100 times that of Mars, which matches the book's statement that the atmosphere on Mars is 100 times thinner.
How big is Earth compared with Mars? 12,756 ÷ 6792 ≈ 1.88 times the diameter. The same table lists surface gravity as 9.8 m/s² on Earth and 3.7 m/s² on Mars, which helps explain why Mars holds only a thin atmosphere.
Magnetic field? The fact sheet lists a global magnetic field for Earth and Mercury, but not for Venus or Mars.
Reading a table like this, and asking which column explains which pattern, is exactly the kind of investigation the book wants from you.
How does life keep going on Earth?
The chapter links back to earlier chapters. Plants make food by photosynthesis and release oxygen; animals eat plants or other animals; decomposers return nutrients to the soil. Land, water, air and living things form one system, and a change in one part, such as cutting a forest, affects rainfall, soil and wildlife.
Reproduction keeps each kind of living thing going. In asexual reproduction one parent produces offspring with the same instructions, as in vegetative propagation from a money plant cutting, potato eyes or ginger. In sexual reproduction, instructions from two parents combine, so offspring are not exact copies. Over many generations, small differences can add up, as with camels and their humps, or bacteria that become resistant to antibiotics.
Which mistakes do students make?
"The closest planet is the hottest." Venus beats Mercury.
"Mars has no atmosphere." It has a thin one; Mercury has essentially none.
Mixing up the greenhouse effect with a glass greenhouse.
Thinking variation needs a new species. Small differences between parents and offspring are variation too.
A revision routine for this chapter
Make your own planet table for all eight planets and circle the one value in each row that breaks the pattern.
Explain each of the four reasons for habitability to someone at home in one sentence each.
Grow a plant from a cutting or a potato eye and record its growth for two weeks.
Answer the chapter's question: what would happen if Earth's magnetic field disappeared?
If a link between chapters feels shaky, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can check the idea it builds on. For the ecosystems chapter just before this one, see How Nature Works in Harmony, and for India's Mars mission, read about Mangalyaan's science.
Start today: copy the four inner planets into your notebook and add a column for "has liquid water at the surface?"
