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The Solar System (AQA GCSE Physics)
Revision Note
Objects in The Solar System
- The Sun lies at the centre of the Solar System
- The Sun is a star which 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
- There are 4 rocky planets: Mercury, Venus, Earth and Mars
- There are 4 gas planets: Jupiter, Saturn, Uranus and Neptune
- 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 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
The objects in our solar system
Examiner Tip
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 Excellent Mother Just Served Us Noodles
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Before Pluto was reclassified as a dwarf planet in 2006, 'my very excellent mother' used to 'serve us' 'Nine Pizzas' and you can still use that one to remember the 'old' solar system!
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Our Place in Space
- Our solar system is just one small part of the Milky Way galaxy
- There are billions of stars in the Milky Way galaxy
- Some of these stars also have planets which orbit them
- The Universe is made up of many different galaxies
Our solar system is just one out of potentially billions in our galactic neighbourhood, the Milky Way. There are estimated to be more than 100 billion galaxies in the entire universe
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:
-
- The time taken to travel a certain distance can be calculated by rearranging to:
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.
Step 1: State the equation for the time taken for light to travel a certain distance
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
- The distance travelled is the radius of the orbit
Step 3: Round up the answer and include units
Examiner Tip
The speed of light is very fast. This is why in our everyday life things like switching on a light seem to be instant. However, this is only because the light travels very fast and the distances are very small. In large, astronomical distances which can be millions or even billions of kilometres, the limit of the speed of light starts to have an effect.
For example, it takes light 8 minutes to travel from the Sun to the Earth. This means we are seeing the Sun as it was eight minutes ago. If the Sun was to disappear, we would not notice till eight minutes later. Although, by that time, time delay would be the least of our worries...
p.s.: The Sun is not going to vanish!
This is one of my all time favourite lessons to teach because students are so interested in space and have so many questions. It gives us the opportunity to discuss the topics that come up in A Level Physics such as Cosmology and Classification of Stars. It also gives us the opportunity to discuss careers in space related areas like:
- Astrobiology - the search for life beyond our planet
- Astrophysics - the study of objects in the Universe like galaxies and stars
- Astrochemistry - the study of elements and chemical reactions in space
- Space medicine - the study of the effects of being in space on the human body
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