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The Solar System (Edexcel GCSE Physics)

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Objects in the Solar System

  • The Solar System consists of:

    • The Sun

    • Eight planets

    • Natural and artificial satellites

    • Dwarf planets

    • Asteroids and comets

The Solar System, downloadable IGCSE & GCSE Physics revision notes

The objects in our solar system

The Sun & the Planets

  • The Sun lies at the centre of the Solar System

    • The Sun is a star that makes up over 99% of the mass of the solar system

  • There are 8 planets and an unknown number of dwarf planets which orbit the Sun

    • The gravitational field around planets is strong enough to have pulled in all nearby objects with the exception of natural satellites

    • The gravitational field around a dwarf planet is not strong enough to have pulled in nearby objects

Satellites

  • There are two types of satellite:

    • Natural

    • Artificial

  • Some planets have moons which orbit them

    • Moons are an example of natural satellites

  • Artificial satellites are man-made and can orbit any object in space

    • The International Space Station (ISS) orbits the Earth and is an example of an artificial satellite

Asteroids & Comets

  • Asteroids and comets also orbit the sun

  • An asteroid is a small rocky object which orbits the Sun

    • The asteroid belt lies between Mars and Jupiter

  • Comets are made of dust and ice and orbit the Sun in a different orbit to those of planets

    • The ice melts when the comet approaches the Sun and forms the comet’s tail

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The Planets

  • The 8 planets in our Solar System in ascending order of the distance from the Sun are:

    • Mercury

    • Venus

    • Earth

    • Mars

    • Jupiter

    • Saturn

    • Uranus

    • Neptune

  • There are 4 rocky planets: Mercury, Venus, Earth and Mars

  • There are 4 gas planets: Jupiter, Saturn, Uranus and Neptune

planets-of-our-solar-system, IGCSE & GCSE Physics revision notes

The eight planets of our Solar System

Distances between Planets in the Solar System

  • The planets and moons of the Solar System are visible from Earth when they reflect light from the Sun

    • The outer regions of the Solar System are around 5 × 1012 m from the Sun, which means even light takes some time to travel these distances

  • The light we receive on Earth from the Sun takes 8 minutes to reach us

    • The nearest star to us after the Sun is so far away that light from it takes 4 years to reach us

    • The Milky Way galaxy contains billions of stars, huge distances away, with the light taking even longer to be seen from Earth

  • The speed of light is a constant 3 × 108 m/s

    • Therefore, using the equation:

speed space equals space distance over time

  • The time taken to travel a certain distance can be calculated by rearranging to:

time space equals space distance over speed

Worked Example

The radius of Mercury's orbit around the Sun is 5.8 × 109 m.

Calculate the time taken for light from the Sun to reach Mercury.

Answer:

Step 1: State the equation for the time taken for light to travel a certain distance

time space equals space distance over speed

Step 2: Substitute in the values

  • The distance travelled is the radius of the orbit

    • Distance, d = 5.8 × 109 m.

  • Speed = the speed of light, v = 3.0 × 108 m/s

time space equals space fraction numerator 5.8 space cross times space 10 to the power of 9 over denominator 3.0 space cross times space 10 to the power of 8 end fraction space equals space 1.933333

Step 3: Round up the answer and include units

time space equals space 19.3 space straight s space

Examiner Tips and Tricks

You need to know the order of the 8 planets in the solar system. The following mnemonic gives the first letter of each of the planets to help you recall them:

My Very Easy Method Just Speeds Up Naming (Planets)

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, (Pluto)

Of course, Pluto has been classed as a dwarf planet since 2006, so you shouldn't include it as one of the 8 planets, but old mnemonics still include it so that the mnemonic makes sense!

The Creation of the Solar System

  • The way humans have thought about the solar system has changed many times throughout history

  • Before the development of the telescope, these ideas were based on what could be seen with the naked eye

  • This restricted the details that could be gathered about the solar system, as asteroids, most of the satellites of other planets, and the most distant planets are not visible to the naked eye

Ancient Model of the Solar System

  • An Egyptian astronomer called Ptolemy (AD 100-168) described one of the earliest ideas for how the solar system is structured

  • Ptolemy’s model and many earlier ideas from scholars of the solar system had the Earth at the centre of it

  • Over time, though, it became apparent that the old model did not quite fit with observation due to more detailed observations of the motions of the planets

    • This was called the geocentric model of the solar system

Modern Model of the Solar System

  • Placing the Sun at the centre of the solar system and having the Earth and other planets orbit it gave a much more accurate explanation of astronomical observations

  • Nicolas Copernicus (1473-1543) first suggested changing the model of the solar system to one with the Sun at the centre

  • Detailed observations with telescopes have given us evidence that this is the correct idea

    • This is now the accepted heliocentric model of the solar system

Geocentric sv Heliocentric, downloadable IGCSE & GCSE Physics revision notes

The geocentric model vs. the heliocentric model

Worked Example

A long time ago, astronomers thought that the Earth was the centre of the Universe. This was called the geocentric model.The evidence for this model came from observations of the sky using the naked eye. After the telescope was invented, astronomers quickly gathered evidence that showed that the geocentric model is not correct. Describe the evidence both for the geocentric model and against the geocentric model.

Answer:

Step 1: Describe the evidence supporting the geocentric model

  • When observing the Sun, Moon, stars or planets with the naked eye, they appear to be moving across the sky and not just orbiting the Sun

  • Instead, they seem to appear to be going around the Earth

  • From simple observation, objects appear to move in the same direction and in a predictable pattern of movement which is the same each day

Step 2: Describe the evidence against the geocentric model

  • The moons of other planets (such as Jupiter) can clearly be seen to be orbiting the other planet and not the Earth

  • Careful observations of the Sun (and other objects) show their movements are, in fact, not quite the same each day

  • Detailed telescope observations show the planets of the Solar System do not move in a simple orbital path around the Earth

  • This is evidenced by the retrograde motion of planets, which can only be explained by a heliocentric model

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    Ashika

    Author: Ashika

    Expertise: Physics Project Lead

    Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.