CDS Geography · Earth in Space, Maps and Coordinates
The Solar System
Who proposed which theory of how the Earth formed, the two families of planets, the planet facts CDS asks, and what syzygy, perihelion and aphelion mean.
Why this matters
Ten CDS questions from 2016 to 2026: 2 EASY, 6 MODERATE, 2 HARD. Five are single planet facts, and both 2026 questions ask who proposed a theory of origin.
Concept 1 of 4: Theories of the origin of the Earth
Nobody saw the Earth form, so scientists proposed theories and named them after their central idea: a cloud of gas, a passing star, a twin star, or small solid bodies sticking together. CDS asks you to match each theory to the person who gave it, so learn them as pairs.
Definition
The theories and their authors:
- Gaseous (nebular) hypothesis: Immanuel Kant (1755). The planets formed from a slowly rotating cloud of gas around the young Sun. Laplace revised it in 1796.
- Planetesimal hypothesis: Chamberlin and Moulton. A passing star pulled matter out of the Sun; it cooled into small solid bodies (planetesimals) that came together as planets.
- Tidal hypothesis: James Jeans and Harold Jeffreys. A passing star raised a tide of matter from the Sun.
- Binary star hypothesis: H. N. Russell. The Sun once had a companion star whose matter formed the planets.
- Big Bang: the universe began from one tiny point that expanded. In 1920 Edwin Hubble gave evidence that the universe is still expanding.
Two theories about the continents and oceans sit beside these:
- Tetrahedral hypothesis: Lowthian Green. A cooling, shrinking Earth took a four-cornered (tetrahedral) shape, with continents at the corners and oceans on the faces.
- Continental drift: Alfred Wegener (1912), with F. B. Taylor an earlier supporter. W. M. Davis gave the cycle of erosion, not an origin theory.
| Theory | Proposed by |
|---|---|
| Gaseous (nebular) hypothesis | Immanuel Kant; revised by Laplace |
| Planetesimal hypothesis | Chamberlin and Moulton |
| Tidal hypothesis | James Jeans and Harold Jeffreys |
| Binary star hypothesis | H. N. RussellQ CDS 2026 (I): Kant gaseous, Russell binary star, Chamberlin planetesimal are all correctly matched. |
| Expanding universe (Big Bang) | Edwin Hubble showed the expansion |
| Tetrahedral hypothesis of continents and oceans | Lowthian GreenQ CDS 2026 (II): Lowthian Green, not Taylor, Wegener or Davis. |
| Continental drift | Alfred Wegener |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (I) 2026 — General Knowledge · Q66Hard
Example 1 · Earth in Space, Maps and Coordinates · The Solar System
Which of the following pairs of theories about origin of the Earth and their propounders is/are correctly matched ?
Select the answer using the codes given below :
| Theory | Propounder |
|---|---|
| 1. Gaseous Hypothesis | Kant |
| 2. Binary Star Hypothesis | Russell |
| 3. Planetesimal Hypothesis | Chamberlin |
Davis and Taylor are distractors for origin theories
W. M. Davis gave the cycle of erosion and F. B. Taylor argued for moving continents. Neither proposed the tetrahedral hypothesis, which is Lowthian Green's.
Planetesimal is Chamberlin, tidal is Jeans
Both theories use a passing star. The one about small solid bodies (planetesimals) is Chamberlin and Moulton's; the one about a tide of matter is Jeans and Jeffreys'.
Concept 2 of 4: The two families of planets
Near the young Sun it was too hot for light gases and ices to stay, so only rock and metal came together: those planets are small, dense and rocky. Far out it was cold, and huge planets grew out of gas and ice. A belt of asteroids lies between the two families.
Definition
Eight planets in two families:
- Inner or terrestrial planets (Earth-like): Mercury, Venus, Earth, Mars. Small, dense, made of rock and metal, with solid surfaces.
- Outer, giant or gas planets (Jovian): Jupiter, Saturn, Uranus, Neptune. Huge, made mostly of hydrogen, helium and ices, with no solid surface.
- The asteroid belt lies between Mars and Jupiter.
- Pluto has been a dwarf planet since 2006.
| Family | Planets | Made of |
|---|---|---|
| Terrestrial (inner) | Mercury, Venus, Earth, Mars | Rock and metal; solid surfaceQ CDS 2020 (II): the four planets closest to the Sun are the terrestrial planets. |
| Giant or gas (outer) | Jupiter, Saturn, Uranus, Neptune | Hydrogen, helium and ices; no solid surfaceQ CDS 2023 (I): Mars is rocky, so any group of gas planets that includes Mars is wrong. |
| Dwarf planet | Pluto (since 2006) | Rock and ice; too small to clear its orbit |
| Asteroid belt | Between Mars and Jupiter | Rocky and metal fragments |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (I) 2023 — General Knowledge · Q88Moderate
Example 2 · Earth in Space, Maps and Coordinates · The Solar System
Which of the following groups of planets is termed as 'gas planets' as they are composed primarily of lighter ices, liquids and gases?
Mars is the last rocky planet
Mars is red and far out, but it is a terrestrial planet. The gas planets begin after the asteroid belt, with Jupiter.
Concept 3 of 4: Planet facts CDS asks
Most planet questions test one striking fact per planet: the smallest, the hottest, the one with acid clouds, the one with rings. Learn one or two such facts for each planet and the statement questions become a quick check.
Definition
The facts to hold:
- Mercury: nearest the Sun, smallest and least massive planet; almost no atmosphere; no moon.
- Venus: the hottest planet; a carbon dioxide atmosphere (about 96%) under thick yellowish clouds of sulphuric acid; no moon; spins from east to west.
- Mars: the red planet (iron oxide dust); carbon dioxide about 95% of its thin air; two small moons, Phobos and Deimos.
- Jupiter: the largest planet; a gas giant with no land to stand on.
- Saturn: its rings are made of lumps of ice and rocky dust; it has the most known moons today.
- Uranus spins on its side; Neptune is the farthest planet from the Sun.
| Planet | Fact to remember |
|---|---|
| Mercury | Smallest and least massive; almost no atmosphereQ CDS 2019 (I): Mercury has the least mass, less than Mars. |
| Venus | Hottest; thick clouds of sulphuric acid; no moonQ CDS 2022 (II): the yellowish clouds of Venus are sulphuric acid. |
| Mars | Red from iron oxide; two moons, Phobos and DeimosQ CDS 2023 (II): carbon dioxide is about 95 to 97% of the air of both Venus and Mars. |
| Jupiter | Largest planet; a gas giant with no solid surface |
| Saturn | Rings of ice and rocky dust; most known moonsQ CDS 2023 (II): Mercury has no atmosphere, Jupiter no land, Saturn's rings are ice and dust; Venus has no moons. |
| Uranus | Spins on its side |
| Neptune | Farthest planet from the Sun |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (II) 2023 — General Knowledge · Q79Moderate
Example 3 · Earth in Space, Maps and Coordinates · The Solar System
Consider the following statements about some planets in the solar system :
1. Mercury has no atmosphere.
2. Venus has two moons.
3. There is no land on Jupiter.
4. Rings of Saturn are composed of lumps of ice and dust.
Which of the statements given above are correct ?
Nearest is not hottest
Mercury is nearest the Sun, but Venus is the hottest planet, because its thick carbon dioxide air traps heat.
Moon counts change
Older books say Jupiter has the most moons. New moons of Saturn were confirmed in 2019 and again later, and Saturn now has the most known moons. Read moon-count options with the date of the paper in mind.
Concept 4 of 4: Syzygy, perihelion and aphelion
Syzygy is just a hard word for three bodies in a straight line: the Sun, the Earth and the Moon. It happens at every new moon and every full moon. Perihelion and aphelion are a different idea: the Earth's nearest and farthest points from the Sun in its yearly orbit.
Definition
Positions in space:
- Syzygy: the Sun, Earth and Moon in a straight line (an angle of 180°). It happens at every new moon and full moon, about twice a month.
- Conjunction: the Moon between the Sun and the Earth (new moon). A solar eclipse can happen.
- Opposition: the Earth between the Sun and the Moon (full moon). A lunar eclipse can happen.
- There is no eclipse at every syzygy because the Moon's orbit is tilted about 5° to the Earth's, so the Moon usually passes just above or below the line.
- Perihelion: the Earth nearest the Sun, about 3 January (about 147 million km).
- Aphelion: the Earth farthest from the Sun, about 4 July (about 152 million km).
- Perigee and apogee are the Moon's nearest and farthest points from the Earth.
| Term | Meaning | When |
|---|---|---|
| Syzygy | Sun, Earth and Moon in a straight line | Every new moon and full moonQ CDS 2016 (II): 'syzygy conjunction occurs at the time of perihelion only' is the wrong statement. |
| Conjunction | Moon between the Sun and Earth | New moon |
| Opposition | Earth between the Sun and Moon | Full moon |
| Perihelion | Earth nearest the Sun | About 3 January |
| Aphelion | Earth farthest from the Sun | About 4 July |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2016 · CDS (II) 2016 — General Knowledge · Q46Hard
Example 4 · Earth in Space, Maps and Coordinates · The Solar System
Which one of the following statements is not correct?
The Earth is nearest the Sun in January
Perihelion comes about 3 January, in the northern winter. Seasons come from the tilt of the axis, not from the distance to the Sun.
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 (4)
Theories of the origin of the Earth7 rows
| Theory | Proposed by |
|---|---|
| Gaseous (nebular) hypothesis | Immanuel Kant; revised by Laplace |
| Planetesimal hypothesis | Chamberlin and Moulton |
| Tidal hypothesis | James Jeans and Harold Jeffreys |
| Binary star hypothesis | H. N. RussellQ CDS 2026 (I): Kant gaseous, Russell binary star, Chamberlin planetesimal are all correctly matched. |
| Expanding universe (Big Bang) | Edwin Hubble showed the expansion |
| Tetrahedral hypothesis of continents and oceans | Lowthian GreenQ CDS 2026 (II): Lowthian Green, not Taylor, Wegener or Davis. |
| Continental drift | Alfred Wegener |
The two families of planets4 rows
| Family | Planets | Made of |
|---|---|---|
| Terrestrial (inner) | Mercury, Venus, Earth, Mars | Rock and metal; solid surfaceQ CDS 2020 (II): the four planets closest to the Sun are the terrestrial planets. |
| Giant or gas (outer) | Jupiter, Saturn, Uranus, Neptune | Hydrogen, helium and ices; no solid surfaceQ CDS 2023 (I): Mars is rocky, so any group of gas planets that includes Mars is wrong. |
| Dwarf planet | Pluto (since 2006) | Rock and ice; too small to clear its orbit |
| Asteroid belt | Between Mars and Jupiter | Rocky and metal fragments |
Planet facts CDS asks7 rows
| Planet | Fact to remember |
|---|---|
| Mercury | Smallest and least massive; almost no atmosphereQ CDS 2019 (I): Mercury has the least mass, less than Mars. |
| Venus | Hottest; thick clouds of sulphuric acid; no moonQ CDS 2022 (II): the yellowish clouds of Venus are sulphuric acid. |
| Mars | Red from iron oxide; two moons, Phobos and DeimosQ CDS 2023 (II): carbon dioxide is about 95 to 97% of the air of both Venus and Mars. |
| Jupiter | Largest planet; a gas giant with no solid surface |
| Saturn | Rings of ice and rocky dust; most known moonsQ CDS 2023 (II): Mercury has no atmosphere, Jupiter no land, Saturn's rings are ice and dust; Venus has no moons. |
| Uranus | Spins on its side |
| Neptune | Farthest planet from the Sun |
Syzygy, perihelion and aphelion5 rows
| Term | Meaning | When |
|---|---|---|
| Syzygy | Sun, Earth and Moon in a straight line | Every new moon and full moonQ CDS 2016 (II): 'syzygy conjunction occurs at the time of perihelion only' is the wrong statement. |
| Conjunction | Moon between the Sun and Earth | New moon |
| Opposition | Earth between the Sun and Moon | Full moon |
| Perihelion | Earth nearest the Sun | About 3 January |
| Aphelion | Earth farthest from the Sun | About 4 July |
Watch out for (6)
- Davis and Taylor are distractors for origin theories→ Theories of the origin of the Earth
- Planetesimal is Chamberlin, tidal is Jeans→ Theories of the origin of the Earth
- Mars is the last rocky planet→ The two families of planets
- Nearest is not hottest→ Planet facts CDS asks
- Moon counts change→ Planet facts CDS asks
- The Earth is nearest the Sun in January→ Syzygy, perihelion and aphelion
Test yourself on a real paper
Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.