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Pluto used to be planet number 9!

For a long time, the Solar System had 9 planets. The last one was tiny, far away, and very cold: Pluto.

But in 2006, scientists made a new rule and said: "Pluto is not a real planet." Poor Pluto! 🥶

Today you'll become a space detective 🔎 and find out the real reason Pluto lost its planet badge. Tap Next to begin your mission!

The 3 secret rules of being a planet

Scientists agreed that to be a real planet, a space object must pass 3 tests. Tap each card to flip it open!

1

Go around the Sun ☀️

It must orbit (circle) the Sun, not a planet.

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2

Be big and round ⚪

It must be heavy enough for its own gravity to squash it into a ball shape.

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3

Clean its own path 🧹

It must be the "boss" of its orbit and clear away other rocks and space junk nearby.

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👆 Tap all 3 cards to reveal the rules.

Pluto passes Rule 1 and Rule 2 ✅✅

Good news for Pluto! Let's check the first two tests:

Rule 1 — Orbits the Sun? ✅ Yes! Pluto loops around the Sun, very far away.

Rule 2 — Round like a ball? ✅ Yes! Pluto is round.

Pluto circling the Sun 🟤

So far so good! But there is one more test... and this is where the trouble starts. 😬 Tap Next.

Rule 3: Be the boss of your orbit 🧹

A real planet is so strong that it has cleared away almost all the other rocks in its path.

A real planet's orbit is clean. Pluto's orbit is full of rocky neighbours (it lives in a crowded zone called the Kuiper Belt). 🪨🪨🪨

Help Earth clear its path! Tap the rocks to sweep them away. A real planet can do this — can you?

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5 rocks left to clear

This is the real reason! 🎯

Earth, Mars and the others cleared their paths — so they pass Rule 3.

But Pluto is too small and too weak. It shares its orbit with thousands of other icy rocks and can't clear them away.

Pluto only passed 2 out of 3 rules.

✅ ✅ ❌

2 ticks is not enough — you need all 3!

Quick check: Which rule did Pluto fail? Tap your answer.

Just how small is Pluto? 🤏

Pluto is even smaller than our Moon! Compare it to Earth below:

Earth 🌍

Pluto 🟤

You could fit many Plutos inside the Earth!

Sort the space objects! Tap an object, then tap where it belongs.

🪐 Planet

🟤 Dwarf planet

Pluto isn't alone — it's a dwarf planet 🟤

Scientists made a brand new group for round-but-small objects: dwarf planets.

Pluto has a whole family of dwarf-planet friends, like Eris, Ceres, Makemake and Haumea. So Pluto isn't lonely at all! 🎉

🟤 ❄️ 🧊
🪨 ⚪

The dwarf-planet club!

Being a dwarf planet is not bad — it's just a different kind of space object. Pluto is still amazing and scientists love studying it. 💙

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Mystery solved, space detective!

You found the real reason Pluto isn't a planet. Here's what you learned:

1️⃣ A real planet must orbit the Sun, be round, AND clear its path.
🧹 Pluto can't clear its path — it shares its crowded orbit with many icy rocks.
🤏 Pluto is very small — even smaller than our Moon.
🟤 So in 2006 Pluto became a dwarf planet, with friends like Eris and Ceres.

Well done! 🌟 Now you know more about Pluto than most grown-ups!