Extension: Touring the Planets
Right now, far out in space, robot spacecraft are flying past distant worlds and beaming photos back to Earth. Some of those signals are caught by giant dish antennas near Canberra, at a place called Tidbinbilla. From there we have seen the volcanoes of Mars, the rings of Saturn and the storms of Jupiter. In this extension lesson you will take a tour of all eight planets of the Solar System, meet the difference between the small rocky planets and the giant gas planets, and find out why Pluto is no longer counted as a planet at all.
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This is an extension lesson. It goes beyond the Stage 4 syllabus dot points for interest and enrichment. There is nothing here you have to learn for an exam, so relax and enjoy the tour of our Solar System.
Q1 · Without looking it up, how many planets do you think are in our Solar System, and can you name any of them in order from the Sun? It is okay to guess.
Q2 · You may have heard that Pluto "used to be a planet." Why do you think scientists would change their minds about something like that?
● Know
- Our Solar System has eight planets that orbit the Sun, in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune
- The four inner planets are small and rocky, and the four outer planets are large and made mostly of gas and ice
- Pluto was reclassified as a dwarf planet in 2006
● Understand
- How the inner rocky planets are different from the outer giant planets
- One or two memorable features of each planet
- Why reclassifying Pluto is a good example of science updating its ideas when new evidence appears
● Can do
- List the eight planets in order from the Sun
- Sort a planet into the inner rocky group or the outer giant group
- Explain in simple terms why Pluto is now called a dwarf planet
Our Solar System is the Sun and everything that orbits it. At the centre is the Sun, a giant ball of hot gas. Going around it are eight planets. A planet is a large round world that orbits the Sun and has cleared its path of other large objects.
The eight planets, in order from the Sun, are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A handy memory sentence is "My Very Easy Method Just Speeds Up Naming", where the first letter of each word matches a planet.
The planets fall into two natural groups. The four inner planets, closest to the Sun, are Mercury, Venus, Earth and Mars. They are small and rocky, so they are also called the terrestrial planets, which just means Earth-like. The four outer planets, far from the Sun, are Jupiter, Saturn, Uranus and Neptune. They are huge and made mostly of gas and ice, so they are called the gas giants and ice giants.
The planet closest to the Sun is ___. The four inner planets are small and ___, while the four outer planets are very ___ and made mostly of gas and ice. Counting outwards from the Sun, the planet right after Earth is ___.
The inner planets are small worlds with solid, rocky surfaces you could stand on. Here is one or two things worth remembering about each.
- Mercury is the smallest planet and the closest to the Sun. It has almost no air, so it is baking hot in the day and freezing at night.
- Venus is the hottest planet of all, even hotter than Mercury. It is wrapped in a thick blanket of cloud and gas that traps heat, a bit like a greenhouse.
- Earth is our home and the only planet we know of with liquid water on its surface and living things. It sits in just the right place, not too hot and not too cold.
- Mars is the red planet. Its rusty red soil gives it the colour. Mars has the tallest known volcano in the Solar System, called Olympus Mons.
Robot rovers from Earth are driving across Mars right now, looking for signs that water and perhaps tiny life once existed there.
- Mercury
- Venus
- Earth
- Mars
- The smallest planet and closest to the Sun
- The hottest planet, wrapped in thick, heat-trapping clouds
- The only planet known to have liquid water and life
- The red planet, home to the tallest known volcano
The outer planets are enormous worlds made mostly of gas and ice. They have no solid surface you could land on. They are so big that all four inner planets could fit inside them many times over.
- Jupiter is the biggest planet of all. You could fit more than a thousand Earths inside it. It has a giant storm called the Great Red Spot that has been swirling for hundreds of years.
- Saturn is famous for its beautiful rings, made of countless pieces of ice and rock circling the planet.
- Uranus is tipped right over and spins on its side, as if it has been knocked over. It has a pale blue-green colour.
- Neptune is the windiest planet, with the fastest winds in the Solar System. It is a deep blue colour and the farthest planet from the Sun.
Because Neptune is so far away, sunlight there is very dim and it takes a spacecraft many years to reach it.
For about 76 years, Pluto was taught as the ninth planet. Then, in 2006, the world's astronomers changed their minds. Pluto did not move or shrink. What changed was the rule for what counts as a planet.
The reason was new evidence. As telescopes got better, scientists found that Pluto is very small and is just one of many icy worlds out beyond Neptune. Some of these objects are about the same size as Pluto. If Pluto stayed a planet, all of those would have to be planets too, and the list would keep growing.
So in 2006 astronomers agreed on a clearer rule. To be a full planet, a world must orbit the Sun, be round, and have cleared its path of other large objects. Pluto orbits the Sun and is round, but it shares its region with lots of other icy bodies, so it has not cleared its path. Pluto was given a new name, dwarf planet, instead.
This is a great example of how science works. When better evidence appears, scientists are willing to update their ideas and agree on a clearer definition, even for something as famous as Pluto.
How do we get photos back from planets that are millions of kilometres away? Spacecraft send faint radio signals to Earth, and we need giant dish antennas to catch them. Australia plays a big part in this.
Near Canberra is the Canberra Deep Space Communication Complex at Tidbinbilla. It is one of only three tracking stations in the world that NASA uses to talk to its planetary spacecraft. The other two are in Spain and the United States. They are spread around the globe so that as the Earth spins, at least one station can always reach a spacecraft.
The big dishes at Tidbinbilla have helped send commands to, and receive images from, missions exploring Mars, the giant outer planets and the far edge of the Solar System. So some of the planet photos you see online passed through a dish in the Australian bush before reaching the rest of the world.
Venus is further from the Sun than Mercury, so you might expect Mercury to be the hottest planet. Yet Venus is actually hotter than Mercury. Predict: what could make Venus hotter, even though it is not the closest planet to the Sun?
How close was your prediction?
At the start you guessed how many planets there are and tried to name them in order. Now write an improved, complete answer.
Your answer must: (1) say how many planets there are and list them in order from the Sun; (2) explain the difference between the inner and outer planets; (3) explain in your own words why Pluto is now called a dwarf planet.
Q1. List the eight planets in order from the Sun, then state which four are the inner planets and which four are the outer planets. (3 marks)
Q2. Describe the difference between the inner rocky planets and the outer giant planets. Give one memorable feature of any inner planet and one of any outer planet. (4 marks)
Q3. Explain why Pluto was reclassified as a dwarf planet in 2006, and explain why this is a good example of science updating its ideas with new evidence. (3 marks)
Answers
▾MCQ 1
B. The correct order from the Sun is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The other lists muddle up the order.
MCQ 2
C. The four inner planets are small and rocky, while the four outer planets are large and made mostly of gas and ice. All eight planets orbit the Sun.
MCQ 3
D. Mars is the red planet, coloured by its rusty soil, and it is home to Olympus Mons, the tallest known volcano in the Solar System.
MCQ 4
A. Pluto is small and shares its region with many other icy objects, so it has not cleared its path. Under the 2006 rule it became a dwarf planet. It did not stop orbiting, grow huge, or crash into Neptune.
MCQ 5
B. The complex at Tidbinbilla uses giant dish antennas to send commands to and receive images from planetary spacecraft for NASA. It is not a launch pad, a garden or a weather bureau.
Short Answer 1
Model answer: In order from the Sun the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are Mercury, Venus, Earth and Mars. The four outer planets are Jupiter, Saturn, Uranus and Neptune.
Short Answer 2
Model answer: The inner planets are small and rocky with solid surfaces, while the outer planets are very large and made mostly of gas and ice with no solid surface. A memorable inner feature could be that Venus is the hottest planet because of its thick atmosphere, or that Mars has the tallest known volcano. A memorable outer feature could be Jupiter's Great Red Spot, Saturn's rings, Uranus spinning on its side, or Neptune being the windiest planet.
Short Answer 3
Model answer: Pluto was reclassified because it is very small and shares its region with many other icy objects, so it has not cleared its path the way the eight planets have. In 2006 astronomers agreed on a clearer rule for what a planet is, and Pluto did not meet it, so it became a dwarf planet. This is a good example of science updating its ideas because Pluto itself did not change. When new evidence showed many similar icy worlds, scientists were willing to agree on a better definition.