If you've seen a t-shirt, mug or meme saying "Pluto: Never Forget, 1930–2006", it's a joke with real astronomy behind it. The slogan treats Pluto like a fallen hero. It borrows the solemn wording of a memorial and applies it to something far less tragic: the day astronomers decided Pluto wasn't a planet any more.
Here's what the dates mean, why Pluto was reclassified, and why so many people still haven't let it go.
What does "Pluto Never Forget" mean?
"Pluto Never Forget" is a light-hearted way of saying you haven't accepted Pluto's demotion. The two dates work like the dates on a gravestone:
- 1930 is when Pluto was discovered and became our ninth planet.
- 2006 is when the International Astronomical Union (IAU) voted to reclassify it as a dwarf planet.
Pluto itself is fine, of course. It's still out there, about 5.9 billion kilometres from the Sun on average. What "died" in 2006 was its planet status, and with it the nine planets that generations of us learned at school.
1930: how Pluto was discovered
Pluto was found by Clyde Tombaugh, a 24-year-old astronomer at Lowell Observatory in Flagstaff, Arizona. On 18 February 1930, while comparing photographic plates of the same patch of sky taken a few nights apart, he spotted a faint dot that had moved. The discovery was announced on 13 March 1930.
The girl from Oxford who named Pluto
Pluto has a British connection. The name was suggested by Venetia Burney, an 11-year-old schoolgirl from Oxford. On 14 March 1930 her grandfather, Falconer Madan, read her the news of the discovery at breakfast. Venetia, who loved classical myths, suggested Pluto, the Roman god of the underworld: a fitting name for a cold, dark world at the edge of the Solar System.
Madan passed the idea to the Oxford astronomer Herbert Hall Turner, who cabled it to Lowell Observatory. The staff there voted for it, and the name was announced on 1 May 1930. It helped that the first two letters, P and L, are the initials of Percival Lowell, the observatory's founder.
Venetia lived to see NASA's New Horizons spacecraft launch towards Pluto in 2006, and one of its science instruments, the Venetia Burney Student Dust Counter, is named after her. So is asteroid 6235 Burney.
2006: why Pluto stopped being a planet
For 76 years Pluto was the ninth planet. Then astronomers started finding other icy worlds in the same region beyond Neptune, known as the Kuiper Belt. The turning point was Eris, discovered in 2005. It's roughly the same size as Pluto and more massive. If Pluto was a planet, Eris surely had to be one too, and so might dozens more objects still waiting to be found.
Rather than let the list of planets keep growing, the IAU agreed a formal definition at its meeting in Prague on 24 August 2006. To count as a planet, an object must:
- orbit the Sun
- be massive enough for its own gravity to pull it into a roughly round shape
- have "cleared the neighbourhood" around its orbit, meaning it's by far the dominant object in its path
Pluto passes the first two tests but fails the third, because it shares its region of space with countless other Kuiper Belt objects. So it was reclassified as a dwarf planet, alongside Eris and Ceres, and soon afterwards given the catalogue number 134340.
Is Pluto still a planet?
Officially, no. Under the IAU definition Pluto is a dwarf planet. But the argument never really ended. Some planetary scientists, including members of the New Horizons team, think "clearing the neighbourhood" is a poor test and would call any large, round world a planet. By that measure Pluto would count, and so would quite a few moons.
That's why you'll also see the more defiant slogan: "I don't care what they say, Pluto is still a planet."
What we've learned about Pluto since 2006
Pluto became far more interesting after its demotion. On 14 July 2015, NASA's New Horizons spacecraft flew past it and sent back the first close-up pictures. They showed:
- a huge, bright, heart-shaped region, now called Tombaugh Regio after Pluto's discoverer
- a smooth plain of frozen nitrogen, Sputnik Planitia, with no impact craters, a sign that the surface is geologically young
- mountains of water ice several kilometres high
- a thin atmosphere with layers of blue haze
New Horizons also carried a small amount of Clyde Tombaugh's ashes, so the man who found Pluto finally got to visit it.
Pluto quick facts
- Discovered: 18 February 1930, by Clyde Tombaugh
- Named by: Venetia Burney, aged 11, from Oxford
- Reclassified: 24 August 2006, as a dwarf planet
- A year on Pluto: about 248 Earth years, so it hasn't completed a single orbit since it was discovered, and won't until around 2178
- Moons: five: Charon, Nix, Hydra, Kerberos and Styx. Charon is about half Pluto's width, so big that the pair orbit a point in the space between them
- Visited by: NASA's New Horizons, 14 July 2015
Pluto Never Forget t-shirts and gifts
Our Pluto Never Forget 1930–2006 t-shirt is a memorial tee for everyone who still counts nine planets. There's also a Pluto Never Forget baby vest, and our "Pluto Is Still A Planet" design comes on a t-shirt, hoodie, sweatshirt, mug, sticker and phone case.
See the full range in our Pluto gifts collection, or browse more space t-shirts. Every item is printed to order, with free delivery on every order.
Pluto Never Forget FAQs
What does 1930–2006 mean on a Pluto shirt?
They're Pluto's years as a planet: discovered in 1930, reclassified as a dwarf planet in 2006.
When did Pluto stop being a planet?
On 24 August 2006, when the International Astronomical Union voted on a new definition of a planet at its General Assembly in Prague.
Why isn't Pluto a planet any more?
It hasn't cleared its orbital neighbourhood. Pluto shares its part of the Kuiper Belt with many other icy objects, so under the IAU's rules it's a dwarf planet.
Who named Pluto?
Venetia Burney, an 11-year-old girl from Oxford, suggested the name in March 1930. It was adopted on 1 May 1930.