Formation of the solar system
Where did the Solar System come from?
When the Solar System formed, there were no people to observe how it was made. Observe means to watch something happening. So how do we know what happened?
Scientists can try to solve a problem like this in two different ways.
- They can look for evidence, in the form of facts from observations or experiments to support their theory, and then try to explain what they have found.
- They can think of a testable theory, called an hypothesis, and then look for evidence to support the hypothesis.
Facts about the Solar System
Look at the diagram of the Solar System.
Here are some facts about the Solar System:
- All the planets in the Solar System follow a path or orbit around the Sun in the same direction.
- The Sun and all the planets (except Venus and Uranus) spin on their axes (singular: axis) in the same direction.
- Most of the moons of the planets orbit their planets in the same direction as the planets rotate around the Sun.
- The direction of spin of the Sun and the planets (except Venus and Uranus) is the same as the direction in which the planets orbit the Sun.
- All the planets orbit the Sun in the same plane. Objects that are in the same plane could all be placed on the same flat surface, just like all the objects on a desk. That means the Solar System looks flat.
- Scientists can use these facts as evidence.
Watching the birth of stars
Scientists can see distant stars forming in other parts of space. These stars are being formed from clouds of dust and gas.
A cloud of dust and gas in space is called a nebula. The picture shows one of these clouds of dust and gas. You can see the young stars in the cloud.
Some young stars can also be seen with a flat disc of dust around them.
Scientists think our Solar System was formed this way.
Using models
Scientists cannot observe a star or Solar System forming in an experiment.
Instead they use computers to create models. A model is a way of representing something that is difficult to observe directly.
The scientists put many of the known laws of physics into a computer program. Then the computer uses this information to predict what will happen, starting with a cloud of dust and gas.
The result is a prediction that a star will form, surrounded by planets.
How do stars and planets form out of dust and gas?
The picture shows what scientists think our Solar System looked like as it was forming.
You saw in Section 3.1 that any object can act as a source of gravity.
All the particles of dust and gas in the pictures have their own weak gravity.
The particles of dust and gas pull on each other with very weak forces due to their own gravity. As they stick together, their total mass increases. As their mass increases, so does the strength of their gravity. That means they attract more dust and gas with a stronger force.
This starts to form a small ball.
Gradually, this ball gets bigger.
If the ball gets beyond a certain size, it will get hot enough to become a star. Otherwise it will become a planet.
Most of the facts about the Solar System support or agree with this hypothesis.
The fact that Venus spins on its axis in the opposite way to all the other planets seems to contradict this hypothesis. Contradict is the opposite of support – it means to go against something.
Scientists think the planet Jupiter almost reached the size to be a star.
It takes millions of years to form a star or a planet.
Questions
1. Use words from the list to copy and complete this sentence.
All the planets in the Solar System orbit the Sun in … different directions
Show Answer
the same direction
2. All the planets in the Solar System orbit the Sun in the same plane.
Show Answer
in the same path (orbit)
3. Which of these is the name given to a cloud of dust and gas in space?
Show Answer
nebula
4. Name the force that can pull particles of dust and gas together in space.
Show Answer
gravity
Think Like a Scientist
Investigation: Using Models in Science
In this task, you will explore how scientists use models to develop and test hypotheses — and why models are sometimes essential in science.
Steps:
1. Read the following example: Scientists use computer models to understand how the Solar System was formed.
2. Reflect on why this method is useful. What challenges do scientists face when studying something that takes millions of years?
3. Consider why a model can be useful even if it is a simplified or artificial version of reality.
4. Think about what makes a model valid, and what its limitations might be.
5. Now, answer the questions below.
Show Answer
Models allow scientists to test predictions without needing real-time cosmic events. They can simulate different conditions and explore outcomes safely and quickly.
(Choose one: an error, a mistake, a limitation, a strength)
Show Answer
A limitation
Show Answer
(Example answers)
• Some planets rotate in the opposite direction.
• The outer planets have unexpected compositions not predicted by simple models.
Show Answer
The standard model assumes uniform rotation and predictable distribution of materials. These facts suggest other events (like collisions) may have shaped the system differently.
- It has been proven to be correct.
- Most, but not all, of the evidence supports it.
- The model that is used has limitations.
- It can be fully tested by experiments.
Show Answer
• Most, but not all, of the evidence supports it.
• The model that is used has limitations.