Pluto Discovery – A Story To Tell

Crossing the Borders: Discovery of Pluto

Story of Pluto discovery is truely a defining moment in astronomy: That incident opened a broader window to our Solar System exploration and ultimately allowed scientists to further understand how Solar System is formed and working. Nevertheless, this story is certainly an entertaining one when you find out how many little chances positioned Pluto to be discovered!

Almost 90 years before now, that was a thrilling news spread worldwide while the world was trying to recover from a world war and running for the next one. Those were the times when US was taking bigger steps on science and astronomy. And a young amateur American astronomer was about to make history.

Interested in ancient stories? Jump over! We will take you to a journey in the dustbin of history.

Still not interested? We did our best!
Jump down to answers of “Frequently Asked Questions” in the end of this article.

Background of Pluto Discovery

None of the discoveries in the history of science happens by snapping a finger. While systematic discovery of our neighbours in the sky has a history of hundreds of years, first step of direct discovery was probably one person raising the head up. She/he was probably just trying to understand what those shining things are..

Actually the history of science goes back to the collective memory of humanity, but this attitude could be ruining it. We don’t want that! 🙂

In any case we don’t need to go back that far since noone can see Pluto from Earth with bare eyes.

Distance between Pluto and Earth
That’s the reason. Photo source there

Discovery of Neptune – The First Step

However, we can easily go back to discovery of Neptune in 1846. Why would we do that? Because an important and new method was used for finding out new planets: Neptune was discovered by estimating its gravitational influence on Uranus orbit. And Neptune was found very close to where it was estimated to be with that practice.

So this was really remarkable because Newton’s gravitational laws were sufficiently working for planets as well!

New findings have always led to new questions in terms of science. And this time it was a simple one: Could there be more planets beyond Neptune?

The gas giant: Neptune. Source: NASA

Jacques Babinet was the first person to follow orbital path of Neptune and to search for similar disturbance. It was 2 years after the discovery and he had the chance to observe only a small portion of Neptune‘s complete orbit around Sun (which is 165 years). That’s why it was not that easy to conclude for him at that time. Although he estimated extremely bigger influence in the orbit, he still concluded that a much more bigger ninth planet was out there. In that aspect, he was substantially right.

Percival Lowell and Lowell Observatory In Search for Ninth Planet

Together with the end of 1800’s and beginning of 1900’s, more astronomers were interested in locating the next planetary target. And the name standing out in this search was Percival Lowell. He was an amateur astronomer and decided to build his own observatory in 1894, especially for observing Mars.

From 1906 till his death in 1916, Percival Lowell was mainly interested in perturbations on Neptune’s orbit. His calculations were indicating that there should have been a planet much more bigger than Earth farther than Neptune.

Percival Lowell
Year: 1916, Mr. Percival Lowell observing Venus. Source: Wikipedia

Percival Lowell died in 1916, but his legacy about Planet X was ready to be taken over!

Following Mr. Lowell’s death, the task and the observatory was stopped for a long time because of some legal issues. After 11 years in 1927, his brother A. Lawrance Lowell was luckily able to settle things down and made the observatory up and running. What is more, from 1927 to 1929, he was able to provide installation of a new telescope; which was going to be a part of planetary discoveries.

Clyde Tombaugh Steps In to Lowell Observatory

The observatory was back, funds were back and they had a new telescope. Time to cook the soup! It was time to initiate the program of finding Planet X.

Clyde Tombaugh
Clyde Tombaugh, discoverer of Pluto. Photo source

The staff started working on this, but in short while the board understood that this is a job to handle with better focus and decided that a dedicated person should better deal with this mission.

On January 1929, a 24-years-old farmer and an amateur astronomer named Clyde William Tombaugh applied to Lowell Observatory with his hand sketches which he adapted from his observations from his own telescope. Lowell management were impressed by his sketches and hired him for this specific job with a $125 monthly salary. You will see how efficient his price/performance was!

His duty was to focus on a specific range of area in the night sky, store sky images on photographic plates and compare them with bare eyes.

Children of Clyde Tombaugh
Mr. Tombaugh unfortunately passed away in 1997. On this photo: Alden and Anette Tombaugh are on the view. They are children of Clyde Tombaugh and were the special guests of New Horizons Spacecraft‘s closest flyby on 14 July 2015. The spacecraft travelled all the way to Pluto together with the ashes of Clyde Tombaugh. 
Photo source here, image credit: John Makely, NBC News

Discovery of Pluto Takes Place!

Before Clyde Tombaugh was hired and even before the Lowell Observatory was re-established, no human being including the astronomers themselves: Gustaf Stromberg and Nicholas U Mayall knew that Pluto was already imaged in 1925. That’s another interesting story..

While that story remained as a secret for everyone, Clyde Tombaugh was working on his night duties photographing the night sky based on the clues from Percival Lowell‘s calculations.

In less than a year, Tombaugh took two different photos of the night sky on 23 and 29 January where Pluto was smiling to the cameras! The night sky views were carved on photographic plates and those plates were on the shelves. Eventually someone had to locate if something was there!

That was another duty of Tombaugh and on 18 February 1930, he realized that he already got it previous month. That is Pluto!

Discovery photo of Pluto
Photos of two seperate photographic plates, proves discovery of planet Pluto. There are two stars visible in the middle of both images. Cursors point out Pluto on both. She moves from left to right side of those stars in 6 days. Photo source here

As Tombaugh located the new planet of the century, the board of observatory considered to observe the planet more and gather more information before making it public. Meanwhile they decided to wait until 13 March 1930, Percival Lowell‘s birthday to announce to the world that the 9th planet was discovered.

The announcement ceremony took place in Ashurst Hall where about 500 people were ready on their seats. Astronomer Carl Lampland was the person announcing the discovery. The hall was not very convenient for such audience and Lowell Observatory states as follows: “Unfortunately, few, if any, of the audience members heard this important message, since the soft-spoken Lampland could not be heard in the echo-filled, uncarpeted room”.

After The Pluto Discovery – A Newspaper Gallery

That was a big news, and the word was spread fast!

We have a collection of newspapers for you: Printed right after the discovery of Pluto. Sources for all images are seperately stated in the end of this article.

Pluto Discovery on Kerbocker
The next day of discovery.. Yeah, scientists initially thought Pluto was bigger than Earth 🙂
Discovery of Pluto - Newspaper
It did!
Pluto Discovery New York Times
The New York Times
True diameter of Pluto is 2.376,6 kilometers / 1.445 miles while its distance to Sun is 29.7 to 49.3 AU
Pluto discovery article
There are surprisingly big amount of proper statements about Pluto here!
Pluto discovery newspaper
Chicago Daily Tribune Announcing the New World
Pluto Discovery Article
Yeah, it is cold.. Below -200º C all the time!
Discovery of Pluto - Article
If someone wrote above today, we’d probably make fun of it.. It’s incredible how our knowledge about Solar System increased enormously in less than a century!
Pluto Discovery Chicago Daily
That’s it!
Coconino Sun
This one is from 21 March 1930
Another page from Coconino Sun

Scientific Background of Pluto Discovery

The reason it only took less than a year for Clyde Tombaugh to locate Pluto is known to be the legacy of Percival Lowell. He was a pioneer calculating the effects of a potential Planet X (that was the name of the “Wanted” planet those times). And his calculations were leading to a specific area where such planet should have approximately been on that phase of the orbit.

However, at this point there are some discrepancies: Pre-discovery calculations about Pluto did not quite match with reality. Percival Lowell’s calculations had an error margin hundreds of times more than the ones for Neptune. That was a huge difference, it worked well for Neptune after all!

How is that possible? Let’s find out.

Pluto was initially (beginning of 1900’s) estimated to have a mass 7 times bigger than Earth’s. Right after the discovery, it was stated that its mass is almost same with the mass of Earth. And now we know Pluto’s mass is 1/456 times of Earth.

Discovery of Pluto newspaper
Scientists can sometimes be wrong..

What is more, Pluto was about 10 times fainter than it was predicted and was located within 6 degrees from where it was estimated to be.

This Pluto fact leads to something else: Scientists are still looking for a much more bigger Planet X in the outer Solar System. And for sure besides any kind of calculation errors, what Percival Lowell tried to find was not Pluto itself. He was probably trying to locate another Planet X which might be the one scientists of this era are still working on. And this subject still remains as a mystery!

We can easily conclude that either Mr. Lowell was looking for another planet or his calculations were wrong! Pluto was discovered while another planet was searched. Pluto was discovered by luck!

How Was Pluto Discovered? The Method of Discovery

It wasn’t easy to be an astronomer on first half of 1900’s. Because they did not have computers, they did not have any complex electronic devices. What they only had were bare eyes, mechanical devices and modern science of that era. Can you imagine how hard it can be to locate a pale planet billions of kilometers away while you don’t exactly know where it is?

Clyde Tombaugh
Clyde Tombaugh with the handmade telescope he built before joining Lowel Observatory. Source: NASA

As human found different ways of photography, not much time lost before directing the lenses to the sky. And for sure thousands of people did it before Clyde Tombaugh. However as mentioned earlier, you can not see Pluto light with your bare eyes or take its photo with an ordinary machine.

The method for spotting Pluto was as follows:

  • Take pictures from different night skies on every possible day
  • Compare pictures from different days.
  • Than check if there is a light source moving or not.
Real picture of photographic plate from Pluto discovery
The plate from 23 January 1930, first photographic plate leading Pluto discovery. Here is the source of this photo. Credit: Smithsonian Institution

Because the stars are incredibly far from our home, you can not observe them shifting positions in few days. This enables astronomers tracking Solar System Objects easily.

On those two images from 23 and 29 January 1930, one can detect only a single light source moving and that was our naughty planet Pluto where rest of the stable light sources were of other stars. Pluto was moving extremely slow and this easily eliminated the possibility of being a nearby asteroid. That was certainly a Trans-Neptunian planet!

How Was Pluto Discovered? Technical Details About Photographic Plates

Let’s go further into details about Tombaugh’s work in search for Planet Nine.

Telescope used for Pluto discovery
This is the telescope used by Tombaugh for the discovery. Source&Credit: Lowell Observatory

As stated above, Tombaugh was reflecting the telescopic images to photographic plates and looking for signs of moving light sources on them.

We can call above mentioned mechanism a specialized microscope. That device provided user to make comparisons on photographic plates. Those plates were 36×43 centimeters wide and the exposures were taken by a telescopic camera with a 33 centimeter diameter lens.

For easy comparison of images on two different plates of different dates, Tombaugh had a device called blink comparator which let user flip mirrors of different images on another easily.

Blink Comparator
That’s the blink comparator used during discovery of Pluto. Photo source here, Credit: Smithsonian Institution

Discovery of Pluto and Its Consequences

Pluto was the first planet discovered 84 years after Neptune. And those were the days when we were not quite sure about Solar System formation. You can clearly see that from the statements on above pieces of newspapers.

Besides that, Pluto was the first planet detected beyond the gas giants, and it meant a lot: As scientists had more understanding, it was realized that Pluto was not alone in that area: There was a huge disk made of thousands of icy rocks which was later going to be called Kuiper Belt. And Kuiper Belt was going to tell a lot more about formation of Solar System.

Above all, knowing how coincidentally Pluto was discovered in the bed of stars and its effects on astronomical comprehension clarifies the importance of finding that small pale dot: PLUTO.

Kuiper Belt
Kuiper Belt is the smoke around the illustrated planet orbits. Source: NASA

Frequently Asked Questions

Here is the part where you can find quick answers to your basic questions about discovery of Pluto. Please let us know in case you have further questions.

  • When was Pluto discovered?
  • Discovery date of Pluto is 18 February 1930.
  • Who discovered Pluto?
  • Clyde William Tombaugh was the discoverer.
  • Where was Pluto discovered?
  • Discovery of Pluto took place in Lowell Observatory, Flagstaff, AZ. The observatory is still open for visiting.
  • How was Pluto discovered?
  • Pluto was discovered by observing a specific range of night sky view on different days, recording thoses views on photographic plates and comparing those views. As the background stars are basically “stable”, possibly moving light sources belong to the planets or asteroids. Since Pluto was far away and moving respectively slower, discovery of Pluto: a planet beyond Neptune was easily confirmed.


  • We aim to build a proper Pluto encyclopedia and your contributions are highly appreciated!
  • Any deficiencies on this article? Please type to below comment box or contact us from this link. We are ready to check and clarify.

Leave a Reply