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NDA Geography · Earth in Space, Maps and Coordinates

Planets and the Solar System

Eight planets orbit the Sun — the four small rocky terrestrial planets inside the asteroid belt and the four giant planets outside it — and the universe itself is explained by the Big Bang theory.

All Earth in Space, Maps and Coordinates notesNDA Geography strategy

Why this matters

4 PYQs, mixing one EASY recall (origin of the universe) with HARD density-ranking. Two anchors carry most marks: the origin theories (Big Bang for the universe; Nebular for the Solar System) and the density facts — EARTH is the densest planet, Jupiter the largest, Saturn the least dense. The terrestrial-planet statements (low density vs the giants, lying inside the asteroid belt, gravity holding gases) are a recurring statement-trap.

Concept 1 of 3

Theories of the origin of the universe and Solar System

Intuition

Two different questions, two different answers. The universe as a whole is explained by the Big Bang theory — everything expanding from a single hot, dense point. The Solar System (the Sun and its planets) is explained by the Nebular hypothesis and its relatives. Don't mix the two: Big Bang = universe; Nebular/Planetesimal/Binary = Solar System.

Definition

Match the theory to what it explains:

Theory / hypothesisExplainsIdea
Big Bang theoryOrigin of the universeUniverse expanded from a hot, dense single point
NDA 2019 — the universe's origin is the Big Bang.
Nebular hypothesisOrigin of the Solar SystemSun and planets formed from a spinning gas-dust cloud (nebula)
Planetesimal hypothesisOrigin of the Solar SystemPlanets built up from small bodies (planetesimals)
Binary / tidal theoryOrigin of the Solar SystemA passing star pulled matter off the Sun
Practice this conceptself-check · 3 quick reps

Try it yourself

Which theory explains the origin of the UNIVERSE: Nebular hypothesis, Binary theory, Big Bang theory, or Planetesimal hypothesis?

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Which theory explains the origin of the universe?
  2. 2.
    Which hypothesis explains the origin of the Solar System from a gas-dust cloud?
  3. 3.
    Is the Big Bang about the universe or just the Solar System?

From the bank · past-year question

Example 1Earth in Space, Maps and CoordinatesEASY
Which one of the following hypothesis/theory explains the origin of the universe?

[Q106 · Sep · 2019]

Big Bang = universe, Nebular = Solar System

The trap puts the Nebular and Planetesimal hypotheses next to the Big Bang and asks about the universe. Those two explain the Solar System; only the Big Bang explains the universe.

Concept 2 of 3

Planet order: terrestrial vs giant planets

Intuition

Outward from the Sun the order is Mercury, Venus, Earth, Mars, then the asteroid belt, then Jupiter, Saturn, Uranus, Neptune. The first four are the small, rocky, dense terrestrial (inner) planets; the last four are the big, low-density giant (outer) planets. The terrestrial planets lie between the Sun and the asteroid belt — a fact the NDA tests directly.

Definition

  • Order outward from the Sun: Mercury, Venus, Earth, Mars | (asteroid belt) | Jupiter, Saturn, Uranus, Neptune.
  • Terrestrial (inner) planets — Mercury, Venus, Earth, Mars: small, rocky, HIGH density, and they lie between the Sun and the asteroid belt.
  • Giant (outer) planets — Jupiter, Saturn, Uranus, Neptune: large, gas/ice, LOW density.
  • Statement-trap fact: terrestrial planets have HIGHER density than the giants (not lower), and their stronger surface gravity (relative to their small mass, helped by being cooler/inner) is part of why they retain or lose certain gases — be careful with the exact wording offered.
SunAsteroid beltMercuryVenusEarthMarsJupiterSaturnUranusNeptuneTerrestrial (rocky, dense)Giant (gas / ice, low density)Earth = densest. Jupiter = largest. Saturn = least dense (floats on water).

Worked example

Two statements: (I) The terrestrial planets lie between the Sun and the asteroid belt. (II) The terrestrial planets have higher density than the giant planets. Are both correct?
  1. Mercury, Venus, Earth and Mars sit inside the asteroid belt — statement I is correct.
  2. Rocky terrestrial planets are denser than the gas/ice giants — statement II is correct.
Answer:Yes — both statements are correct.
Practice this conceptself-check · 4 quick reps

Try it yourself

Name the four terrestrial planets in order from the Sun.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Name the four terrestrial planets.
  2. 2.
    What lies between the terrestrial and giant planets?
  3. 3.
    Do terrestrial planets have higher or lower density than the giants?
  4. 4.
    Which planet comes right after Mars going outward?

From the bank · past-year question

Example 2Earth in Space, Maps and CoordinatesHARD
Consider the following statements about terrestrial planets: I. Mercury, Venus, Earth and Mars are terrestrial planets having low densities as compared to other planets. II. They lie between the Sun and the belt of asteroids. III. Their lower gravity can hold the escaping gases. Which are correct?

[Q119 · Apr · 2026]

Terrestrial planets have HIGHER density, not lower

A statement claims terrestrial planets have 'low densities as compared to other planets'. The opposite is true — the rocky terrestrials are the densest planets; the gas giants are low-density. So that statement is treated as incorrect.

Terrestrial planets lie INSIDE the asteroid belt

The reliable true statement is that the terrestrial planets lie between the Sun and the belt of asteroids. Anchor on that one when a multi-statement question mixes it with density errors.

Concept 3 of 3

Density ranking — Earth is the densest planet

Intuition

Of all eight planets, Earth has the highest density (about 5.5 g/cm3), just ahead of Mercury and Venus. The giants are far lighter for their size — Saturn is the LEAST dense (it would float on water). So in 'highest density' or 'arrange by density' questions, Earth sits at the top and the gas giants at the bottom.

Definition

  • Earth is the densest planet (~5.51 g/cm3). Mercury and Venus are close behind; Mars is a bit lower.
  • The giant planets are low density: Jupiter ~1.33, Saturn ~0.69 g/cm3. Saturn is the least dense planet (less dense than water).
  • A typical descending-density order among Earth, Venus, Saturn, Jupiter is Earth > Venus > Jupiter > Saturn (Jupiter, though low-density, is slightly denser than Saturn).
  • Don't confuse density (mass per volume) with size: Jupiter is the largest planet but Earth is the densest.

Worked example

Arrange in descending order of density: Earth, Saturn, Venus, Jupiter.
  1. Earth is the densest planet — first.
  2. Venus, a rocky terrestrial, is next.
  3. Among the two giants, Jupiter (~1.33) is denser than Saturn (~0.69).
  4. So Saturn, the least dense, is last.
Answer:Earth > Venus > Jupiter > Saturn.
Practice this conceptself-check · 4 quick reps

Try it yourself

Which planet has the highest density: Mercury, Venus, Jupiter or Earth?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Which planet has the highest density?
  2. 2.
    Which planet is the least dense?
  3. 3.
    Largest planet vs densest planet?
  4. 4.
    Order by density: Earth, Venus, Jupiter, Saturn.

From the bank · past-year question

Example 3Earth in Space, Maps and CoordinatesHARD
Which one of the following is the correct sequence of arrangement of the given planets in descending order of their density (in gm/cm3^3)?

[Q60 · Apr · 2023]

Densest is EARTH, not Jupiter

Jupiter is the largest planet, which tempts students into picking it for 'highest density'. But density is mass per volume — Earth is the densest. Size and density are different questions.
Drill 1 more on density ranking — earth is the densest planet

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (1)

Theories of the origin of the universe and Solar System4 rows
Theory / hypothesisExplainsIdea
Big Bang theoryOrigin of the universeUniverse expanded from a hot, dense single point
NDA 2019 — the universe's origin is the Big Bang.
Nebular hypothesisOrigin of the Solar SystemSun and planets formed from a spinning gas-dust cloud (nebula)
Planetesimal hypothesisOrigin of the Solar SystemPlanets built up from small bodies (planetesimals)
Binary / tidal theoryOrigin of the Solar SystemA passing star pulled matter off the Sun

Watch out for (4)

Mastery check — 1 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Earth in Space, Maps and CoordinatesMODERATE
Which one of the following planets has the highest density ?

[Q120 · Apr · 2021]

Drill every past-year question on this subtopic

4 questions from the bank — paginated, with cart and Word-export support.

Drill the 4 questions