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Space · Giants and dwarf planets

Why Pluto is no longer a planet

Pluto did not change, the word did

Only spacecraft can see it

Why Pluto is no longer a planet
The three conditions for being a planet, and the exact point where Pluto no longer meets them.

The numbers

When it was decided
August 24, 2006At a meeting of the International Astronomical Union
The rule it fails
It has not cleared its own orbit

How it happens, step by step

  1. Go around the Sun

    The first condition is the easiest: you have to orbit the Sun and not another planet.

  2. Be round

    The second: be big enough that your own gravity pulls you into an almost round shape.

  3. Clear the orbit

    The third: have swept your own path clean, with no other bodies of a similar size around.

  4. Pluto stops here

    Pluto passes the first two tests but not the third: it shares its zone with a great many icy bodies.

  5. A new category

    For Pluto and others like it, a new class was created in 2006: the dwarf planets.

To get an idea

To be a planet takes three conditions, and Pluto meets only two of them.

The difference is that Pluto shares its orbit with a great many other icy bodies.

Fun facts

These are not in the book: there was no room on paper.

  1. The person who invented it thought of it as a kind of planet, not a category separate from planets.

  2. The decision was made amid a lot of arguing and controversy, and the debate is not over.

  3. One objection is that there is no exact threshold for saying when an orbit has been cleared.

  4. Some astronomers would like to call every round body a planet and rename the others classical planets.

  5. The dwarf planets beyond Neptune even have a name all their own: plutoids.

  6. Ceres is a dwarf planet but not a plutoid, because it sits between Mars and Jupiter.

Test yourself

The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.

Question 1 of 5★ is the one from your book

What changed in 2006?

Question 2 of 5

How many conditions does it take to be a planet?

Question 3 of 5

Which condition does Pluto fail?

Question 4 of 5

Why was a change needed?

Question 5 of 5

Is the debate over?

Riddles

I am not a thing but a rule, and I have three parts. Whoever follows all three gets an ancient title; whoever follows only two gets one invented just recently. What am I?

In 2006 I did not move a single stone, and yet I took a place of honor away from a distant world and gave it to a new category. I am made only of words. What am I?

I am the job a world must have finished to be called great: sweeping its own path clean. Some say nobody knows when I am truly complete. What am I?

Try it yourself

Activity 1

Line up round objects of different sizes and decide where big ends and small begins. Whatever line you choose will be a little arbitrary.

Activity 2

Look up how many dwarf planets are recognized today: the number has changed since the category was invented, and it may change again.

The words

plutoid
A dwarf planet that orbits farther out than Neptune.

Sources: Wikidata (Q3419), Wikipedia (Pianeta nano).