On 14 July 2015, NASA's New Horizons spacecraft flew past Pluto at 14 kilometres a second. Tucked inside it was a small metal container with a label that read: "Interred herein are remains of American Clyde W. Tombaugh, discoverer of Pluto and the solar system's third zone." Tombaugh's ashes had travelled almost five billion kilometres to fly past the world he had spent his whole life studying.
Pluto's discovery story is one of the most human stories in all of astronomy. It starts with an unhappy millionaire, runs through a farm boy who built his own telescopes out of car parts, and ends with an 11-year-old girl eating breakfast in Oxford.
Pluto was discovered by Clyde Tombaugh, a 24-year-old American astronomer, at the Lowell Observatory in Arizona on 18 February 1930. He spotted it by flicking between two photographs of the night sky and noticing a tiny dot that had moved.
Percival Lowell — The Search for "Planet X"
The story actually starts decades before Pluto was found. An American astronomer named Percival Lowell was convinced there was a mystery ninth planet hiding beyond Neptune.
Lowell came from a wealthy family. In 1894 he used his fortune to build an observatory in Flagstaff, Arizona, and from there he tried for years to find what he called "Planet X" — X for "unknown," not the number ten. He thought a hidden planet must be pulling on the orbits of Uranus and Neptune in tiny ways.
Lowell died in 1916 without ever finding it. His observatory kept searching after his death.
Clyde Tombaugh — The Farm Boy With Homemade Telescopes
In 1929, the Lowell Observatory needed someone new to do the long, painful job of searching. They got a letter from a 23-year-old farm boy in Kansas named Clyde Tombaugh.
Tombaugh hadn't been to university. He was a farmer. But he had built his own telescopes from old car parts and farm machinery, and he had sent in some incredibly detailed sketches of Jupiter and Mars. The observatory was so impressed they invited him to come and work for them. Tombaugh had never even left Kansas before. He took the train to Arizona and started.
The Job — Photograph the Sky, Look for Anything That Moves
Tombaugh's job was monotonous and exhausting. Every clear night, he would take long-exposure photographs of small patches of the sky. Then he would take the same patches again a few nights later. Stars stayed put. Anything that moved between photos was a possible planet.
The machine he used to find moving objects was called a blink comparator. It took two photos of the same patch of sky and flickered them on and off, on and off, like a very early flip-book. If anything had moved between the two photos, it would jump back and forth and catch your eye.
The Night Tombaugh Found Pluto
Tombaugh searched for almost a year. By 18 February 1930, he had already flipped through thousands of pairs of plates. On that day, he sat down at the blink comparator to look at two photographs taken weeks apart in January.
One dot was jumping back and forth between them. It wasn't a star. It wasn't an asteroid. It was something far, far away — moving slowly, the way an object in a very wide orbit would move. Tombaugh checked, double-checked, and then went next door to tell his boss what he'd found.
Imagine flicking back and forth between two photos. Thousands of dots. They all stay in the same places. Except one. One tiny dot, in a slightly different spot in each picture. That tiny moving dot was a world that humans had never seen before. It was Pluto.
Venetia Burney, Aged 11, Names a Planet
The news exploded around the world. Humans had found a new planet. There was just one problem — nobody had agreed on a name. Suggestions poured in from everywhere. Minerva. Cronus. Atlas. Zeus.
On the morning of 14 March 1930, in Oxford, England, an 11-year-old girl called Venetia Burney was eating breakfast with her grandfather. Her grandfather, Falconer Madan, was reading the newspaper and mentioned the new planet to her. Venetia had been learning about the Greek and Roman gods at school. After a moment's thought, she said: "Why not Pluto?"
Pluto was the Roman god of the dark underworld — a perfect match for a tiny, freezing, far-away world.
Her grandfather thought it was a wonderful idea. He knew an astronomer at Oxford and passed the suggestion along. The Lowell Observatory loved it too — especially because the first two letters, P and L, also happened to be the initials of Percival Lowell. On 24 March 1930, the name was officially announced. More on Pluto's name and mythology.
A farm boy found it.
An 11-year-old named it.
Even the letters matched the man who started the search.
Venetia Lived Long Enough to See the Spacecraft Launch
Venetia Burney lived a long, quiet life as a schoolteacher in England. She lived to be 90. In 2006 — the same year Pluto was reclassified as a dwarf planet, and the same year NASA's New Horizons spacecraft launched to visit Pluto — Venetia spoke to reporters about her childhood breakfast idea. She lived to see the spacecraft begin its journey toward the world she had named, and only died in 2009, six years before it arrived.
Tombaugh's Ashes Travelled With New Horizons
Clyde Tombaugh died in 1997 at the age of 90. Almost ten years later, when NASA launched New Horizons toward Pluto, a small container of his ashes was tucked inside. On 14 July 2015, his ashes flew past the world he had discovered. Tombaugh's ashes are now drifting beyond the Kuiper Belt — the only human remains ever to leave the solar system.
Read about the New Horizons mission that carried Tombaugh's ashes to Pluto, or the mythology behind Pluto's name, or jump back to the main Pluto facts page.
Frequently Asked Questions
Who discovered Pluto?
Pluto was discovered by Clyde Tombaugh, a 24-year-old American astronomer, at the Lowell Observatory in Arizona on 18 February 1930. He spotted Pluto by comparing pairs of photographs of the night sky using a special machine called a blink comparator.
Who named Pluto?
Pluto was named by an 11-year-old girl from Oxford, England, called Venetia Burney. She suggested the name to her grandfather over breakfast in 1930 because Pluto was the Roman god of the dark underworld. The Lowell Observatory loved the idea — and it stuck.
When was Pluto discovered?
Pluto was discovered on 18 February 1930 by Clyde Tombaugh. The discovery was announced publicly on 13 March 1930. The name 'Pluto' was officially adopted on 24 March 1930.
Where was Pluto discovered?
Pluto was discovered at the Lowell Observatory in Flagstaff, Arizona, USA. The observatory was founded by Percival Lowell, who had spent years searching for a mysterious 'Planet X' before he died in 1916. Clyde Tombaugh finally found it 14 years later.
What is a blink comparator?
A blink comparator is a machine that flickers two photographs of the night sky on and off, on and off, like an early flip-book. Stars stay still, but any object that has moved between the two photos appears to jump back and forth. That's how Tombaugh spotted Pluto in 1930.