The Solar System

Our Solar System

The solar system consists of the Sun and everything that orbits it — eight planets, dwarf planets, moons, asteroids, comets, and dust. It formed about 4.6 billion years ago from a giant rotating cloud of gas and dust.

The Sun contains 99.86% of all the mass in the solar system. Its gravity holds everything in orbit. It is a medium-sized star made mostly of hydrogen and helium, generating energy through nuclear fusion.

The eight planets

The planets are divided into two groups: inner (terrestrial) planets — rocky, closer to the Sun — and outer (gas/ice giant) planets — much larger, farther away.

#	Planet	Type	Approx. distance from Sun	Moons
1	Mercury	Terrestrial	57.9 million km	0
2	Venus	Terrestrial	108 million km	0
3	Earth	Terrestrial	150 million km	1
4	Mars	Terrestrial	228 million km	2
5	Jupiter	Gas giant	778 million km	95
6	Saturn	Gas giant	1.43 billion km	146
7	Uranus	Ice giant	2.87 billion km	28
8	Neptune	Ice giant	4.50 billion km	16

Mnemonic: My Very Energetic Mother Just Served Us Nachos (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)

Gravity and orbits

Gravity is the force of attraction between objects with mass. The Sun's enormous gravity keeps the planets in their nearly circular orbits. Planets closer to the Sun orbit faster and have shorter orbital periods (years).

Earth's orbital period is 365.25 days (one year). Mercury takes only 88 Earth days to orbit the Sun; Neptune takes 165 Earth years!

Planets orbit in the same direction — counter-clockwise when viewed from above Earth's north pole. This is because they all formed from the same rotating cloud of gas.

The asteroid belt and dwarf planets

Between Mars and Jupiter lies the asteroid belt — a region of rocky bodies ranging from pebbles to hundreds of kilometres across. Beyond Neptune is the Kuiper Belt, home to dwarf planets including Pluto. Pluto was reclassified as a dwarf planet in 2006 because it has not "cleared the neighbourhood" around its orbit.

Why does Earth support life?

Earth is in the "habitable zone" — the right distance from the Sun for liquid water to exist. It has a protective atmosphere, a magnetic field that deflects harmful solar radiation, and a stable axial tilt that produces seasons.

Answer in your exercise book. Use diagrams where helpful.

Question 1 · (2 marks)

List the eight planets in order from the Sun. You may use the mnemonic "My Very Energetic Mother Just Served Us Nachos."

Question 2 · (2 marks)

Distinguish between a terrestrial planet and a gas giant. Give one example of each.

Question 3 · (1 mark)

What force keeps the planets in orbit around the Sun?

Question 4 · (2 marks)

Why is Pluto no longer classified as a planet? Name the category it now belongs to.

Question 5 · (3 marks)

Explain THREE reasons why Earth is able to support life, while other planets in the solar system cannot.

Question 6 · (4 marks)

Study the table below, then answer the questions that follow.

Planet	Distance from Sun (million km)	Orbital period (Earth years)
Mercury	57.9	0.24
Earth	150	1
Jupiter	778	12
Neptune	4 500	165

(a) Calculate how many times farther from the Sun Neptune is compared to Earth. Show your working. (1)
(b) Calculate how many times longer Neptune's orbital period is compared to Earth's. Show your working. (1)
(c) Compare your two answers above. Is the increase in orbital period proportional to (the same size as) the increase in distance? (1)
(d) Using what you know about gravity, explain why a planet much farther from the Sun has a proportionally much longer year. (1)

Our Solar System

The eight planets

Gravity and orbits

The asteroid belt and dwarf planets

Why does Earth support life?

Solar System Explorer