IGCSE §8: Astrophysics · IGCSE
The Solar System
Revision notes on The Solar System for the OCR A-level Physics specification (H556). Free to read, with 4 practice questions in the app.
The Sun is a star at the centre of the solar system, and it holds almost all of the system's mass. Everything else orbits it because of the Sun's gravity.
What the solar system contains:
Planets — eight of them, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The four inner ones are small and rocky; the four outer ones are large and gaseous.
Dwarf planets — bodies such as Pluto, which orbit the Sun but have not cleared other objects from their orbital path.
Moons (natural satellites) — bodies that orbit a planet rather than the Sun directly. Earth has one; Jupiter and Saturn have dozens.
Asteroids — rocky bodies, most of them in the asteroid belt between the orbits of Mars and Jupiter.
Comets — bodies of ice and dust in highly elongated orbits, which develop a tail of gas and dust when they approach the Sun and are heated.
Satellite — anything in orbit around a larger body. A natural satellite is a moon; an artificial satellite is one we have put there.
Orbit shapes:
- Planets and moons follow almost circular orbits — slightly elliptical, but close to circular.
- Comets follow highly elliptical orbits, coming very close to the Sun at one end and travelling far beyond the outer planets at the other.
Why a comet's speed changes so dramatically. As it falls towards the Sun, energy is transferred from its gravitational potential store to its kinetic store, so it speeds up. It moves fastest when closest to the Sun and slowest when furthest away. On the outward leg the transfer runs the other way and it slows down again.
This is the same physics as a ball thrown upwards, on an enormous scale. It also explains why comets spend most of their orbital period far from the Sun and are visible only briefly — they race through the inner part of the orbit and crawl through the outer part.
Example: Halley's Comet takes about 76 years to complete one orbit, but is visible from Earth for only a few months of it. The rest is spent in the slow, distant stretch beyond Neptune.
4 questions on The Solar System
Reading is not revising. This topic has 4 questions in the app — multiple choice, calculations, and written answers marked against the mark scheme, point by point. Miss one and it comes back later with its options reshuffled, so you have to know it rather than remember where the answer sat.
- Every topic of Edexcel International GCSE Physics (4PH1)
- Flashcards and every equation, linked to the topic it comes from
- Full mock papers in the real paper structure
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