What is in the Solar System, and what keeps the planets in orbit?
The structure of the Solar System, the role of gravity in orbits, and how a star is formed and stays stable.
A focused answer to the WJEC GCSE Science Double Award Unit 6 topic on space, covering the structure of the Solar System, how gravity keeps planets and moons in orbit, and how a star forms and stays stable.
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What this dot point is asking
WJEC Double Award Unit 6 wants you to describe the structure of the Solar System, explain the role of gravity in orbits, and describe how a star forms and stays stable.
The structure of the Solar System
There are eight planets orbiting the Sun. Moons are natural satellites that orbit planets. The Sun contains almost all the mass of the Solar System, which is why its gravity dominates.
Gravity and orbits
A more massive object has a stronger gravitational pull, which is why the Sun (very massive) holds the planets, and planets hold their moons. Artificial satellites orbit the Earth in the same way, held by Earth's gravity.
How a star forms
A star forms in stages:
- A cloud of dust and gas (a nebula) is pulled together by gravity.
- As the material falls together, it gets hotter and denser.
- When it is hot and dense enough, nuclear fusion of hydrogen begins, and the star starts to shine.
Why a star is stable
When a star eventually runs out of hydrogen fuel, this balance is lost and the star changes, beginning the later stages of its life.
Galaxies and the scale of the Universe
Our Solar System is just a tiny part of a much larger Universe. The Sun is one of billions of stars in a galaxy called the Milky Way, and the Universe contains billions of galaxies. The distances are so vast that they are measured in light-years (the distance light travels in a year). Light from the Sun takes about 8 minutes to reach Earth, but light from other stars takes years, and from distant galaxies millions of years. Having a sense of this scale - planet, star, galaxy, Universe - helps you put the Solar System in context, which is a common exam theme.
How ideas about the Solar System developed
Ideas about the Solar System have changed over time as evidence improved. People once believed the Earth was at the centre (the geocentric model), but observations, especially with the telescope, showed that the planets orbit the Sun (the heliocentric model). This is a good example of how a scientific model is revised when new evidence does not fit. Recognising that the Sun-centred model replaced the Earth-centred one because of better evidence is a common "how science works" point.
Try this
Q1. What force keeps the planets in orbit around the Sun? [1 mark]
- Cue. Gravity (the Sun's gravitational pull).
Q2. What process begins when a forming star becomes hot and dense enough? [1 mark]
- Cue. Nuclear fusion (of hydrogen).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe the main objects in the Solar System and what holds them in orbit.Show worked answer →
A Unit 6 describe question worth 4 marks. Reward: the Solar System has the Sun (a star) at the centre (1); eight planets orbit the Sun, and moons orbit some planets (1); there are also smaller objects such as asteroids and comets (1); everything is held in orbit by the gravitational force of the Sun (and the planets for their moons) (1). Markers credit the Sun, planets and moons, smaller bodies, and gravity holding them in orbit. A common error is to forget that gravity provides the force for orbits.
WJEC style4 marksExplain how a star such as the Sun forms and why it is stable for billions of years.Show worked answer →
A Unit 6 explain question worth 4 marks. Reward: a star forms when a cloud of dust and gas (nebula) is pulled together by gravity (1); the material gets hotter and denser until nuclear fusion begins, joining hydrogen nuclei (1); the star is stable because the outward pressure from fusion balances the inward pull of gravity (2). Markers credit gravity pulling the cloud together, fusion starting, and the balance of forces for stability. A common error is to say the star is stable because it is very big.
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