PHYSICS · Stage 7 · Chapter 5: The Earth in Space
Physics 5.4 Our Solar System — Rocky and Gas Giant Planets
5 min read 4 parts Lesson 4 of 11
Lesson summary
5.4 Our Solar System — Rocky and Gas Giant Planets — Alhussan International Academy — Physics (Stages 7–9) notes: Describe what is in the Solar System, how it formed, and why objects orbit the Sun.Solar system = planets (and other bodies) that orbit a star.We live on a planet tha…
- Dwarf planets
- Order from the Sun
- Seeing them
- Orbit times
- Composition
- Two-way pull
Part 1 of 4What is a solar system
- Solar system = planets (and other bodies) that orbit a star.
- We live on a planet that orbits a star called the Sun.
- Structure — our Solar System contains four inner planets, an asteroid belt and four outer planets.
- Planets — objects that have cleared their orbits of dust, gas and rock.
- Dwarf planets — they are in the asteroid belt and beyond the outer planets.
- Objects in the asteroid belt orbit the Sun but have not cleared their orbit, so they are not planets.
- Until 2006 Pluto was called a planet, but astronomers found other objects like Pluto beyond Neptune, so they called Pluto a dwarf planet.
- Order from the Sun — Mercury, Venus, Earth, Mars, then the asteroid belt, then Jupiter, Saturn, Uranus and Neptune.
- Seeing them — some planets can be seen with the naked eye, but other objects orbiting the Sun need a telescope.
Part 2 of 4Inner and outer planets
| Feature | Inner planets | Outer planets |
|---|---|---|
| Planets | Mercury, Venus, Earth, Mars | Jupiter, Saturn, Uranus, Neptune |
| Made of | Mostly rock | Mostly gas (hydrogen, helium) and ice |
| Size | Small | Large |
| Rings | No | Yes, made of rock, dust and ice |
| Moons | Mercury 0, Venus 0, Earth 1, Mars 2 | Jupiter 79, Saturn 82, Uranus 27, Neptune 14 |
| Shine by | Reflected light | Reflected light |
| Life | Only on Earth | No |
- Mercurythe smallest planet and closest to the Sun.
- Venusit has a very thick atmosphere and is the hottest planet.
- Earththe only planet known to have life.
- Marscold and rocky.
- Orbit timesthe outer planets take far longer to orbit the Sun because they are further away and travel a longer path.
Part 3 of 4How the Solar System formed
- Origin — it formed about 4.6 billion years ago (roughly 5 billion) from a swirling cloud of dust and gas.
- Gravity — it pulled the dust and gas together to make the Sun at the centre and all the planets.
- Some of the material formed rings around the outer planets.
- Composition — planets that formed near the hot young Sun were rocky, while those further out collected large amounts of gas.
Part 4 of 4Why objects orbit the Sun
1Gravity1 point
= a force that acts between all objects with mass.
- There is a force of gravity between all the planets and the Sun, and this force keeps the planets orbiting the Sun.
2Two-way pull1 point
— the Sun pulls on the Earth and the Earth pulls on the Sun with the same size of force.
- The mass of the Sun is much larger than the mass of the Earth, so the Earth is continuously pulled in, which keeps it in orbit.
3Other objects1 point
— asteroids and comets also orbit the Sun because of the force of gravity.
- The orbits of Pluto and comets are different from the orbits of the planets.
4If gravity was switched off
— the planets would fly off into space.
5If gravity was much stronger
— objects would be pulled much closer to the Sun.
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