The Solar System and Orbital Motion

This whole topic (4.8 Space physics) is Separate Physics only. It is not assessed in Combined Science: Trilogy.

Our solar system has a single star at its centre: the Sun. Orbiting the Sun are the eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), a number of dwarf planets (such as Pluto and Ceres), and huge numbers of smaller bodies such as asteroids and comets.

Many planets are themselves orbited by moons. A moon is a natural satellite: a natural body that orbits a planet. Earth has one moon; Jupiter and Saturn have dozens each.

A planet orbits a star. A moon (natural satellite) orbits a planet.

The solar system is only a tiny part of a much larger structure. The Sun is one of many billions of stars in the Milky Way, our galaxy. A galaxy is a vast collection of stars held together by gravity, and the universe contains billions of galaxies.

Body	What it orbits	Example
Star	Centre of the galaxy	The Sun
Planet	A star	Earth, Jupiter
Dwarf planet	A star	Pluto, Ceres
Moon (natural satellite)	A planet	The Moon (Earth's), Titan (Saturn's)

Stars, including our Sun, form from a nebula: a cloud of dust and gas in space.

The story of the Sun's birth is a story about gravity. Every particle of gas and dust in the nebula pulls on every other particle. Over a very long time, gravity draws the material together, and the cloud begins to collapse inwards on itself.

As the cloud shrinks, the material becomes more and more concentrated at the centre. Squeezing the gas into a smaller space makes it hotter and denser. Eventually the centre becomes so hot and so dense that the nuclei of hydrogen atoms begin to join together. This process is called nuclear fusion.

The Sun formed from a nebula (a cloud of dust and gas) that was pulled together by gravity until it became hot and dense enough for nuclear fusion to begin.

Fusion releases an enormous amount of energy. Once fusion starts at the core, the object becomes a true star, and that energy is what makes the Sun shine and gives out heat and light.

A star like the Sun is being pulled inwards by its own gravity all the time. If gravity were the only force acting, the star would keep collapsing. It does not, because fusion at the core pushes back.

The energy released by fusion heats the core and creates an outward force (an outward pressure) that resists the inward pull of gravity.

A stable star is in equilibrium: the inward force of gravity is balanced by the outward force from the energy released by fusion, so the star stays the same size.

This balance lasts for a very long time. For a star the size of the Sun, this stable period (called the main sequence) lasts for billions of years. The star only changes when it starts to run out of the hydrogen fuel that powers fusion, which upsets the balance and drives the later stages of its life cycle.

Think of it as a tug-of-war that stays level: gravity pulling in, fusion energy pushing out, neither side winning while fuel lasts.

The same force that formed the Sun also holds the solar system together. Gravity provides the force that keeps planets and satellites in their orbits.

Each planet is constantly trying to travel in a straight line. The Sun's gravity pulls it towards the Sun, and the combination of the two makes the planet follow a curved path: a roughly circular orbit. Without gravity, a planet would simply fly off in a straight line into space.

The same idea applies at every scale:

• Gravity from the Sun keeps the planets in orbit around it.
• Gravity from a planet keeps its moons in orbit around it.
• Gravity from a planet keeps artificial satellites in orbit around it.

Gravity is the force that keeps planets, moons and artificial satellites in their (roughly circular) orbits.

The exam often asks you to describe similarities and differences between planets, moons and artificial satellites, so it helps to set them side by side.

Similarities:

• All three are held in their orbits by gravity.
• All three follow roughly circular orbits.

Differences:

• A planet orbits a star; a moon and an artificial satellite orbit a planet.
• Planets and moons are natural; an artificial satellite is human-made and launched from Earth.

Feature	Planet	Moon (natural satellite)	Artificial satellite
Orbits	A star	A planet	A planet
Natural or human-made	Natural	Natural	Human-made
Held in orbit by	Gravity	Gravity	Gravity
Example	Earth	The Moon	Communications or GPS satellite

Artificial satellites are used for jobs such as communications, weather monitoring, navigation (GPS) and observing space or the Earth.

(Higher Tier only) The following explanation of circular orbits is required for Higher Tier candidates only.

In a circular orbit, gravity always pulls the orbiting object towards the centre of the orbit, which is at right angles (90°) to the direction it is moving.

Because the force acts sideways to the motion and not along it, gravity does not speed the object up or slow it down. Instead it constantly changes the direction of travel.

Remember that velocity is speed in a given direction (a vector), while speed is just how fast the object moves (a scalar). Changing direction changes the velocity even if the speed stays the same.

(Higher Tier) For a circular orbit, the force of gravity changes the velocity (by changing direction) without changing the speed.

There is also a link between orbital speed and orbital radius for a stable orbit:

(Higher Tier) For a stable orbit, if the speed of the object changes, the radius of the orbit must change too. A faster stable orbit has a smaller radius; a slower stable orbit has a larger radius.

So an object cannot simply speed up and stay in the same orbit. To orbit stably at a higher speed it must move to a lower (smaller-radius) orbit. This is why satellites in low orbits move faster than those in high orbits.

1. Saying planets orbit because there is no force on them

The opposite is true. A planet stays in orbit because gravity acts on it. Gravity provides the force that bends its path into a curve. With no force it would fly off in a straight line.

2. Confusing what orbits what

Planets orbit the Sun (a star). Moons and artificial satellites orbit planets. Do not say a satellite orbits the Sun in a GCSE answer unless the question is specifically about a solar-orbiting probe.

3. Forgetting that a nebula is dust and gas

A nebula is a cloud of dust and gas, pulled together by gravity. Writing only "gas" or only "dust" is incomplete.

4. Not stating both parts of the stability balance

A stable star needs both forces named: gravity pulling inwards and the energy from fusion pushing outwards. State that they are balanced (in equilibrium).

5. Claiming gravity changes an orbiting object's speed (Higher Tier)

In a circular orbit gravity acts at right angles to the motion, so it changes velocity (direction) but not speed. Only say gravity changes the speed if the orbit is not circular.