Welcome, space scientists!
For 76 years, every kid in school learned that Pluto was the ninth planet. Then, in 2006, a group of astronomers voted and Pluto was a planet no more. Was that fair? By the end of this mission, you'll know the whole story and be ready to argue your side.
Your mission
My badges
0 of 7 badges
Meet Pluto
A small, icy world at the far edge of our solar system.
Pluto's ID card
Pluto lives way out past Neptune, so far away that sunlight takes about 5 hours to get there. It is made of rock and ice, and it has a giant heart-shaped patch of frozen ice on its surface.
The heart is called Tombaugh Regio, named after the man who discovered Pluto. You'll meet him on the next page.
How big is Pluto, really?
These circles are drawn to scale. Pluto is smaller than our Moon, and it's even narrower than the United States is wide!
Game: Fact or fiction?
Read each statement. Is it a fact or is it fiction?
Fill in the blanks
The Discovery
How Pluto was found, named, and called the ninth planet.
The hunt for Planet X
In the early 1900s, an astronomer named Percival Lowell believed a hidden world was out past Neptune. He called it Planet X and built an observatory in Flagstaff, Arizona to search for it. He never found it.
Years later, the observatory hired Clyde Tombaugh, a young farm kid from Kansas who built his own telescopes. His job was to take photos of the same patch of sky a few days apart. Then he used a machine called a blink comparator to flip back and forth between the two photos. Stars stay in the same spot. But anything that jumps is moving, and could be a planet!
On February 18, 1930, when he was just 24, Tombaugh spotted a tiny dot that jumped. He had found Pluto!
Game: Be Clyde Tombaugh
These are two "photos" of the same stars, taken days apart. Press Blink to flip between them. Find the one dot that moves and click it. Find it in 3 star fields to win.
A name from an 11-year-old
Everyone wanted to name the new world. The winning idea came from Venetia Burney, an 11-year-old girl in Oxford, England. She suggested Pluto, the Roman god of the underworld, because it was such a dark, cold, faraway place.
The astronomers loved it for another reason too: the first two letters, P-L, are Percival Lowell's initials.
So why was it called a planet?
It was found right about where Planet X was expected, and it seemed to be alone out there. Astronomers at first thought it might be about as big as Earth. With no better information, Pluto became the ninth planet in 1930, and it kept that title for 76 years.
Fill in the blanks
Quick quiz
New Neighbors
The discoveries that made astronomers ask, "Wait, what is a planet?"
Clue 1: Pluto is tiny (1978)
In 1978, astronomer James Christy discovered Pluto's biggest moon, Charon. By watching Charon go around Pluto, scientists could finally figure out how much Pluto weighs. The answer surprised everyone: Pluto was far smaller than they thought. Even Mercury, the smallest planet, has more than 20 times as much mass.
Clue 2: Pluto has lots of company (1992)
In 1992, astronomers David Jewitt and Jane Luu found another icy object out past Neptune. Then another, and another. Today we know about thousands of them. This zone of icy worlds is called the Kuiper Belt (say it: KY-per). Pluto isn't alone out there. It is one of the biggest members of the Kuiper Belt crowd.
Hubble Space Telescope photos also found four more tiny moons around Pluto between 2005 and 2012: Nix, Hydra, Kerberos, and Styx.
Clue 3: Meet Eris (2005)
In 2005, astronomer Mike Brown and his team announced a Kuiper Belt world almost as wide as Pluto, and with more mass than Pluto. It was later named Eris.
Now astronomers had a big problem. If Pluto is a planet, then Eris should be one too. And what about all the other large objects they might find? The solar system could end up with 50 planets or more! They needed a clear rule.
Fun fact: the New Horizons spacecraft blasted off to visit Pluto in January 2006, when Pluto was still a planet. By the time it flew past Pluto in 2015, Pluto was a dwarf planet.
Game: Put the story in order
Use the arrows to put these events from first (top) to last (bottom). Then press Check.
Quick quiz
Who Decides?
The group, the vote, and the three rules.
Meet the IAU
The International Astronomical Union (IAU) is a group of professional astronomers from all over the world. Since 1919, it has been in charge of naming things in space and deciding official science words, like what counts as a planet.
In August 2006, the IAU held a big meeting in Prague, in the Czech Republic. On August 24, 2006, the last day of the meeting, the astronomers in the room held up yellow cards to vote on a new definition of "planet." The IAU has thousands of members, but only a few hundred were still there to vote.
The three rules for being a planet
An object that passes rules 1 and 2 but fails rule 3 (and isn't a moon) is called a dwarf planet. Pluto shares its path with lots of Kuiper Belt objects, so it fails rule 3.
Game: The planet test machine
Pick an object and watch the machine test it against the three rules.
Game: Planet, dwarf planet, or neither?
Use the three rules to sort each object.
You're in Prague. Cast your vote!
It's August 24, 2006. The new rules say a planet must orbit the Sun, be round, and clear its neighborhood. Do you vote to accept these rules?
Quick quiz
The Great Debate
Scientists still argue about Pluto. Here are both sides.
Pluto should be a planet
- It is round, has 5 moons, and has a thin air layer with blue haze.
- New Horizons found ice mountains, glaciers, and a surface that is still changing. That's what planets do.
- Alan Stern, leader of the New Horizons mission, says any round world orbiting the Sun should count as a planet.
- Earth, Mars, and Jupiter share their paths with asteroids too, so "clearing the neighborhood" is a fuzzy rule.
- Only a few hundred of the IAU's thousands of members voted.
- In 2024, Arizona made Pluto its official state planet.
Pluto should stay a dwarf planet
- Eris has more mass than Pluto. If Pluto is a planet, Eris must be too, and so might dozens more.
- Pluto belongs to a group: it's one of thousands of icy Kuiper Belt objects.
- Pluto is smaller than 7 moons in our solar system, including our own Moon.
- Science changes when we learn more. Ceres was called a planet in 1801 and was later reclassified, too.
- A clear rule helps scientists sort the millions of objects in space.
- The IAU's rule is still the official definition today.
Game: Which side does it support?
Each statement is evidence. Decide which side of the debate it helps.
Game: Fact or opinion?
A fact can be proven. An opinion is what someone thinks or feels. Your speech needs both: an opinion, backed up by facts.
The recipe for a persuasive paragraph
Search engines like Google help you find websites, but they are not the source. Your evidence must come from the website itself, like NASA.
Quick quiz
The Big Test
Ten questions from every lesson. Get 8 or more right on your first try to earn the Space Scholar badge.
Only your first answer counts, so think before you tap.
My Speech
Plan, research, and write your persuasive speech. Your work saves automatically on this computer.
About me
Part 1: Pluto fact check
Part 2: Pick your side
Should Pluto be called a planet again? You'll have to prove it!
Part 3: Research
Use at least three websites. For each one, fill in the author (skip it if there isn't one), the title of the page, the web address, and the year it was published. Then write notes and quotes you can use as evidence.
Good places to start
Part 4: Plan your paragraph
Part 5: Write your paragraph
Press the button to put your plan together, then fix it up so it flows like real writing. You need at least 6 sentences.
0 sentences
Part 6: Get ready to speak
Speech day checklist
- Practice reading your paragraph out loud until you know it well.
- Point to your visual aid when it helps prove a reason.
- Look up at your audience, speak clearly, and skip the "ums."
- Take a deep breath and smile. You're the expert now!
Part 7: After your speech
How did it go?
Bonus: turn your speech into a friendly letter to NASA asking them to consider your side!
Speech Star badge checklist
Send my work to Google Docs
This puts everything you wrote into a Google Doc you can edit, print, and turn in.