The Solar System
Introduced| English | Meaning |
|---|---|
| Solar System/ˈsəʊlə ˈsɪstəm/ | The Sun together with the planets, dwarf planets and smaller bodies orbiting it. |
| planets/ˈplænɪts/ | planets |
| asteroids/ˈæstərɔɪdz/ | asteroids |
| comets/ˈkɒmɪts/ | comets |
| rocky/ˈrɒki/ | Describes the inner planets, which have solid surfaces, unlike the gas giants. |
| gas giants/ɡæs ˈdʒaɪənts/ | gas giants |
| satellites/ˈsætəlaɪts/ | An object that orbits a larger body under gravity, whether natural or artificial. |
| ellipses/ɪˈlɪpsɪz/ | ellipses |
| accretion/əˈkreʃn/ | The gradual gathering of matter by gravity into a larger body. |
The Sun's family
- At the centre of the Solar System 太阳系 is one star — the Sun.
- Around it move eight planets 行星, plus moons, asteroids 小行星 and comets 彗星.
- Gravity holds the whole family together.
The eight planets
- In order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
- The four inner planets are small and rocky 岩石.
- The four outer planets are large and gaseous (no solid surface).
- Jupiter and Saturn are gas giants 气态巨行星; Uranus and Neptune are ice giants.
Jupiter, the largest planet, with its swirling bands of gas
Watch the planets race
The closer a planet is to the Sun, the faster it orbits — Mercury in 88 days, Jupiter in 12 years — because the Sun's gravity is stronger close in.
Put these planets in order, nearest the Sun first.
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From the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune — so of these, Mercury, Earth, Jupiter, Neptune.
How do the inner and outer planets differ?
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The four inner planets are small and rocky; the four outer planets are large and gaseous (gas giants and ice giants).
Other objects
- Dwarf planets (like Pluto) and asteroids — most asteroids lie in a belt between Mars and Jupiter.
- Moons (natural satellites 卫星) that orbit the planets.
- Comets — small icy bodies on very stretched orbits.
A giant telescope fires a laser to study the night sky
Orbits and gravity
- The Sun's gravity reaches out and keeps the planets in orbit; it is weaker further out, so outer planets orbit more slowly.
- Most orbits are ellipses 椭圆 (stretched circles), with the Sun at one focus (not the centre).
- A planet moves faster when closer to the Sun: as its gravitational PE falls, its kinetic energy rises (energy is conserved).
What keeps the planets in their orbits around the Sun?
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The Sun holds most of the mass of the Solar System, and its gravity pulls the planets into orbit.
A planet on an elliptical orbit moves fastest when it is:
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As the planet comes closer, its gravitational PE falls and its kinetic energy rises (energy is conserved), so it speeds up.
Light travel time and how it formed
- Space is huge, so we use light travel time: $\text{time} = \dfrac{\text{distance}}{\text{speed}}$, with light at $3.0 \times 10^8\ \text{m/s}$.
- Sunlight takes about 8 minutes to reach Earth.
- The Solar System formed by accretion 吸积: gravity pulled a spinning cloud of gas and dust into a disc; most fell to the centre to form the Sun, and the planets grew from the rest.
- Near the hot young Sun only rock and metal stayed solid → rocky inner planets; far out, gases collected too → giant outer planets.
The Solar System
The Sun is about $1.5 \times 10^{11}\ \text{m}$ away. How long does its light take to reach Earth, in seconds? (light speed $3.0 \times 10^8\ \text{m/s}$)
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$\text{time} = \dfrac{d}{v} = \dfrac{1.5 \times 10^{11}}{3.0 \times 10^8} = 500\ \text{s}$ (about 8 minutes).
Why are the inner planets rocky?
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Close to the hot Sun, gases could not condense — only rock and metal stayed solid, so the inner planets are rocky.
You've got it
- eight planets: four rocky inner, four gaseous outer (gas giants + ice giants)
- the Sun's gravity holds the planets; outer ones orbit slower
- orbits are ellipses with the Sun at a focus; planets move faster when closer
- light from the Sun takes ~8 min; the system formed by accretion of gas and dust