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The Discovery of Pluto

Using Slooh’s Online Telescope and integrated Quest learning activities, you can create your own unique timelapse of Pluto and learn more about its discovery. In The Footsteps of Clyde Tombaugh is one of 60+ curriculum-aligned STEM Quest learning activities on Slooh for students 4th grade to college.

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The Discovery of Pluto

Clyde Tombaugh's Early Life and Career

Clyde Tombaugh, son of farmer Muron Dealvo Tombaugh, had his early college education hopes dashed when a hailstorm crushed the farm's crops. This setback, however, did not stop the young and astronomically keen Clyde Tombaugh from building telescopes and embarking on a career of observing planetary and minor planetary bodies. Expecting only to receive helpful critiques, his submissions of observational drawings of planets Jupiter and Mars to the Lowell Observatory paid off when he was offered a job in 1929.

Clyde Tombaugh, discoverer of Pluto here shown with his homemade 9-inch telescope. Source: Wikipedia

While employed at Lowell from 1929 to 1945, Clyde didn't just make his historic discovery; he also worked towards obtaining a master’s degree in astronomy at the University of Kansas and, during World War II, taught U.S. Navy personnel navigation skills. His passion, though, was the search for planets. Luckily for him, he was working at a time when astronomical observations were aided by photography on large glass plates, so dim objects could be detected more easily than ever before.

Discovering Pluto

After Neptune was discovered in 1846, astronomers Percival Lowell and William Pickering realized that there must be a body beyond Neptune causing anomalies in Uranus’ orbit. They called this unknown object Planet X, and Lowell established his observatory to try to find this mystery planet. Tombaugh's task, once he joined the staff at the Lowell Observatory, was to photographically search the ecliptic, the apparent path in the sky that the Sun follows over the year, for this new planet.

The young astronomer spent his nights guiding Lowell's 13-inch telescope while exposing photographic glass plates for hours at a time. Each morning, he processed the plates before comparing each one with plates of the same area of the sky taken several nights before. He would then 'blink' these two plates on a device known as a blink comparator. 'Blinking' is the process in which the device switches between two photographs rapidly so that any moving object still within the magnifying lens' field of view may be detected by its apparent movement. If movement is seen, it infers that a celestial object must be closer to Earth than the stars beyond the solar system. Therefore, the detected body could be a planet or a minor planet, as most stars will show no perceptible movement in the time between photographs.

On the 18th of February, 1930, Tombaugh noticed such a movement between two photographic plates he'd taken on the 23rd and 29th of January that year. Subsequent observations allowed him to determine its orbit, which was confirmed to be beyond the orbit of Neptune and near the area of the sky where its presence had been predicted. Clyde Tombaugh had discovered Pluto! Interestingly, Pluto was not the cause for Uranus’ orbit. Upon further study, astronomers found that Neptune was large enough to account for the entire disturbance, and Pluto was too small to have a noticeable effect on Uranus. If they had realized that earlier, Pluto may not have been discovered as quickly as it was!

Tombaugh compared his photographic plates using this blink comparator. Source/Credit: Wikipedia, Wikimedia/Taken by user Pretzelpaws with a Casio Exilim EX-Z750 camera.

The name Pluto was officially adopted on May 1, 1930. This name was suggested by English schoolgirl Venetia Burney because it was the name of the Roman god of the underworld, who was able to render himself invisible as Pluto long had been. Pluto's first two letters also honor the observatory's long-since-dead founder, Percival Lowell.

On August 24, 2006, the International Astronomical Union (IAU) reclassified Pluto, grouping it with two similarly sized "dwarf planets" rather than with the eight "classical planets". A dwarf planet is neither a planet nor a natural satellite. It is in direct orbit around a star and is large enough to be roughly spherical, but it is not massive enough to clear its orbit of other material, and, therefore, cannot be defined as a planet, although Pluto does remain the largest object in the Kuiper belt.

Further Discoveries

Not content with his discovery of Pluto, Tombaugh continued searching for another planet. Before and after his Pluto discovery, he observed hundreds of asteroids. Tombaugh was credited with the discovery of fourteen of these asteroids. He was also interested in UFO observations, claiming to have observed three unexplained phenomena. Although he thought it was unlikely that they were extra-terrestrial, he kept an open mind as to their nature, encouraging fellow scientists to also always consider all possibilities.

New Horizons

NASA's Pluto/Kuiper Belt Mission, New Horizons, launched in 2006. It used a gravity assist slingshot maneuver around the gas giant Jupiter in 2007 and finally reached Pluto in July 2015. New Horizons investigated Pluto, and its group of five tiny moons, in unprecedented detail. After that, it continued to study other Kuiper Belt objects. Now, over 90 years after its discovery, decades after the death of its discoverer, and years after Pluto's demotion from planet to dwarf planet in 2006, we have finally visited this tiny world, which is only one-sixth of the mass of Earth's Moon.


More About Slooh's In the Footsteps of Clyde Tombaugh Quest

In this Quest, you will use Slooh's telescopes at the Canary Islands or Chile observatories to capture the dwarf planet Pluto. You'll follow in the footsteps of Clyde Tombaugh by using the same techniques he used to identify this tiny world in 1930.

Along the way, you'll learn more about Clyde Tombaugh and the now demoted dwarf planet Pluto. You'll then use Slooh's time-lapse animation tool to create an animated GIF showing the tiny, frozen world as it moves against the Milky Way galaxy's rich backdrop of stars.

Learning Objectives

By the end of this Quest, students will be able to answer the following questions:

  • What is Pluto, and who discovered it?

  • What technique was used to discover Pluto?

  • When was Pluto reclassified as a dwarf planet, and what does that mean?

Vocabulary Words

Standards Addressed


RST.9-10.1, RST.9-10.2, RST.9-10.3, RST.9-10.4, RST.9-10.5, RST.9-10.6, RST.9-10.7, RST.9-10.10, WHST.9-10.2D, WHST.9-10.2E, WHST.9-10.F


​HSN.Q.A.1, HSN.Q.A.2

Related Slooh Quests

  1. In the Footsteps of Kepler - Planetary Motion

  2. Solar System Explorer

  3. Stars Like Ours

About Slooh’s Astronomy NGSS Aligned Learning Activities

Slooh’s Online Telescope is a learning platform designed to support any educator in teaching astronomy to meet NGSS requirements by collecting and analyzing real-world phenomena. No previous experience with telescopes is necessary to quickly learn how to use Slooh to explore space with your students.

You can join today to access Slooh's Online Telescope and all 60+ Quest learning activities if you are able to make astronomy a core subject of study for the semester or year. If you only have a few weeks to study astronomy, we also have a curriculum designed to fit your busy academic schedule and budgetary limitations. To learn more about our offers, click here.