Our Solar SystemAQA GCSE Physics: Revision notes
Section 1
What makes up our Solar System?
Our Solar System consists of:
- one star — the Sun
- eight planets orbiting the Sun
- dwarf planets (smaller bodies that orbit the Sun but have not cleared their orbital path)
- natural satellites (moons) orbiting planets
Our Solar System is itself only a small part of a much larger galaxy, the Milky Way, which contains billions of stars.
Distinguish 'natural satellite' (a moon) from an 'artificial satellite' (human-made, e.g. for communications) — both can orbit a planet, but only the natural one is a moon.
Section 2
How did the Sun form?
The Sun formed from a nebula — a huge cloud of dust and gas — pulled together by gravitational attraction.
As the cloud of matter contracted under gravity, it heated up. At the start of the Sun's life, this heating caused fusion reactions to begin, which release enormous amounts of energy.
Once fusion starts, the Sun reaches an equilibrium: the inward pull of gravity (trying to collapse the star) is balanced by the outward push of energy released by fusion reactions (trying to expand the star). This balance is what keeps a star a stable size for most of its life.
Think of a star like a tug of war: gravity pulls matter inward, fusion energy pushes it outward — as long as neither side wins, the star stays a stable size.
Section 3
What keeps planets and satellites in orbit?
Gravity provides the force that keeps planets, moons and artificial satellites moving in (roughly) circular orbits.
- Without gravity, an orbiting object would simply travel off in a straight line
- Gravity constantly pulls the object towards the centre of its orbit, changing its direction of travel
Comparing objects in orbit:
| Object | Orbits | Natural or artificial? |
|---|---|---|
| Planet | The Sun | Natural |
| Moon | A planet | Natural |
| Artificial satellite | A planet (usually Earth) | Artificial (human-made) |
All three types share the same underlying physics: gravity provides the centripetal force needed for a circular orbit.
Section 4
Why does an object in a stable circular orbit have constant speed but changing velocity? (HT)
For a genuinely circular orbit, the gravitational force acts towards the centre of the orbit at all times.
- This force does not speed up or slow down the object, because it acts at right angles to the direction of travel — so the speed stays constant
- However, the force continuously changes the direction of motion — and since velocity is a vector (speed and direction), the velocity is always changing
- If the object's speed changes (for example, due to an external factor), the radius of the orbit must also change to keep the orbit stable — a faster orbiting object needs a larger radius to remain balanced by the same gravitational pull, and vice versa
A common error is saying a satellite in a circular orbit has 'no force acting on it' because its speed doesn't change — in fact a resultant force (gravity) is always acting, it's just always directed towards the centre, so it changes direction, not speed.
Must Know
- Our Solar System = one star (the Sun) + eight planets + dwarf planets + natural satellites (moons); it is part of the Milky Way galaxy
- The Sun formed from a nebula (cloud of dust and gas) pulled together by gravity
- Fusion reactions in a star create an equilibrium between gravitational collapse and the outward push of fusion energy
- Gravity provides the force that keeps planets, moons and artificial satellites in circular orbits
- (HT) In a stable circular orbit, gravity changes the object's velocity (direction) but not its speed
- (HT) If the orbital speed changes, the orbital radius must change too, to keep the orbit stable
That's the notes covered.
Carry on to the next subtopic.