From Planet to Dwarf: Why Is Pluto Not a Planet?

The decision to reclassify Pluto was based on evolving definitions and criteria for what constitutes a planet. Here’s a brief overview of why Pluto is no longer considered a full-fledged planet.


In the vast expanse of our solar system, Pluto had long held its place as the ninth planet, a status conferred upon it since its discovery in 1930 by astronomer Clyde Tombaugh. For decades, it was considered one of the familiar planetary siblings, joining the ranks alongside Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. This arrangement of planets provided a sense of order and familiarity in our cosmic neighborhood.

However, the story of Pluto’s planetary identity took an unexpected turn in the early 21st century. The catalyst for this shift was the International Astronomical Union (IAU), the authoritative body responsible for classifying celestial objects. In 2006, the IAU introduced new criteria, marking a significant departure from the historical classification system. This reevaluation was not an arbitrary decision but a response to evolving scientific understanding and the need for precise definitions in the field of astronomy.

In this article, we will explore the reasons behind Pluto’s transition from the prestigious status of a planet to its current designation as a dwarf planet. This shift in classification not only altered the way we perceive Pluto but also served as an emblematic example of how science continually refines its understanding of the universe.

Explanation of the New Planetary Classification Criteria

In the early 21st century, a significant revision of the criteria used to classify celestial objects as planets was introduced by the International Astronomical Union (IAU). This revision was not prompted by caprice but was a response to the need for more precise definitions in the realm of astronomy. The updated criteria aimed to provide a clearer and more standardized framework for classifying objects within our solar system.

Under these revised criteria, three fundamental conditions were established for an object to qualify as a planet:

  1. Orbiting the Sun: The first criterion requires that the object in question must orbit the Sun, just like other celestial bodies within our solar system. This condition underscores the solar-centric nature of our planetary system.
  2. Sufficient Mass and Spherical Shape: The second condition necessitates that a celestial object must possess enough mass for its self-gravity to overcome rigid body forces, allowing it to assume a nearly round or spherical shape. This condition reflects the principle that planets are substantial bodies that have gravitationally molded them into rounded forms.
  3. Clearing the neighbourhood: The third condition is perhaps the most pivotal and relevant to the reclassification of Pluto. To be considered a planet, an object must have “cleared the neighborhood” around its orbital path. In essence, this means that a planet should be gravitationally dominant in its vicinity and not share its orbital zone with other bodies of comparable size. This condition helps distinguish planets from other celestial objects that may inhabit the same region of space.

In the following section, we will delve further into Pluto’s qualifications, or lack thereof, based on these criteria, ultimately shedding light on why Pluto is no longer considered a planet.

The Kuiper Belt is a region of the solar system beyond Neptune that contains many small, icy objects, including Pluto. It is often referred to as the “third zone” of the solar system, after the inner terrestrial planets and the outer gas giants. Pluto’s presence within the Kuiper Belt and its orbital characteristics were factors that led to its reclassification as a dwarf planet by the IAU.

Pluto’s Shortcomings

Meeting the First Two Criteria

As we delve into the reevaluation of Pluto’s planetary status, it’s essential to recognize that Pluto did indeed meet the first two criteria outlined by the International Astronomical Union (IAU) in its new classification framework. Let’s examine these criteria and Pluto’s adherence to them:

  1. Orbiting the Sun: Pluto, like other celestial objects in our solar system, follows an orbital path around the Sun. This fundamental requirement was fulfilled, as Pluto is part of our solar family and orbits the same central star.
  2. Sufficient Self-Gravity: Pluto also exhibited sufficient self-gravity to overcome rigid body forces and assume a nearly round shape. This spherical configuration is characteristic of planets and celestial bodies that have achieved gravitational equilibrium. Pluto’s ability to meet this criterion was not in question.

Falling Short on the “Clearing the Neighborhood” Criterion

Pluto’s inability to meet the third IAU criterion was the key factor in its reclassification as a dwarf planet:

  1. Clearing the Neighborhood: This condition states that a planet must have “cleared the neighborhood” around its orbital zone. In simpler terms, a planet should be gravitationally dominant in its immediate vicinity, and it should not share its orbital space with other celestial bodies of comparable size.

Pluto’s downfall in retaining its planetary status primarily stemmed from this criterion. Unlike the larger planets in our solar system, such as Jupiter and Earth, Pluto’s orbital path takes it through a region known as the Kuiper Belt. The Kuiper Belt is a vast expanse filled with numerous smaller celestial objects, some of which are similar in size to Pluto. This meant that Pluto did not stand out as gravitationally dominant in its orbital neighborhood.

The IAU’s rationale for introducing this criterion was to ensure that planets have a decisive gravitational influence within their orbital zones, shaping their surroundings and maintaining orbital order. Pluto’s inability to meet this criterion underlines the unique and evolving nature of celestial classifications and the need for precise definitions in the field of astronomy.

NASA’s New Horizons spacecraft conducted a flyby of Pluto in July 2015. This historic mission provided valuable data and images of Pluto and its moons, helping scientists learn more about this distant world. The mission revealed detailed information about Pluto’s surface features, atmosphere, and geological characteristics.

The Decision to Reclassify Pluto

Overview of the Historic Vote by the IAU

The reclassification of Pluto from the status of a planet to that of a dwarf planet was not a unilateral decision but the result of a historic vote conducted by the International Astronomical Union (IAU) in 2006. This significant event marked a turning point in our understanding of celestial classifications. Let’s delve into the key aspects of this decision:

  1. The IAU’s Responsibility: The IAU is an authoritative body in the field of astronomy, responsible for establishing standards and definitions for celestial object classifications. It convened to address the evolving understanding of our solar system and the need for more precise criteria for planetary designations.
  2. The Debate and Deliberations: Prior to the vote, astronomers, scientists, and experts engaged in extensive debates and deliberations regarding Pluto’s status. The question at hand was whether Pluto should continue to be recognized as a planet or be reclassified due to the revised planetary criteria.

The Outcome: Pluto’s Reclassification as a Dwarf Planet

The culmination of these deliberations was the decision made by the IAU during their 2006 assembly. The outcome of this decision was twofold:

  1. Reclassification: By a majority vote, the IAU officially reclassified Pluto as a dwarf planet. This decision was based on the assessment that Pluto did not fulfill the “clearing the neighborhood” criterion. As a result, Pluto transitioned from being the ninth planet in our solar system to its new designation as a dwarf planet.
  2. Impact and Implications: The reclassification of Pluto had far-reaching implications, not only in terms of its status but also in how we perceive celestial objects in our solar system. It triggered discussions about the evolving nature of scientific understanding and the need for precise definitions in astronomy.

The decision to reclassify Pluto was met with a mix of support and controversy. While some applauded the IAU’s commitment to scientific rigor, others lamented the demotion of a celestial object that had held a cherished place in our cosmic family for decades. This decision continues to be a point of interest and debate among astronomers and the broader public, highlighting the dynamic nature of scientific knowledge.

Pluto is not the only dwarf planet in the solar system. Several other objects in the Kuiper Belt and beyond have been classified as dwarf planets. Some examples include Eris, Haumea, Makemake, and Ceres. These objects are comparable to Pluto in that they meet the IAU’s criteria for dwarf planet classification.


In closing, the journey of Pluto’s reclassification from a planet to a dwarf planet is a testament to the evolving nature of scientific understanding. This process has unfolded over decades and has involved a series of significant developments that have reshaped our perception of celestial objects within our solar system.

Pluto’s initial classification as the ninth planet in 1930 marked a milestone in the exploration of our solar system. For many years, it held a cherished place in the lineup of planets, capturing the imagination of astronomers and the public alike.

The International Astronomical Union’s (IAU) introduction of new planetary classification criteria in 2006 brought a shift in perspective. These criteria aimed to provide precise definitions for celestial objects and necessitated a reevaluation of Pluto’s status based on scientific rigor.

FAQs Why Is Pluto Not a Planet?

How has our understanding of Pluto evolved since its discovery?

Our understanding of Pluto has evolved significantly since its discovery in 1930. Initially, it was considered the ninth planet in the solar system. However, advancements in telescopes and space exploration missions, such as New Horizons, have provided detailed data about Pluto’s composition, surface features, and environment. This information has led to a more nuanced classification of Pluto as a dwarf planet and a deeper understanding of its place in the solar system.

Is there ongoing research or exploration related to Pluto?

While there are no current missions specifically targeted at Pluto, ongoing astronomical observations and studies continue to advance our understanding of this dwarf planet and the Kuiper Belt region. Scientists also study data from the New Horizons mission to gain further insights into Pluto’s properties and its place in the solar system.

Why isn’t Pluto considered a planet anymore?

Pluto is no longer considered a planet primarily due to a redefinition of the term “planet” by the International Astronomical Union (IAU) in 2006. According to the IAU’s new definition, a celestial body must meet three criteria to be classified as a planet. Pluto met the first two criteria (orbiting the Sun and having sufficient mass for a nearly round shape) but failed the third criterion, which requires it to have cleared its orbital zone of other debris. Pluto shares its orbital zone with other objects in the Kuiper Belt, so it was reclassified as a dwarf planet.