Maarg Manthan · Topic 1.2

Star Formation and Formation of Planets

Geography › Earth's Origin, Interior and Motions · Topic 1.2

Stars form inside giant, cold clouds of gas and dust called nebulae. Gravity pulls clumps of gas together until the core becomes hot and dense enough for hydrogen to fuse into helium, and a star is born. The planets of a star system form from the leftover disc of gas and dust around the young star. Dust grains stick together to form planetesimals, planetesimals gather into protoplanets, and protoplanets become planets. Close to the star only rock and metal could survive the heat, so the inner planets are small and rocky, while the cold outer planets grew large by collecting gas and ice.

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

At a Glance

Birthplace of stars Nebula of hydrogen, helium and dust
Star begins to shine When hydrogen fusion starts in the core
Planet building blocks Planetesimals
Inner planets Small and rocky (terrestrial)
Outer planets Large and gaseous (Jovian)
Exam link UPSC Prelims, Geography: Earth’s Origin, Interior and Motions

Where Does It Fit?

This topic follows the Big Bang by narrowing the story from the whole universe to a single star and its planets. It explains why the Sun and the planets exist and why the inner and outer planets differ, which is needed before the Earth’s own evolution is described. Previous topic: The Earth’s Origin: Early Theories and the Modern Big Bang Theory. Next topic: The Solar System and the Evolution of the Earth.

How Does a Star Form?

In short: a star forms when gravity squeezes a cloud of gas until its core is hot enough for nuclear fusion.

Nebula: a vast cloud of hydrogen, helium and dust drifts in a galaxy. A shock, such as the blast from a nearby supernova, can set parts of it collapsing.
Clumping: gravity draws gas into dense clumps. As a clump shrinks, it spins faster and heats up, and flattens into a spinning disc.
Protostar: the hot, dense centre of the clump is a protostar. It shines by the heat of its own contraction, not yet by fusion.
Ignition: when the core reaches roughly ten million degrees, hydrogen nuclei fuse into helium and release huge energy. The star has now joined the main sequence.
Balance: the outward pressure of fusion balances the inward pull of gravity, and the star shines steadily for most of its life.

The Sun formed in this way about 4.6 billion years ago inside a nebula of the Milky Way. It is an ordinary medium-sized star.

What Are the Stages in the Life of a Star?

A star’s length of life depends on its mass. Massive stars burn fast and die young, while small stars burn slowly and last for tens of billions of years.

Stage	What happens
Main sequence	Hydrogen is fused to helium in the core; the Sun is in this stage and will remain so for several billion more years
Red giant	When core hydrogen runs out, the outer layers swell and cool and the star turns red
White dwarf	A star like the Sun sheds its outer layers and the small hot core remains and slowly cools
Supernova	A very massive star collapses and explodes, scattering heavy elements into space
Neutron star or black hole	The core left by a supernova becomes either a dense neutron star or a black hole

The Indian-born astrophysicist Subrahmanyan Chandrasekhar showed that a white dwarf cannot be heavier than about 1.4 times the mass of the Sun. This limit carries his name and he shared the Nobel Prize in Physics in 1983. Supernovae also matter to the Earth, because they scatter the carbon, oxygen, silicon and iron that later form planets and living things.

How Did the Planets Form?

The disc of gas and dust around the young Sun, called the solar nebula, was the raw material of the planets. Planet formation is described in steps.

Dust to planetesimals: grains in the disc collided and stuck together, and grew into bodies a few kilometres across, called planetesimals.
Planetesimals to protoplanets: gravity drew planetesimals into larger lumps through repeated collisions, a process called accretion. The largest lumps swept up the rest of the material along their orbits.
Protoplanets to planets: in a few million to tens of millions of years, a small number of large bodies were left, and these became the planets.
Clearing the disc: the young Sun gave off a strong stream of charged particles, the solar wind, which blew the remaining light gases out of the inner solar system.

Why Are the Inner and Outer Planets Different?

Temperature in the solar nebula fell with distance from the Sun. Near the Sun, only rock and metals could remain solid. Beyond a distance called the frost line, between the orbits of Mars and Jupiter, water and other compounds could freeze, giving much more solid material to build large cores that then trapped the abundant hydrogen and helium.

Feature	Terrestrial (inner) planets	Jovian (outer) planets
Members	Mercury, Venus, Earth, Mars	Jupiter, Saturn, Uranus, Neptune
Size	Small	Very large
Make-up	Rock and metal	Mostly hydrogen, helium and ices
Gases	Light gases were lost to the solar wind and weak gravity	Strong gravity and cold held the gases
Density	High	Low
Moons and rings	Few moons, no rings	Many moons, ring systems

As an analytical point, the Earth’s rocky nature, its thin atmosphere and its mix of metals all follow from its position in the inner solar system. The Earth’s later history is taken up in The Solar System and the Evolution of the Earth.

Why Is This Topic Important?

Solar system Explains planet differences
Prelims Stages of a star and names are asked
Elements Shows where heavy elements come from

What Came Next?

The next topic describes the members of the solar system and the way the Earth itself changed after it formed. Next topic: The Solar System and the Evolution of the Earth.

Key People and Terms

Nebula Cloud of gas and dust
Protostar Young star before fusion starts
Nuclear fusion Joining of light nuclei, releasing energy
Accretion Growth by collision and sticking
Frost line Distance beyond which ices can form
Chandrasekhar limit Upper mass of a white dwarf

Exam Corner

Points to Remember

A star starts to shine when hydrogen fusion begins in its core.
Planets grow by accretion from planetesimals.
Inner planets are terrestrial; outer planets are Jovian.
The solar wind removed light gases from the inner planets.
The Chandrasekhar limit is about 1.4 solar masses.

Do Not Confuse With

Protostar and star A protostar has no fusion; a star does
Planetesimal and planet Small building block versus finished body
Terrestrial and Jovian Rocky inner planets versus gaseous outer planets

Memory Hook

Nebula clumps, fusion lumps; dust sticks, planets mix.

Mains Angle

Questions ask why the inner planets are rocky and the outer planets gaseous. These points add depth.

Process Collapse, disc and accretion
Contrast Temperature gradient and the frost line
Link Elements made in earlier stars

A Question You May Face

An original practice question, not a past paper question.

“Explain how planets form from a solar nebula and why the terrestrial and Jovian planets differ in size and composition.”

How to Answer

Begin Nebula, collapse and disc.
Build Planetesimals and accretion.
Contrast Temperature and gravity.
Close Link to the Earth.

GS Relevance

Prelims: stages of star life, planetesimals, terrestrial and Jovian planets. GS Paper 1: origin and evolution of the solar system.

Frequently Asked Questions

How is a star formed?

A star forms when a cloud of gas and dust, called a nebula, collapses under its own gravity. The centre becomes hot and dense, and when it reaches about ten million degrees hydrogen starts to fuse into helium, releasing energy. The star then shines steadily on the main sequence.

What are planetesimals?

Planetesimals are small solid bodies, a few kilometres across, that formed from dust grains sticking together in the disc around a young star. Through repeated collisions and gravity they gathered into larger protoplanets, which later became the planets of the solar system.

Why are the inner planets rocky?

The inner planets formed close to the hot young Sun, where only rock and metal could stay solid. Their gravity was weak, and the solar wind swept away light gases such as hydrogen and helium, so they stayed small, dense and rocky.

Why are Jupiter and Saturn gaseous?

Jupiter and Saturn formed in the cold outer disc, beyond the frost line, where ices added much solid material. Their large cores had strong gravity that captured abundant hydrogen and helium, so they grew into giant planets that are mostly gas.

What is the Chandrasekhar limit?

The Chandrasekhar limit is the maximum mass, about 1.4 times the mass of the Sun, that a white dwarf star can have. The Indian-born astrophysicist Subrahmanyan Chandrasekhar derived it, and he shared the Nobel Prize in Physics in 1983.

PYQ Practice — Statement Analysis

1 Hydrogen fusion in the core marks the beginning of a star.

True

A protostar becomes a star when fusion starts.

2 Terrestrial planets are mostly made of hydrogen and helium.

False

They are made of rock and metal; Jovian planets are gaseous.

3 Planetesimals joined together by accretion to form planets.