Have you ever wondered why Pluto is no longer considered a planet?

This blog takes you on a journey through the fascinating history of Pluto — from its discovery in 1930 to the advancements in telescope technology that challenged its planetary status. Along the way, you'll learn how Pluto’s reclassification helped scientists better define what it means to be a planet and led to the creation of the “dwarf planet” category. With photos that span nearly a century, we’ll explore how Pluto captured our imagination, sparked scientific debate, and continues to teach us about our ever-evolving solar system.

2015 Image of Pluto taken by NASA's New Horizon spacecraft. 

Introduction to Pluto's Story

Clyde Tombaugh’s discovery of Pluto in 1930. Notice the white arrow that points to Pluto and its movement against the background of stars much farther away (over the course of about a week). 

Have you ever wondered why Pluto was once considered a planet - then wasn’t? Did you know Pluto had the rollercoaster ride when it comes to its planetary status? In this blog, we’ll dive into the fascinating history of Pluto, explore why it was considered a planet in the first place, and learn some fun facts along the way- like how Pluto hasn’t even made one full orbit around the Sun since its discovery in 1930! (That won’t happen until the year 2178). 

Let's begin with how Pluto first earned its planetary title. In the early days of planetary astronomy, scientists were curious about possible unseen planets affecting the orbits of the known planets. The hunt was on, and in 1930, Clyde Tombaugh discovered Pluto by comparing two images of the night sky to spot tiny differences (using a blink comparator to see changes in any object over the course of a few days). Even with the best technology of the 1930s, Pluto only appeared as a small dot- but that tiny dot was enough to earn the title of the ninth planet. Soon, however, scientists in the late 1900s began to notice some issues… and Pluto’s planetary future would start to wobble. Before we get to the big “demotion,” let’s pause and look at some cool facts about Pluto!

Quick Facts on Pluto 

Discovery of Pluto’s Moon Charon in 1973 (on this up close examination, notice the bulge on the left negative that only appears every so often, indicating an orbiting moon).

As you can see in the early photos, Pluto from Earth is very, very small- with an average distance of 3.7 billion miles from the Sun. That’s more than 39 times further than Earth, and with those distances comes some extreme cold, to nearly 400 degrees (F) below zero! Here are some other facts about Pluto. In 1973, Pluto was discovered to have a moon. Technology has not improved much from the 1930s, but telescopes now were able to detect a slight bump when imaging Pluto, which turned out to be a really large moon that orbited closely (so close that 1970s telescopes could not “resolve” Pluto and its newly discovered moon into separate spheres).  This is the largest moon - planet combo in the solar system! Charon (Pluto's moon) is so large that they actually orbit around a point between the two. Still, in the 1980s and 1990s, Pluto was considered a planet, but that will change here real soon! As Hubble started taking better photos of Pluto, scientists discovered that Pluto actually had more moons (5 total!). That was not all, other “planets” were soon discovered, and this forced scientists to look at the rules that make up planetary status. Let's look at those rules. 

What makes a Planet... a Planet? Let's get to the bottom of this demotion. 

1994 Hubble Space Telescope image of Pluto and moon Charon- resolved into two different spheres. Notice how close they are to each other! The Hubble Space telescope was able to take much clearer images since it did not have to peer through Earth’s atmosphere when imaging. 

It didn’t take long before new rules had to be established on what makes a planet a planet. Teams of scientists had been steadily discovering many Pluto-like planets in the outer Kuiper Belt. These planets were similar in size and orbit, so in 2006 the International Astronomical Union (IAU) came up with the three determining factors on what makes a planet a planet:

• - It must be an object which independently orbits the Sun (can’t be moons orbiting another planet) 

As you start to understand, Pluto does not meet the third determining criteria to be a planet. 

Let's Explore the Third Reason!

Diagram of Pluto and Eris (discovered in 2005). If you look at the diagram, you will notice something very interesting- the orbits of Pluto and Eris are tilted and stretched out compared to the flat orbits of the inner and outer planets. 

This discovery of more and more “planets” in the outer area of our solar system helped scientists realize that Pluto fits into a new category: Dwarf Planets. Why is this? Pluto cannot  clear or dominate its own orbit, just like the other dwarf planets that are found in the Kuiper belt (a region of our solar system found beyond the orbit of Neptune). Remember the third rule? 

If you compare Pluto, Eris, and other discovered dwarf planets, you will notice a big difference, and understand more about why they do not clear their orbit:

• - Planets like Earth or Jupiter are hundreds of thousands of times larger than anything else in their orbits
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• - Dwarf Planets reside in a space that also has many other icy rocks and small bodies orbiting the Sun 
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• - Since Dwarf Planets are so small, their orbits can change/ be affected by all of the icy rocks and other small chunks of debris orbiting the Sun 
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Many other dwarf planets have been discovered as well, all sharing the same stretched out, tilted, and chaotic orbits (unlike the nice, even orbits of the eight inner and outer planets). 

What sets Pluto apart from the other Dwarf Planets?

• In 2015, the New Horizons spacecraft visited Pluto on a flyby, and took some amazing photos!.

In 2015, after traveling for nearly a decade, NASA’s New Horizons spacecraft upgraded the quality of images we had of Pluto (going from grainy, fuzzy dots to up-close images of the complex world that makes up Pluto and its moons). New Horizons captured mountains of ice, glaciers, valleys and a thin atmosphere of nitrogen as it flew by. Interestingly enough, New Horizons was also tasked with exploring another Kuiper Belt object to gain new insights into how these building blocks of planets formed and survived in the most distant and remote areas of our solar system. Thanks to this mission- Pluto is no longer just a tiny dot in the night sky- it's a fascinating world set apart from all of the other dwarf planet discoveries. 

Pluto's Lasting Importance in Astronomy and Exploration 

This is a Panoramic view of Pluto’s icy mountains and flat ice plains. New Horizons image 15 minutes after its closest approach to Pluto. Note the distinct haze layers in Pluto’s atmosphere that can be seen backlit by the Sun. 

Though Pluto was reclassified as a dwarf planet in 2006, it has proven to help students and scientists alike to think about how science is always changing. When Pluto was demoted, many people had strong feelings about it! This just shows that it was a powerful example of how important scientific decisions are not made by one person- they are made from a peer review that involves discussions and consensus. In science, models are used to describe what we observe- and models can change! This teaches an important lesson: Science is not just a collection of facts, but rather a process of discovery. 

By studying Pluto, scientists have discovered a number of other dwarf planets and dwarf planet candidates.  Scientists have unlocked new understandings about the outer edges of our solar system and the Kuiper Belt. When new discoveries and evidence comes to light, ideas must sometimes change. This change and flexibility is a core strength of science- it means we are always improving our understanding of the universe! To conclude, even though Pluto is now solidly listed as a small dwarf planet, it will always have a big spot in our hearts! 

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