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When large objects such as planets pass near one another, the force of gravity attracts them and causes them to deviate slightly from their usual orbits. This phenomenon is called perturbation in astronomy. It was such perturbations in Neptune’s orbits that led a French mathematician called Urbain Le Verrier in the 1840s to predict a planet further from the Sun than Neptune.

Solar System AnimationPluto

It wasn’t until 1906 that a wealthy American astronomer, Percival Lowell, founded the Lowell Observatory and a large-scale search for “Planet X” began. Percival died in 1916, but his widow Constance Lowell fought for his search to continue, and in 1929 23-year old Clyde Tombaugh was handed the task of continuing the search.

Clyde took photographs of the night sky over time, then examined them to see whether any of the objects in them had shifted. He used a machine called a blink comparator which switched quickly between two images, allowing him to more easily see any differences in position. On February 18th 1930, he made his discovery!

Can you spot Pluto in the recreation of the blink comparator’s view below? The images used are the original images taken by Clyde in 1930, 6 days apart!

Blink Comparator Model
Credit: Lowell Observatory Archives
As the person who discovered it, it was up to Clyde to decide a name for the ‘new’ planet. He received over 1,000 suggestions from around the world! In the end it was Venetia Burney, an 11 year old girl from Oxford, who suggested the name of the god of the underworld ‘Pluto’ in a conversation with her librarian grandfather, who passed it along to an astronomy professor, who cabled it across the Atlantic to the Lowell Observatory! A vote was held between three options; ‘Minerva’, ‘Cronus’, and ‘Pluto’. Pluto won unanimously and the planet was officially named Pluto on May 25th of that year.

In some languages, the name Pluto is not transliterated but replaced with their own gods of the underworld, including Meiōsei or 冥王星, "Star of the King of the Underworld" in Japanese, Yama, the God of Death in Hindi, and Whiro, the god of the underworld in Maori.

Everything seemed settled, until along came the 1990s and the discovery of lots of other celestial (positioned in the sky) objects near Pluto, some which were nearly as large as Pluto itself! Pluto, and the objects which share roughly the same orbit became known as the ‘Kuiper Belt’ and the rumblings began. For a short time the number of planets grew quickly; Ceres (below right) was named a planet, as was Juno, and Vesta (below left).

Should they all be named planets? If not, why should Pluto get special recognition? What makes a planet a planet?


People who had grown up with the seemingly indisputable fact that there were 9 planets in the solar system were upset at the prospect of losing one, and it became quite a controversy when some museums and planetariums chose to say there were only 8 planets in the Solar System in their displays!

In 2006 the International Astronomical Union had the final say on the matter, issuing an official definition for a ‘planet’. It states that a celestial body needed to meet three conditions to be considered a planet in our solar system:

1. The object must be in orbit around the Sun.
2. The object must be massive enough to be rounded by its own gravity.
3. It must have cleared the neighbourhood around its orbit.

Which of these conditions do you think Pluto meets?

Click to reveal answer!

1. YES - Even though Pluto’s orbit is more elliptical than planets like Earth and Mars and Neptune, it does still orbit the Sun.
2. YES – Pluto, unlike Vesta shown above, is almost spherical in shape.
3. NO – Pluto’s mass is only 0.07 times that of the mass of the objects in its orbit. (Earth’s mass is 1.7 million times the rest of the mass in its orbit.)

The IAU also decided that bodies that met criteria 1 and 2, but not 3, would be named dwarf planets. As such, Pluto is now a dwarf planet.

And that’s the story of how Pluto was discovered, reigned as a planet for 76 years, and was subsequently demoted to dwarf planet, all whilst remaining exactly the same… But no matter what it’s called, Pluto will always have a special place in our history!

So... how much do you know about Pluto?
Question 1: Why did they predict there was another planet further away from the Sun than Neptune?
correct: Neptune’s orbit was perturbed
incorrect: There was a prophecy
incorrect: They found a photograph of it
Question 2: Who discovered Pluto?
correct: Clyde Tombaugh
incorrect: Percival Lowell
incorrect: Constance Lowell
Question 3: What name was used for Pluto during the search for it?
correct: Planet X
incorrect: Minerva
incorrect: Cronus
Question 4: What piece of equipment was used to compare the photos of Pluto in the night sky, leading to its discovery?
correct: Blink comparator
incorrect: Telescope
incorrect: Calculator
Question 5: Which of the below is not an alternate name for Pluto?
correct: Hades
incorrect: Whiro
incorrect: Yama
Question 6: What is the collective name for the objects which share roughly the same orbit as Pluto?
correct: Kuiper Belt
incorrect: Asteroid Belt
incorrect: Oort Cloud
Question 7:How many conditions need to be met for an object to be categorised as a planet?
correct: 3
incorrect: 4
incorrect: 2
Question 8: Which condition for planetary status does Pluto not meet?
correct: It must have cleared the neighbourhood around its orbit
incorrect: The object must be in orbit around the Sun
incorrect: The object must be massive enough to be rounded by its own gravity
Question 9: According to the IAU definition, can other stars have ‘planets’?
correct: No
incorrect: Yes
Question 10: In what year was Pluto classified as a dwarf planet?
correct: 2006
incorrect: 1930
incorrect: 1990