Maarg Manthan · Topic 1.3

The Solar System and the Evolution of the Earth

Geography › Earth's Origin, Interior and Motions · Topic 1.3

The solar system consists of the Sun, eight planets, dwarf planets, their moons, asteroids, comets and the icy bodies of the Kuiper belt. The Sun holds about 99.8 per cent of the mass of the system. The Earth formed about 4.6 billion years ago as a hot, molten ball. Heavy iron sank to form the core while lighter rock rose to form the mantle and crust, a process called differentiation. A giant collision with a Mars-sized body is thought to have thrown out the debris that formed the Moon.

6 min read · UPSC Prelims · Geography · Earth's Origin, Interior and Motions

At a Glance

Planets Eight, since the 2006 IAU definition
Sun-Earth distance About 149.6 million km (1 AU)
Earth’s age About 4.6 billion years
Earth’s early state Hot and molten, then differentiated
Moon’s origin Giant impact hypothesis
Exam link UPSC Prelims, Geography: Earth’s Origin, Interior and Motions

Where Does It Fit?

This topic completes the story of the formation of the solar system by listing its members and then follows the Earth’s own early evolution. It sets up the later topics on the Earth’s layers, its atmosphere and oceans, and its geological history. Previous topic: Star Formation and Formation of Planets. Next topic: Layered Structure of the Earth (Five Layers).

What Are the Members of the Solar System?

In short: the solar system has one star, eight planets, five officially recognised dwarf planets, many moons and countless smaller bodies.

Planet	Group	Notable feature
Mercury	Terrestrial	Nearest to the Sun and smallest planet; no real atmosphere
Venus	Terrestrial	Hottest planet because of a thick carbon dioxide atmosphere; spins in the opposite direction to most planets
Earth	Terrestrial	Densest planet; the only one known to have life and liquid water on its surface
Mars	Terrestrial	Red because of iron oxide; has the largest volcano of the solar system, Olympus Mons
Jupiter	Jovian (gas giant)	Largest planet, with a very strong magnetic field
Saturn	Jovian (gas giant)	Famous ring system; the least dense planet, lighter than water
Uranus	Jovian (ice giant)	Axis tilted about 98 degrees, so it rolls on its side as it orbits
Neptune	Jovian (ice giant)	Farthest planet, with the strongest winds

The number of known moons of Jupiter and Saturn keeps rising as new ones are confirmed, so the exact counts should be checked in the latest reports before use.

What Are Dwarf Planets, Asteroids, Comets and the Kuiper Belt?

In August 2006 the International Astronomical Union defined a planet as a body that orbits the Sun, is large enough to be nearly round, and has cleared the neighbourhood around its orbit. Pluto failed the last test and was moved to the new class of dwarf planets, along with Ceres, Eris, Haumea and Makemake.

Asteroid belt: a band of rocky bodies between the orbits of Mars and Jupiter. Ceres, the largest object in the belt, is a dwarf planet.
Kuiper belt: a ring of icy bodies beyond Neptune, which includes Pluto.
Oort cloud: a vast, distant shell of icy bodies thought to be the source of long-period comets.
Comets: icy bodies that grow a glowing tail when they come close to the Sun.
Distances: the average Sun-Earth distance of about 149.6 million km is one astronomical unit (AU). Sunlight takes a little over eight minutes to reach the Earth.

India’s space programme studies these bodies directly. Chandrayaan-3 made a soft landing near the Moon’s south pole on 23 August 2023, and Aditya-L1, launched in September 2023, studies the Sun from a point near the Sun-Earth line.

How Did the Earth Evolve After It Formed?

The young Earth was very different from the planet today. It was a hot, rocky ball with no oceans and an atmosphere of hydrogen and helium, which was soon lost.

Heating: impacts of planetesimals, the squeezing of material by gravity and the decay of radioactive elements made the Earth so hot that it partly melted.
Differentiation: in the molten state, dense materials, mainly iron and nickel, sank to the centre to form the core, while lighter silicate material rose and formed the mantle and a thin crust. This layering is described in Layered Structure of the Earth (Five Layers).
Cooling: the surface cooled and hardened into a crust. Volcanoes released gases from the interior, which began to build a new atmosphere and, later, the oceans, as in Evolution of the Lithosphere, Atmosphere and Hydrosphere.

How Was the Moon Formed?

Several ideas have been put forward, such as capture of a passing body or splitting of the Earth. The most widely accepted explanation is the giant impact hypothesis, sometimes called the ‘big splat’. About 4.5 billion years ago, a body about the size of Mars, often named Theia, struck the young Earth a glancing blow. The debris thrown into orbit gathered together to form the Moon.

Size: the Moon’s diameter is about 3,474 km, a little more than a quarter of the Earth’s.
Distance: it orbits at an average distance of about 384,400 km.
Composition: Moon rocks closely match the Earth’s mantle and crust in their make-up, which supports the impact idea.
Effects on the Earth: the Moon causes tides and slows the Earth’s rotation very slowly, as explained in Tides: Tidal Currents, Types and Importance.

Why Is This Topic Important?

Facts Planet features are asked often
Layers Differentiation explains the interior
Current affairs Chandrayaan and Aditya missions

What Came Next?

The next topic turns to the layers of the Earth that arose from the differentiation described here. Next topic: Layered Structure of the Earth (Five Layers).

Key People and Terms

Astronomical unit Mean Earth-Sun distance
Dwarf planet Round body that has not cleared its orbit
Kuiper belt Icy ring beyond Neptune
Differentiation Separation of the Earth into layers by density
Theia Name given to the Mars-sized impactor
Olympus Mons Largest volcano on Mars

Exam Corner

Points to Remember

Pluto became a dwarf planet in 2006.
The Earth is the densest planet; Saturn is the least dense.
Venus is the hottest planet; Mercury is nearest the Sun.
The Moon is thought to have formed in a giant impact.
One astronomical unit is about 149.6 million km.

Do Not Confuse With

Asteroid belt and Kuiper belt Between Mars and Jupiter versus beyond Neptune
Hottest and nearest planets Venus is hottest; Mercury is nearest
Giant impact and capture theory Debris from a collision versus a captured body

Memory Hook

Eight planets, five dwarfs; iron sinks and rock floats; Theia hits, Moon is born.

Mains Angle

Questions ask for the stages of the Earth’s early evolution. These points add depth.

Layering Heat and density sorting
Moon Impact origin and its effects
Space missions Indian missions to the Moon and Sun

A Question You May Face

An original practice question, not a past paper question.

“Describe the stages of the early evolution of the Earth and explain how the Moon is believed to have formed.”

How to Answer

Start Hot molten Earth.
Layers Differentiation.
Moon Giant impact and evidence.
End Atmosphere and oceans follow.

GS Relevance

Prelims: planets and their features, dwarf planets, origin of the Moon, differentiation of the Earth. GS Paper 1: salient features of the world's physical geography.

Frequently Asked Questions

How many planets are there in the solar system?

There are eight planets: Mercury, Venus, Earth and Mars, which are terrestrial, and Jupiter, Saturn, Uranus and Neptune, which are Jovian. Pluto was reclassified as a dwarf planet by the International Astronomical Union in August 2006, along with Ceres, Eris, Haumea and Makemake.

What is differentiation of the Earth?

Differentiation is the separation of the young, molten Earth into layers by density. Heavy materials such as iron and nickel sank to form the core, while lighter silicates floated upward and formed the mantle and crust. It explains why the interior of the Earth is layered.

How did the Moon form?

The most widely accepted view is the giant impact hypothesis. About 4.5 billion years ago a Mars-sized body, often named Theia, struck the young Earth. The debris thrown into orbit joined together to form the Moon, whose rocks resemble the Earth's mantle.

What is the asteroid belt?

The asteroid belt is a region between the orbits of Mars and Jupiter that contains a large number of rocky bodies called asteroids. Its largest member, Ceres, is a dwarf planet. The belt is thought to be made of material that never joined to form a planet.

What is an astronomical unit?

An astronomical unit is the average distance between the Earth and the Sun, about 149.6 million km. It is used as a convenient unit for distances inside the solar system, and light takes a little over eight minutes to travel this distance.

PYQ Practice — Statement Analysis

1 Pluto is one of the eight planets of the solar system.

False

It was reclassified as a dwarf planet in 2006.

2 Venus is the hottest planet in the solar system.

True

Its thick carbon dioxide atmosphere traps heat.

3 The asteroid belt lies between the orbits of Mars and Jupiter.