Solar System & Orbital MotionOxford AQA IGCSE Physics: Revision notes

The Solar System is centred on the Sun, a medium-sized star.

• Earth is one of eight planets that orbit the Sun
• Other objects in the Solar System include moons (which orbit planets), asteroids (rocky bodies, mostly found in the asteroid belt), dwarf planets (smaller than planets but still orbit the Sun directly, e.g. Pluto), and comets (icy bodies with highly elliptical orbits)

These objects differ in relative size and the way they move — planets follow roughly circular orbits around the Sun, moons orbit planets, and comets follow long, stretched-out (elliptical) orbits.

The universe is far larger than just the Solar System.

• The universe contains thousands of millions of galaxies
• Each galaxy contains thousands of millions of stars
• Our Sun is just one of thousands of millions of stars within the Milky Way galaxy

This scale means the Solar System is an extremely tiny part of the universe as a whole.

Gravity provides the centripetal force that keeps planets and satellites (both natural, like the Moon, and artificial, like satellites) moving in orbit.

• This centripetal force acts towards the centre of the orbit
• It continuously changes the direction of the orbiting object's velocity
• It does not change the object's speed

This is why an object can orbit at a constant speed while its direction is constantly changing — the force is always at right angles to the direction of motion.

The centripetal force due to gravity decreases as separation (distance) increases.

• This means objects orbiting further away from the object they orbit experience a weaker gravitational pull
• A weaker centripetal force results in a lower orbital speed
• So planets or satellites further from the Sun/Earth orbit more slowly than those closer in

At any particular separation (orbital radius), there is one particular orbital speed that keeps an object in a stable circular orbit at that radius.

• If the orbital speed changes, the orbital radius must also change to keep the orbit stable
• This principle is applied in the real world when engineers design where to place satellites — for example, choosing a specific orbital radius to give a satellite the orbital period needed for its purpose (such as geostationary satellites, which orbit at the same rate as the Earth spins)