The Solar System

Teacher Tips

Hook

• It helps to hold off correcting anyone during the six images. The point isn’t to sort them, it’s to get learners to notice they’re already using a rule they can’t name. “What rule are you using to decide?” is the question to ask.
• The star tends to expose the most interesting confusion. Learners who call it a planet are reasoning from “round bright object in space”, which is exactly the reasoning the IAU rejected. You might ask “What’s the difference between something that makes its own light and something that only reflects it?”
• The Moon will split the room, and that’s the most useful disagreement of the session, so it’s worth leaving unresolved. Take both sides, write the arguments up, and leave it hanging. Twenty minutes later Rule 1 settles it for them.

ITZA Pathways

• You could ask directly: “What would happen to Earth if it stopped moving sideways tomorrow?” The answer, that it would fall straight in, is what makes orbit make sense.
• It helps to circulate with one question ready: “If you discovered a new object tomorrow, how would you decide what to call it?” It primes the activity, and it shows you straight away who has grasped that classification is a decision rather than a fact.

Classroom Activity

• It’s worth covering the Earth slide properly before moving on. Say all three rules out loud against the one object everybody already agrees about. Without that model, learners tend to name the object instead of classifying it, and naming isn’t the skill you’re after.
• It helps to ask for every verdict to say which rule fails, not just yes or no. “Not a planet” on its own doesn’t tell you much. “Not a planet, because it orbits a planet rather than a star” is better. Where an object passes, encourage learners to recite all three rules.
• Rule 3 is where the errors tend to appear. Learners read “cleared its path” as “nothing else is anywhere near it”, which isn’t what it means and would disqualify most of the solar system. It helps to say early that it means the object is the dominant thing in its lane, not that the lane is empty.

Plenary

• Part 2 asks what changed. Expect “Pluto changed” or “the scientists were wrong”. Both are worth hearing before you push. Pluto is exactly as it was in 1930. What changed is that we found thousands of similar objects and had to decide what that meant.
• The lock-in is from Alan Stern, the scientist who leads the New Horizons mission to Pluto and one of the best-known opponents of the definition this session teaches. He still argues Pluto is a planet, and saying so makes a stronger ending than the quote alone, because it shows the argument is live.