Solar System Planets and the Greenhouse Effect (13.2) — "Our Home: Earth, a Unique Life Sustaining Planet"

1. 12 marks

   Name the eight planets of the solar system in order of increasing distance from the Sun, and state which four are rocky versus gas-dominated.

   Hint. The book gives this order directly, then splits the eight into two groups of four.

   ✦ In order of increasing distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Mercury, Venus, Earth and Mars are relatively small and rocky planets; Jupiter, Saturn, Uranus and Neptune are large planets, mostly made of gases, since the chapter groups all eight this way by size and composition.

2. 23 marks

   What general rule would you expect for planet temperature versus distance from the Sun, and which planet breaks that rule?

   Hint. Activity 13.2's own table reveals a surprising exception once filled in.

   ✦ Since all planets get their energy from the Sun, the general expectation is that a planet close to the Sun would be very hot, and planets get colder as you move farther away. Venus breaks this rule: even though it is only the second planet from the Sun (with Mercury closer), Venus has the highest average temperature of any planet and is the hottest planet in the solar system.

3. 33 marks

   Explain, step by step, why Venus is hotter than Mercury even though Mercury is closer to the Sun.

   Hint. The cause isn't distance at all — it's what Venus's atmosphere is made of and what that does to escaping heat.

   ✦ 'Venus is the hottest planet not because it is the closest to the Sun, but because its thick atmosphere traps heat.' Venus's air is almost entirely carbon dioxide, which does not let heat escape — this is called the greenhouse effect, and it makes Venus even hotter than Mercury, which is relatively closer to the Sun but lacks Venus's thick, heat-trapping atmosphere.

4. 43 marks

   Explain how the greenhouse effect works on Earth, and why the chapter describes its role here as important rather than harmful.

   Hint. On Earth, the same mechanism that overheats Venus plays a much gentler, life-supporting role.

   ✦ On Earth, gases like carbon dioxide in the atmosphere trap heat by absorbing the radiation given off by the Earth after it gets warmed by the Sun — the same basic mechanism at work on Venus. On Earth, though, this effect is far milder, and the chapter describes it as important rather than harmful because it 'plays an important role in maintaining just the right temperature on Earth' — without it, Earth would lose too much heat to space and become too cold for liquid water to exist.

5. 53 marks

   A step further: explain how a garden greenhouse for growing plants keeps things warm differently from the planetary greenhouse effect on Venus and Earth.

   Hint. One mechanism involves gas absorbing radiation directly; the other simply seals warmed air inside a physical enclosure.

   ✦ On Venus or Earth, gases like carbon dioxide in the atmosphere trap heat by absorbing the radiation given off by the planet after it is warmed by the Sun. A plant greenhouse, on the other hand, traps warmed air simply because it is a closed space, usually with glass walls — it heats up during the day, and the air stays in because the heat does not escape easily through the glass. So while both keep things warm, they do it through entirely different mechanisms — one through gas absorbing radiation, the other through physically sealing in warm air.

6. 62 marks

   Why might a student find it surprising that some planets in Table 13.2 have 'No' listed for having an atmosphere?

   Hint. Not every planet's gravity is strong enough to hold gases in place.

   ✦ A student might expect every planet to have some kind of atmosphere, since Earth's own atmosphere feels like a given — but a planet's ability to hold an atmosphere depends on having strong enough gravity, which in turn depends on its size and density. Smaller planets or those with weaker gravity, like Mercury, simply cannot hold gases close enough to form a lasting atmosphere, which is exactly the size-and-gravity relationship the chapter explores further in section 13.3.2.

7. 72 marks

   Where would you look to find the missing information in Table 13.2, according to the chapter's own instructions for Activity 13.2?

   Hint. The activity names three specific sources students can use.

   ✦ The chapter suggests collecting this information 'from books in your school library, trusted websites, or discuss with your teachers' — since combining these gives a mix of print sources, verified online sources, and direct consultation with a knowledgeable adult.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 13 (hecu113.pdf), Reprint 2026-27, pages 210-228. Questions are referenced from the NCERT textbook for identification.