Session 4: The Solar System
Start thinking: Which of these are planets? How do you know?
Only two are planets.
Planet
Planet
Comet
Dwarf planet
Moon
Star
Jupiter
The Moon
Pluto
Earth
Halley’s Comet
Betelgeuse
Today’s Big Question:
What makes a planet a planet?
What makes a planet a planet?
PATHWAY 1
How Did The Solar System Form?
10 mins
PATHWAY 2
What Makes A Planet A Planet?
10 mins
Finished? Complete the ‘How Worlds are Made & What Counts as a Planet’ Minigame
Planet Guess Who?
To be a planet, a cosmic object must meet three criteria:
Orbit + shape + clear path = planet
How do we know that Earth is a planet?
Orbit + shape + clear path = planet
Planet Guess Who?
For each object, read the factfile and use the three rules to decide if it is a planet. You may be able to work out the names of some of them.
Planet Guess Who?
OBJECT A
Fact 1: I orbit a large ringed world, going round it once every 16 days. I never travel around the Sun on my own.
Fact 2: I am 5,150 kilometres across, wider than Mercury. My gravity has pulled me into a ball. Dozens of other bodies circle the same ringed world alongside me.
Fact 3: I have a thick orange atmosphere, and lakes of liquid methane on my surface. It rains on me.
Am I a planet? Can you work out what I am?
Planet Guess Who?
I am Titan, a moon of Jupiter.
Fact 1: I orbit a large ringed world, going round it once every 16 days. I never travel around the Sun on my own. Titan orbits Jupiter, not the sun, so it cannot be a planet.
Planet Guess Who?
OBJECT B
Fact 1: I orbit the Sun. One trip takes me 3.6 years.
Fact 2: I am about 525 kilometres across and not round. A large impact near my south pole gouged out a crater almost as wide as I am and left me lopsided.
Fact 3: I am one of the biggest things in my path. Around me are thousands of other rocks.
Am I a planet? Can you work out what I am?
Planet Guess Who?
I am Vesta, an asteroid.
Fact 2: I am about 525 kilometres across and not round. Vesta’s gravity hasn’t shaped it into a sphere, so it cannot be a planet.
Fact 3: I am one of the biggest things in my path. Around me are thousands of other rocks. Vesta has not cleared its path of debris, so it cannot be a planet.
Planet Guess Who?
OBJECT C
Fact 1: I orbit the Sun a very long way out. One lap takes 558 years.
Fact 2: I am 2,326 kilometres across and round. That makes me very slightly narrower than the famous icy world that used to be called the ninth planet, but I contain about a quarter more material than it does. I am the heavier of the two.
Fact 3: Scattered icy bodies share my region of space.
Am I a planet? Can you work out what I am?
Planet Guess Who?
I am Eris, a dwarf planet.
Fact 3: Scattered icy bodies share my region of space. Eris has not cleared its path of debris, so it cannot be a planet.
When I was discovered in 2005, scientists faced a choice. If Pluto counted as a planet, so would I. Instead, they changed the rules.
Planet Guess Who?
OBJECT D
Fact 1: Thousands of icy objects share my path.
Fact 2: I circle a small icy world out beyond the eighth planet. We both swing around a point in empty space between us.
Fact 3: I am 1,212 kilometres across and round. My partner is only about twice my width.
Am I a planet? Can you work out what I am?
Planet Guess Who?
I am Charon, a moon of Pluto.
Fact 1: Thousands of icy objects share my path. Charon has not cleared its path, so cannot be a planet.
Fact 2: I circle a small icy world out beyond the eighth planet. Charon doesn’t orbit a star - instead it swings around Pluto.
Planet Guess Who?
OBJECT E
Fact 1: I orbit the Sun once every 11.9 years.
Fact 2: I am round, and I am the largest object in the solar system other than the Sun.
Fact 3: The other objects in my orbit are held in place by my gravity, so they are not in my path.
Am I a planet? Can you work out what I am?
Planet Guess Who?
I am Jupiter, a planet.
It meets all three criteria for a planet.
Planet Guess Who?
OBJECT F
Fact 1: I orbit no star at all. I drift alone through the galaxy, about 80 light years from Earth.
Fact 2: I have six times the mass of Jupiter. I am round, and made of gas.
Fact 3: There is nothing around me. My neighbourhood is empty space.
Am I a planet? Can you work out what I am?
Planet Guess Who?
I am PSO J318.5-22, a ‘rogue planet’.
Fact 1: I orbit no star at all. Because PSO J318.5-22 doesn’t orbit a star, it cannot be a planet.
It is called a rogue planet because it has many features of a planet, but it doesn’t orbit a star.
WE HAVE COVERED:
Learner Journal
Part 1: Three Things I Learnt Today
Open your Learner Journal. Write down three things you learnt. These can be facts, ideas, or things that surprised you.
Write your response in your Learner Journal, then finish sections 3-5.
Learner Journal
Part 2: Reflective Question
Pluto was a planet in every textbook for 76 years before the rules changed. What changed?
“Every time we send a spacecraft to explore a new world in our solar system, nature turns out to be far more inventive and surprising than our imaginations were.” Alan Stern, space scientist at NASA (2015)
The solar system built itself from the leftovers (the 0.1%) of a star that died before our Sun was born. Dust became rocks. Rocks became planetesimals. Planetesimals became protoplanets. Protoplanets became worlds. And the rules for deciding what counts as a world are stricter than you might think: you have to orbit a star, be round, and have cleared your path.
Lock-In Quote
Extension Activities
Extension 1
Extension 2
The Worst Weather in the Solar System
How Do Stars Die?