P6.1.2P6. Space Physics

The Solar System — Combined Science 0653 - Physics Revision Notes & Topical Questions

5 min read 12 parts Lesson 3 of 6 1 quiz

📚 Combined Science 0653 - Physics Hub / Topical Revision

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Lesson summary

P6.1.2 The Solar System — COMBINED SCIENCE notes: Describe the bodies in the Solar System, how it formed, and how gravity controls orbits.Contents — one star (the Sun), eight planets, dwarf planets, natural and artificial satellites, asteroids and cometsPlanets in order from the …

Planets in order from the Sun
Natural satellite
Inner planets
Outer planets
Planet mass
Value of g

Part 1 of 10Members of the Solar System

Members of the Solar System

Contents

Contents one star (the Sun), eight planets, dwarf planets, natural and artificial satellites, asteroids and comets

Planets in order from the Sun

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
The four inner planets are small and rocky.
The four outer planets are large gas giants.

Dwarf planet

Dwarf planet an object like a planet but much smaller

Mass

Mass the Sun holds more than 99% of the mass of the Solar System, so its gravity dominates

Part 2 of 10Satellites, asteroids and comets

Natural satellite

Natural satellite an object that orbits a planet, and a moon is one type

Artificial satellite

Artificial satellite a man-made object that orbits another body, for example the International Space Station

Asteroid

Asteroid a small rocky object that orbits the Sun, mostly in the belt between Mars and Jupiter

Comet

Comet an object of dust and ice in a highly elliptical orbit around the Sun, found mostly in the outer Solar System
The ice melts near the Sun and forms the tail.

Part 3 of 10What orbits what

Orbiting body	Body it orbits
Planet	The Sun
Moon	A planet
Asteroid	The Sun
Artificial satellite	The Earth

Part 4 of 10Formation of the Solar System

Formation of the Solar System

Origin

Origin the Sun and planets formed about 4.5 billion years ago from a nebula, a cloud of gas and dust
The nebula contained elements made in an earlier supernova.

Accretion

Accretion attractive gravitational forces joined particles into larger objects
As the cloud collapsed it spun faster, became hotter and formed an accretion disc.
The Sun formed at the centre and the planets formed in the disc.

Part 5 of 10Inner and outer planets

Inner planets

Inner planets formed in the hot region near the Sun
Only materials with high melting points, such as metals, could be solid.
The nebula held few heavy elements, so these planets stayed small and rocky.
Mercury, Venus, Earth and Mars are rocky and small.

Outer planets

Outer planets formed in the cooler region further out
Low-melting-point materials such as hydrogen, helium, water and methane could be solid there.
There was far more of this material, so the planets grew very large.
Jupiter, Saturn, Uranus and Neptune are gaseous and large.

Part 6 of 10Gravitational field strength

Planet mass

Planet mass the greater the mass, the greater the gravitational field strength at its surface

Distance

Distance the field strength decreases as the distance from the planet increases

Value of g

Value of g it differs from planet to planet because of their different masses and radii

Part 7 of 10Orbits around the Sun

Gravitational attraction

Gravitational attraction the force always acts towards the centre of the Sun and keeps planets in orbit
It causes the orbiting body to move in a circular path.

Distance from the Sun

Distance from the Sun as it increases, the Sun's field strength and the planet's orbital speed decrease
Planets further out have longer orbital periods.
The Sun's gravitational attraction acts as the centripetal force.

Orbital speed

Orbital speed v = 2πr / T, using the radius from the centre of the Sun and the period in seconds

Part 8 of 10Elliptical orbits

Planets

Planets slightly elliptical orbits that are almost circular

Comets

Comets highly elliptical orbits with the Sun at one focus of the ellipse
A comet speeds up as it approaches the Sun and slows down as it moves away.
Gravitational potential energy changes to kinetic energy on approach and back again on the way out.

Part 9 of 10Light travel in the Solar System

Speed of light
- 3.0 × 10⁸ m/s, so time = distance / speed of light
- Light takes about 8 minutes to reach the Earth from the Sun.
- Light takes about 5 hours to reach the outer Solar System.
- The nearest star after the Sun is about 4 light-years away.

Part 10 of 10More from the specification

Hemisphere
- Half of the Earth, northern or southern.

Key terms

Term	Meaning
Solar System	The Sun and the objects that orbit it, including planets, dwarf planets, moons, asteroids and comets.
Nebula	A cloud of gas and dust from which the Sun and planets formed.
Accretion	The joining of particles into larger objects by gravitational attraction.
Asteroid	A small rocky body orbiting the Sun, mostly in the belt between Mars and Jupiter.
Comet	A body of dust and ice in a highly elliptical orbit around the Sun.
Gravitational field strength	The force per unit mass on an object in a gravitational field, which depends on the planet's mass and radius.
Orbital period	The time for an object to complete one orbit.

Avoid these mistakes

Saying the Sun's gravity instead of the Sun's gravitational attraction — write the force of gravitational attraction

Saying outer planets move faster — the further a planet is from the Sun, the slower it moves

Saying the inner planets are made of gas — the inner four are rocky and the outer four are gaseous

Thinking comets orbit with the Sun at the centre — the Sun is at one focus of a highly elliptical orbit

Last updated: 14 October 2026