The Solar System (Edexcel GCSE Physics)

• The Solar System consists of:
  • The Sun
  • Eight planets
  • Natural and artificial satellites
  • Dwarf planets
  • Asteroids and comets

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

• 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

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:

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

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

Step 3: Round up the answer and include units

• 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

The geocentric model vs. the heliocentric model

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