The Solar System
The Sun's family
- At the centre of the Solar System is one star — the Sun.
- Around it move eight planets, plus moons, asteroids and comets.
- 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.
- The four outer planets are large and gaseous (no solid surface).
- Jupiter and Saturn are gas giants; Uranus and Neptune are ice giants.
Put these planets in order, nearest the Sun first.
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?
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) that orbit the planets.
- Comets — small icy bodies on very stretched orbits.
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 (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?
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:
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: 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 Sun is about $1.5 \times 10^{11}\ \text{m}$ away. How long does its light take to reach Earth, in seconds? (light speed $3.0 \times 10^8\ \text{m/s}$)
$\text{time} = \dfrac{d}{v} = \dfrac{1.5 \times 10^{11}}{3.0 \times 10^8} = 500\ \text{s}$ (about 8 minutes).
Why are the inner planets rocky?
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