What the Solar System contains
Definition
Solar System
The Solar System is the Sun together with the eight planets, their natural satellites, the dwarf planets, the asteroids and the comets that orbit it.
Definition
Star
A star is a large ball of hot gas held together by its own gravity that releases energy by nuclear fusion in its core.
Definition
Planet
A planet is a large body orbiting a star that is massive enough for gravity to pull it into a nearly spherical shape and that has cleared other objects from its orbital path.
- The Sun sits at the centre of the Solar System and is the only star in it, which is why it is described as our star.
- The Sun holds far more mass than everything else in the Solar System put together, so its gravitational field controls the motion of every other member.
- There are eight planets in orbit around the Sun, and many of them have their own natural satellites in orbit around them.
- Alongside the planets the Solar System also contains dwarf planets, asteroids and comets.
- A star releases its own energy by nuclear fusion and so gives out light, while a planet has no fusion of its own and is seen only by the sunlight it reflects.
- The Solar System is one small part of a much larger structure, since the Sun is only one of the huge number of stars that make up the Milky Way galaxy.
The eight planets in order
- Counting outwards from the Sun the order is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
- A sentence such as My Very Easy Method Just Speeds Up Names fixes the order, because the first letters follow the eight planets exactly.
- The four nearest the Sun, Mercury, Venus, Earth and Mars, are small and rocky and are called the inner planets.
- The four furthest out, Jupiter, Saturn, Uranus and Neptune, are much larger and made mostly of gas and ice, and are called the outer planets.
- The spacing is very uneven, with Mercury about 585858 million km\text{km}km from the Sun, the Earth about 150150150 million km\text{km}km and Neptune about 450045004500 million km\text{km}km.
- Planets further from the Sun take longer to complete one orbit, so a year on Neptune lasts about 165165165 Earth years while a year on Mercury lasts about 888888 Earth days.
- Planets further from the Sun also receive less energy from it each second over each square metre of surface, so they are colder.
Moons and other smaller bodies
Definition
Natural satellite
A natural satellite is a naturally formed body, such as a moon, that orbits a planet.
Definition
Dwarf planet
A dwarf planet is a nearly spherical body that orbits the Sun but has not cleared other objects from its orbital path.
Definition
Asteroid
An asteroid is a small rocky body that orbits the Sun, most of which are found in a belt between the orbits of Mars and Jupiter.
Definition
Comet
A comet is a small body of ice, dust and rock that orbits the Sun along a highly elliptical path.
- A natural satellite orbits a planet rather than the Sun, and our own Moon is the standard example to quote.
- Mercury and Venus have no natural satellites at all, whereas Jupiter and Saturn each have dozens of them.
- A dwarf planet such as Pluto or Ceres is round and orbits the Sun, but it shares its region of space with other sizeable objects, which is why it is not counted as one of the eight planets.
- Most asteroids lie in the asteroid belt between the orbits of Mars and Jupiter, and they are irregular lumps of rock and metal rather than spheres.
- A comet is mostly ice and dust, and heating by the Sun turns some of that ice to gas, producing the bright tail that is seen when a comet passes close to the Sun.
- Every one of these bodies is held in place by the gravitational attraction of the Sun, apart from the natural satellites, which are held by the planet they circle.
Example
Time for sunlight to reach Earth
- The Earth orbits at about 1.5×1011 m1.5 \times 10^{11}\ \text{m}1.5×1011 m from the Sun and light travels at 3.0×108 m/s3.0 \times 10^{8}\ \text{m/s}3.0×108 m/s.
- Rearranging v=xtv = \dfrac{x}{t}v=tx gives t=xvt = \dfrac{x}{v}t=vx.
- Substituting gives t=1.5×10113.0×108t = \dfrac{1.5 \times 10^{11}}{3.0 \times 10^{8}}t=3.0×1081.5×1011.
- The time taken is 500 s500\ \text{s}500 s, which is about 888 minutes and 202020 seconds.
Example
Scaling the Solar System down
- A model places the Earth 1.0 m1.0\ \text{m}1.0 m from the Sun, and the position of Neptune on the same scale is wanted.
- The real distances are 150150150 million km\text{km}km for the Earth and 450045004500 million km\text{km}km for Neptune.
- The ratio of the two distances is 4500150=30\dfrac{4500}{150} = 301504500=30.
- Neptune therefore belongs 30 m30\ \text{m}30 m from the model Sun, which shows how thinly the outer Solar System is spread.
Exam technique
Naming and ordering the members
- Learn the eight planets as an ordered list rather than a set, because questions often supply four names and ask for the correct sequence outwards.
- Use the exact word the question asks for, since a natural satellite, a dwarf planet and an asteroid are three separate categories and each earns its own mark.
- Give a named example when a question asks you to identify a type of body, such as Ceres for a dwarf planet or the Moon for a natural satellite.
- Watch for numbers written in standard form in data questions, and convert kilometres to metres before using any equation.
Common Mistake
- Do not count Pluto as one of the eight planets, because it is classed as a dwarf planet.
- Do not call the Sun a planet, since it is the star at the centre of the Solar System.
- Do not confuse a natural satellite with an artificial one, because a natural satellite such as the Moon was not built and launched by people.
- Do not treat the Solar System and the Universe as the same thing, since the Solar System is one star and the bodies around it.
Self review
- List the eight planets in order of increasing distance from the Sun.
- Name four types of body other than planets that belong to the Solar System.
- State one difference between a planet and a dwarf planet.
- Explain why a star is visible from a great distance but a planet is not.
- State where most asteroids in the Solar System are found.
