Lesson Overview
This lesson explains how the Solar System formed from a cloud of gas and dust over 4.6 billion years ago. Learners will explore the solar nebula theory, how the Sun and planets developed, and why the Solar System has its current structure.
Lesson Content
The Solar Nebula
The formation of the Solar System began with a vast cloud of gas and dust known as the solar nebula. This nebula was composed mainly of hydrogen and helium, along with small amounts of heavier elements created by earlier generations of stars.
A disturbance — such as a nearby supernova shockwave — likely caused parts of the nebula to collapse under gravity. As the cloud collapsed, it began to spin faster and flatten into a rotating disk.
Formation of the Sun
As material collected at the centre of the collapsing nebula, pressure and temperature increased. Eventually, the central region became dense and hot enough for nuclear fusion to begin.
This marked the birth of the Sun, which quickly became the dominant mass in the forming Solar System. Once fusion began, intense solar radiation and solar winds cleared much of the remaining gas from the inner Solar System.
Protoplanetary Disk and Accretion
Surrounding the young Sun was a rotating protoplanetary disk. Within this disk, dust grains collided and stuck together through a process called accretion.
Over time:
Dust formed clumps
Clumps became planetesimals
Planetesimals merged to form planets
This process occurred at different speeds depending on distance from the Sun and available material.
Why Planets Are Different
Temperature played a major role in shaping the Solar System:
Inner Solar System: High temperatures allowed only rock and metal to remain, forming terrestrial planets
Outer Solar System: Cooler temperatures allowed gases and ices to accumulate, forming giant planets
This explains why the inner planets are rocky while the outer planets are large and gas-rich.
Clearing the Solar System
As planets formed, their gravity reshaped the Solar System:
Some objects were pulled into planets or moons
Others were scattered into belts and distant regions
Remaining debris became asteroids, comets, and dwarf planets
This process led to the relatively stable system observed today.
Evidence Supporting Solar System Formation Models
Scientists support the solar nebula model using:
Observations of protoplanetary disks around other stars
Chemical composition of meteorites
Orbital alignment of planets
Computer simulations of planetary formation
By studying young star systems, astronomers can see similar processes occurring elsewhere in the galaxy.
Why Understanding Formation Matters
Understanding how the Solar System formed helps scientists:
Explain planetary differences
Understand Earth’s origins
Predict planet formation around other stars
Identify potentially habitable worlds
It also provides context for studying the evolution of galaxies and stars.
Key Terms Introduced
Solar nebula
Protoplanetary disk
Accretion
Planetesimal
Nuclear fusion