Pluto and the Kuiper Belt

By: Julie Watt, Peter Meimaris, Rafaele Rigolo, Sanwar Hossain, Evan Leier

Pluto’s discovery in 1930 by Clyde W. Tombaugh brought about a revolutionary shift in the way astronomers viewed our solar system.1 For nearly a century afterwards, Pluto was considered to be the ninth planet in our solar system. However, with the discovery of QB1 in 1992, as well as Sedna, and then Eris in 2005, scientists were forced to re-evaluate their position in regards to Pluto’s planetary status.2

Pluto’s change of classification from a planet to a dwarf planet, as well as constant new discoveries about the objects in the Kuiper Belt, demonstrate that the constant re-evaluation of established scientific facts is important to the advancement of our understanding of the universe we inhabit.


The Kuiper Belt

Kuiper Belt Illustration Courtesy of Space Facts/Laurine Moreau


The Kuiper Belt is an icy area located in the outer regions of our solar system. The Kuiper Belt is 20 Astronomical Units (AU) wide and can be found past Neptune and up to 50 AU away from the Sun. The Kuiper Belt is home to dwarf planets, icy celestial bodies, traveling comets, and a mixture of icy light hydrocarbons. While scientists have currently discovered over a thousand astronomical objects in the Belt that span over 100 km in diameter, it is estimated that there are over 100,000 objects in the Belt that fill this category. 1

Though the discovery of Pluto in 1930 was significant in its own merit, the discovery of the Kuiper Belt – a breakthrough which can be traced back to the discovery of Pluto – is an example of how one scientific discovery can be a gateway to larger realizations of the universe around us.

  • Pluto was the first object discovered in the Kuiper Belt. Clyde W. Tombaugh discovered Pluto after Percival Lowell theorized the existence of a planet that exerted some gravitational influence on Uranus and Neptune that caused the wobbles in their orbit. 1
  • Due to Pluto’s size, scientists questioned whether it could truly have so much influence on the outermost planets. After Pluto’s discovery, the mystery of whether undiscovered larger planets existed beyond Pluto remained. 1
  • In 1951, Gerard Kuiper speculated about the existence of a disc where the material was too widely dispersed to form planets, but was condensed into smaller bodies, resulting in a belt. He also predicted that some of these objects would wander into the inner solar system to become comets. The Belt is named the Kuiper Belt after Gerard Kuiper. 1
  • In 1980, Julio Fernandez proposed that to account for the number of comets discovered by scientists, there must be a comet belt, or something of that nature, around 35-50 AU from the Sun. 1
  • In 1988, a team of Canadian astronomers made up of Martin Duncan, Tom Quinn, and Scott Tremaine, determined that the Oort cloud does not account for all the comets that were being recorded by scientists, the short orbit comets in particular. 1
  • In 1992 came the first discovery of a Kuiper Belt object since Pluto and its respective moons with the finding of QB1. David Jewitt and Jane Luu had been searching the outer solar system for 5 years. Using the Mauna Kea observatory at the University of Hawaii, they finally found success with the discovery of QB1. 2
  • Eris was discovered in 2005, and was once thought to be larger than Pluto. While it is smaller in volume, it has 27% more mass than Pluto. This discovery was one of the stepping stones to the reclassification of Pluto. 2

Kuiper Belt and Solar System Courtesy of Don Dixon

The discovery of Pluto led to a significant rise in interest about the surrounding area. Many scientists hoped to find answers for the astronomical mysteries within the outer solar system, and hoped that future discoveries would aid in their understanding of the universe. Many of the discoveries made in the Kuiper Belt were preceded by computer simulations, which provided a clear starting point for the observation of the objects found in the belt.



Pluto as seen by New Horizons Courtesy of NASA

Pluto is a dwarf planet that resides in the Kuiper Belt; an icy area found in the outer regions of our solar system. Pluto is made up of ⅓ ice and ⅔ rock. Of the 5 moons that revolve around Pluto, Charon is the biggest. Pluto and Charon were found to be in a binary orbit system, with the center of gravity slightly off of Pluto. A day on Pluto is about 6.5 days/nights on Earth. Pluto’s orbit around the Sun takes 248 Earth years. Earth’s air would freeze on Pluto and since it has one-fifteenth the gravity of Earth, a person who weighs 100 pounds on Earth would weigh about 7 pounds on Pluto. 3

The discovery of Pluto was a big step in understanding our solar system, and through their initial observations the scientific community agreed that Pluto was a planet, as there was no alternative at the time.  However, that initial discovery and classification is only the beginning of Pluto’s story.

  • Many years passed after the discovery of Pluto. It seemed to be alone at the edge of the solar system with no observed planets beyond it. However, astronomers James Christy and Robert Harrington noticed a bump with Pluto in pictures. That bump was determined to be Charon, which was named Pluto’s moon in 1978. 3
  • Since Charon was only about half the size Pluto, further observations showed that both Pluto and Charon orbit around a common centre of mass, which was not Pluto itself. This introduced the idea of binary systems in the Kuiper belt, with scientists predicting that there would be more objects like Pluto and Charon in the Kuiper Belt. 4

    Pluto Orbit Courtesy of Unknown Artist

  • Pluto is known for its eccentric orbital path. In 1979, scientists found Pluto’s orbit intersected with Neptune’s when it became closer to the Sun than Neptune. 5
  • In 2005, the Hubble Space Telescope was used to discover Nix and Hydra, two of Pluto’s moons. Kerberos was found in 2011 and Styx in 2012. 5
  • New Horizons was launched to get closer to the Kuiper Belt and study more of the objects within it in 2006. 5
  • The IAU, in 2006, declared Pluto to be a dwarf planet along with Eris and Ceres. 5

At the start of the twentieth century, scientists had begun to question whether there were other objects similar to Pluto near the edge of the solar system. Years later, and in relatively quick succession, scientists found that Pluto was not alone at the edge of the solar system, starting with the discovery of QB1. The number of planetary objects there has changed from 1 to a theoretical 70,000 in the span of the past century alone. 5

Pluto and Moons Courtesy of ZME Science


Pluto’s Change in Status

With all the new developments at the end of the 20th and the beginning of the 21st century, one thing became clear: astronomers needed to take a fresh look at the edge of our solar system. If the definition of a planet was not re-evaluated, we may have had many new planets added to our solar system. The discovery of Eris by astronomers was one of the main reasons to consider whether Pluto should be demoted. A committee was created for this reason, and astronomers deliberated the nomenclature with which to define a planet. In 2006 at the IAU General Assembly Meeting in Prague, a decision was reached for the definition of a dwarf planet within our solar system. 6

  1. A planet orbits around the sun.
  2. A planet’s gravity is enough to become spherical.
  3. A planet needs to have “cleared the neighbourhood” around its orbit.

Pluto and Eris Courtesy of Northern Arizona University

The 3rd criterion is where Pluto, as well as other dwarf planets, fall short. Pluto’s orbit intersects with many other objects in the Kuiper belt, and therefore it has not “cleared the neighbourhood”. In comparison to Pluto being only 0.07 times the mass in its orbit, Earth is 1.7 million times the mass of the objects in its orbit. The more mass a planet has, the more gravity it has in its respective orbit. Eris is one of ~70,000 other icy bodies in the Kuiper belt that rival Pluto in size. 7 That is why Pluto never fit in with the other planets: it belonged to this other group of outcasts. After heated debate, astronomers officially voted to demote Pluto to a dwarf planet in 2006. 8 With new technology advancing the field of astronomy, astronomers are constantly revising what they believed about the Kuiper belt. From 1930 to 1992, Pluto was considered a planet because nothing else was found beyond Neptune’s orbit.

Science is the process of gaining more and more information about the universe in which we live. With this idea in mind, classifying Pluto as a planet in the 20th century made sense because there was nothing telling astronomers otherwise. With many years and the developments of new technologies, astronomers rethought Pluto’s original status of ‘planet’ and more accurately reclassified as a Pluto dwarf planet.


Dwarf Planets

“A dwarf planet is an object in orbit around the Sun that is large enough (massive enough) to have its own gravity pull itself into a round (or nearly round) shape. Generally, a dwarf planet is smaller than Mercury. A dwarf planet may also orbit in a zone that has many other objects in it. For example, an orbit within the asteroid belt is in a zone with lots of other objects.” 6 After the discovery of Eris and the classification of Pluto and Eris to dwarf planets, Ceres was also added to this family. Makemake and Haumea were also confirmed to be dwarf planets that orbit the Sun. When a new celestial body is discovered in the Kuiper Belt, astronomers work hard to determine as many of its characteristics as they can. A dwarf planet is nearly spherical and orbits the sun.

Pluto’s reclassification was considered a demotion by many; and people were shocked to see the ninth planet struck from the record. Rather than a demotion, our scientific knowledge evolved and this is demonstrated by the reclassification.  Mike Brown says it best during an interview: “Pluto would never have been called a planet if it were discovered today, if we knew about the eight planets and then, just now, started going out and discovering objects in the Kuiper Belt. Here’s a thought experiment. Imagine that Pluto doesn’t exist. Then in 2005 I discover Eris, which is essentially the same size as Pluto. Not a single person on Earth, I suspect, would suddenly jump and say, ‘You just discovered a planet.’” The new discoveries of many aspects of the Kuiper Belt put Pluto in this odd position by having been discovered earlier than expected. 9

Technologies used to observe the Kuiper Belt?

As Pluto was slowly studied with constantly improving technology, our understanding of Pluto followed that advancement. There were many different forms of technology used, but the focus should fall on the major ones.

A device known as a blink comparator is used in researching celestial bodies in the Kuiper Belt, this device rapidly flips back and forth between two photographs. Stars and galaxies essentially remain unmoving in the images, but anything closer could be visually identified by its motion across the sky.  Tombaugh spent approximately a week studying each pair of photographs, some of which contained over 150,000 stars, others nearly a million. Tombaugh noticed the difference between two photos taken a month prior on Feb 18, 1930. Lowell Observatory announced the discovery of a ninth planet on March 13 of that year after confirming that it was Pluto. 10 Scientists wanted to know more after this, and to look further outward towards the edge of our solar system required better technology. There were many other astronomical objects that could be better studied with an advancement.

Photographic Plates showing Pluto and Charon Courtesy of NASA/SSE

After half a century, Charon was discovered. On June 22nd, 1978, the USNO’s (United States Naval Observatory) Flagstaff telescope observed Charon because of a slight elongation off Pluto appearing periodically. The telescope used photographic plates, and is 1.55-meters. The plates dated back to 1965, with the same elongation. Pluto’s mass had to be recalculated due to this new discovery. 11 This discovery came with more questions, like was Pluto truly the only planet out there or is the Kuiper belt full of objects just like Pluto.

The search for finding Kuiper Belt Objects was ongoing, but accelerated after Charon’s discovery. The 2.2 m telescope used to find 1992 QB1 at the University of Hawaii took five years. Dave Jewitt and Jane Luu used the telescope in hopes of finding something beyond Pluto. 12 Astronomers quickly found many objects past Neptune and soon, Eris. Eris’ discovery by Mike Brown in 2005 was preceded by a survey of the solar system at the Palomar Observatory.13

Pluto and Charon Courtesy of Hubble Space Telescope

All of Pluto’s moons after Charon were found using the Hubble Space telescope. The Hubble overcomes the troubles of Earth’s atmosphere and gives clearer images of objects in the universe. The high-resolution pictures given by the telescope are used by scientists to make accurate predictions of composition of Planets and their satellites.14 Even with the Hubble’s images, scientists wanted to know more. They wanted to know the composition of the now dwarf planet and gain further knowledge of our solar system’s structures.

New Horizons, Pluto and Charon Courtesy of NASA

So, the probe New Horizons was sent into space on January 2006 to gather images of Pluto and its surroundings. In July 2015, New Horizons made its closest approach to Pluto. New Horizons was able to capture up-close and detailed images of the surface of Pluto that had never been seen before.

These photos revealed Pluto in more detail and scientists were surprised at how Pluto looked different from the predictions, with a lot more complex geography than expected. 15 The continued exploration of New Horizons will collect more information and may change the perspective astronomers have on many of the established models of thinking.

With the development of new technology comes new understanding. Without using new telescopes like the Hubble, we may never have found Pluto’s odd orbit and 2 other moons. Technology pushes forward knowledge and the search for patterns.


Newer Discoveries

As astronomers continue to study the solar system, patterns become apparent. The Kuiper Belt was merely theoretical until 1992. Following that were more discoveries of Kuiper Belt objects. Since 2006, other donut shaped icy debris belts have been found. These other Kuiper-esque belts are around nine other observed star systems. There are variances in the width of the belts, but this discovery shows that certain phenomena in our solar system are more common in the universe than we previously thought. 1

Another interesting hypothetical discovery is Planet 9, or Planet X, that is expected to have an elongated elliptical orbit. The object could be 10 times larger than Earth and 20 times farther from the Sun than Neptune. The size of this planet X would be similar to Neptune’s. The theory is based on mathematical modelling and computer simulations, which have shown themselves to be accurate in prior simulations of theoretical models. Mike Brown and Konstantin Batygin predict that the large object could account for the strange orbits of many, but at least 5, Kuiper Belt objects. 16


Now, and Into the Future

Although it took almost a century, Pluto has been reclassified as a dwarf planet along with four other celestial bodies in our solar system. This change was a result of years of technological advancement and reevaluation of the known facts of the time. Pluto was once thought to be alone, but was soon joined by its moons and eventually other dwarf planets challenging its position as a planet. Some people did not understand the decision, but it was a necessary step to help astronomers in their future findings.

The fact that Pluto’s very nature was unknown for several years goes to show that certain scientific facts can never be taken for granted. While the known facts pointed to a model scientists were confident was ideal, a flurry of discoveries changed that.  Therein lies the essence of the scientific method, a willingness to question what is taken for granted to arrive at a better understanding of the universe as we know it. Pluto was not proven to be a dwarf planet, it merely did not fit the definition of a planet, and that realization prompted many astronomers to consider what truly constitutes a planet.

Without looking for Pluto, we would not have the same knowledge of the Kuiper Belt as today. The Kuiper Belt was once thought to be just a belt, a sea of dirty ice. However, with a new set of eyes on the model, it was soon discovered that the shape is more like a torus.1 While this was confirmed by direct observation, much of the knowledge of the Kuiper Belt had to undergo the scientific method. For example, the realization that short orbit comets were not accounted for by the Oort cloud as originally thought was due to a team of scientists re-evaluating the facts considered to be true and testing it. The discovery of Eris was closely tied to Pluto’s fate. If scientists decided to leave the definition of a planet as it was before 2006, many new planets would have been added to our model of the solar system. This may have been more confusing to the public than the reclassification because many Pluto sized objects are predicted to be within the Kuiper Belt. Although the demotion may not make sense to people, it is a result of re-evaluating the knowledge of Pluto and taking new discoveries into consideration to classify Pluto with others like itself.

Furthermore, the discovery of icy debris belts in other systems shows that our solar system geography is rather common. The Kuiper Belt highlights how constant revision of current knowledge is, paradoxically, required for future discoveries. Scientific knowledge is constantly changing, from Aristotle’s ‘proof’ of a geocentric universe, through Kepler’s laws of planetary motion, and into our current understanding of quantum physics thanks to the research and ideas of scientists like Einstein and Planck. The patterns and natural laws of the universe are constant, but our understanding of them comes in small steps. Through diligent work in all aspects of astronomy, the steps do indeed come, and science ultimately moves forward.



1Matt Williams, What is the Kuiper Belt, WWW Document, Accessed on March 9, 2017, (

2Nola Taylor Redd, Kuiper Belt Objects: Facts about the Kuiper Belt & KBOs, WWW Document, Accessed March 7, 2017, (

3Flint Wild, What is Pluto?, WWW Document, Accessed on March 9, 2017, (

4David Jewitt, Binary Kuiper Belt Objects, WWW Document, Accessed on March 10, 2017, (

5Erik Gregerson, 10 Important Dates in Pluto History, WWW Document, Accessed on March 10, 2017, (

6International Astronomical Union, Pluto and the Developing Landscape of Our Solar System, WWW Document, Accessed on March 9, 2017, (

7Fraser Cain, Why is Pluto No Longer a Planet, WWW Document, Accessed on March9, 2017, (

8Marcus Woo, How Pluto is Changing Out Understanding of the Solar System, WWW Document, Accessed on March 9, 2017, (

9Mike Wall, The Man Who Killed Pluto: Q & A with Astronomer Mike Brown, WWW Document, Accesed March 10, 2017, (

10Nola Taylor Redd, Clyde Tombaugh: Astronomer Who Discovered Pluto, WWW Document, Accessed March 7, 2017, (

11Matt Williams, Charon: Pluto’s Largest Moon, WWW Document, accessed March 16, 2017, (

12What Lurks in the Outer Solar System?, WWW Document, Accessed March 16, 2017, (

13Eris: In Depth, WWW Document, Accessed March 28, 2017, (

14Nora Taylor Redd, Hubble Space Telescope: Pictures, Facts & History, WWW Document, Accessed March 16, 2017, (

15New Pluto Images from NASA’s New Horizons: It’s Complicated, WWW Document, Accessed March 17, 2017, (

16Hypothetical ‘Planet X”: In Depth, WWW Document, Accessed March 10, 2017, (