Why Pluto Lost Its Planetary Crown

Why Pluto Lost Its Planetary Crown
Posted on: July 28th, 2023

The classification of Pluto as a planet has been a subject of debate among scientists since its discovery in 1930. In 2006, the International Astronomical Union (IAU) redefined the criteria for a celestial body to be considered a planet. According to their definition, a planet must:

  1. Orbit the Sun.
  2. Be massive enough for its gravity to make it roughly spherical in shape.
  3. Clear its orbit of other debris.

Pluto meets the first two criteria but does not meet the third, as it shares its orbit with other objects in the Kuiper Belt, a region beyond Neptune filled with small icy bodies. As a result, the IAU reclassified Pluto as a “dwarf planet.”

While some scientists still argue for Pluto\’s planetary status, the official classification by the IAU is that Pluto is a dwarf planet. The debate, however, highlights the complexities and nuances involved in defining celestial bodies.

Why all the fuss about Pluto?

Why not just keep Pluto as a planet since it has already been for so long? The problem is that if we let Pluto stay a planet, then many other dwarf planets would have to be considered planets as well. As of right now, there are 5 dwarf planets and many more contenders for dwarf planet classification. We could end up with as many as 70,000 planets.

Four out of the five known dwarf planets, including Pluto, are located in the Kuiper Belt, a region beyond Neptune’s orbit that is home to numerous celestial objects. It is highly likely that the Kuiper Belt contains even more objects that could be classified as dwarf planets, but they have not yet been discovered. Some have been but have yet to be classified. Let’s meet the dwarf planets.

The dwarf planets


Pluto, once considered the ninth planet in our solar system, was reclassified as a dwarf planet in 2006 by the International Astronomical Union (IAU). It is located in the outer solar system, specifically in the Kuiper Belt. Pluto was discovered in 1930 by American astronomer Clyde Tombaugh.

Pluto has a diameter of approximately 1,473 miles (2,370 kilometers), making it the largest known dwarf planet. Its orbit around the Sun is highly elliptical, taking about 248 Earth years to complete one full orbit. At times, Pluto’s orbit brings it closer to the Sun than Neptune, although this is a temporary arrangement in their respective orbital paths.

Pluto’s Surface

The surface of Pluto is composed of a mixture of rock and ice, with vast plains, mountains, and large glaciers. Nitrogen, methane, and carbon monoxide ice have been observed on its surface, giving it a reddish-brown color. Its thin atmosphere consists mainly of nitrogen, with traces of methane and carbon monoxide, and it undergoes seasonal changes as Pluto moves closer to or farther from the Sun.

Pluto has five known moons: Charon, Nix, Hydra, Styx, and Kerberos. Charon, the largest of the moons, is so massive compared to Pluto that the two are often considered a binary system, with their common center of mass located outside of Pluto.

NASA’s New Horizons spacecraft conducted a historic flyby of Pluto in July 2015, providing detailed images and valuable data about the dwarf planet’s surface, atmosphere, and moons. The mission greatly expanded our understanding of Pluto and revealed previously unknown features, such as vast plains, towering mountains, and evidence of past geological activity.

Despite the wealth of information gathered by New Horizons, much is still left to be discovered about Pluto. Future missions and ongoing analysis of the data from the New Horizons flyby will continue to enhance our understanding of this fascinating dwarf planet.


Ceres is the only dwarf planet situated in the inner solar system, specifically between Mars and Jupiter. If not for Jupiter’s immense gravitational pull, Ceres might have developed into a full-fledged planet. However, due to Jupiter’s influence, Ceres remains one of the largest objects within a belt of asteroids that share the same orbit around the Sun. Jupiter’s strong gravitational force prevented Ceres from accumulating enough surrounding asteroids to grow large enough to qualify as a planet.

Ceres, discovered by Italian astronomer Giuseppe Piazzi in 1801, was initially considered the first and largest asteroid before being reclassified as a dwarf planet by the International Astronomical Union (IAU) in 2006. As the largest object in the asteroid belt, Ceres has a diameter of about 590 miles (940 kilometers) and is composed of a mixture of rock and ice. Its mass accounts for roughly 40% of the total mass of the entire asteroid belt.


The surface of Ceres features a diverse landscape, including large craters, mountains, and plains. The Occator Crater is one of its most prominent features, containing bright deposits of sodium carbonate, which hint at past or ongoing geologic activity. Scientists speculate that Ceres might possess a significant amount of water beneath its surface, possibly as a layer of ice or even as a subsurface ocean. The potential presence of water raises questions about the dwarf planet’s ability to support life.

In 2015, NASA’s Dawn spacecraft arrived at Ceres, providing valuable data and images of its surface and improving our understanding of its composition and history. The mission ended in November 2018 when the spacecraft ran out of fuel. As scientists continue to analyze data from the Dawn mission and conduct further research, future missions to Ceres could reveal more about its composition, geological activity, and potential for water or life.


Eris, a dwarf planet located in the scattered disc beyond the Kuiper Belt, is one of the most distant known objects orbiting the Sun. Discovered in 2005 by a team of astronomers led by Mike Brown, Eris played a significant role in redefining the concept of a planet and led to the reclassification of Pluto as a dwarf planet.

Slightly smaller in diameter than Pluto at about 1,445 miles (2,326 kilometers), Eris is the second largest dwarf planet in our solar system. Its highly elliptical orbit takes approximately 558 Earth years to complete, with its distance from the Sun ranging between 38 to 98 astronomical units (AU). One AU represents the average distance between Earth and the Sun (about 93 million miles or 150 million kilometers). Eris has a rotation period of around 25.9 hours.

Eris has one known moon, Dysnomia, which was discovered in 2005. Much smaller than Eris, Dysnomia has an estimated diameter of about 93 miles (150 kilometers). Eris is primarily composed of rock and ice, and its surface is likely covered in a layer of frozen methane. The surface is relatively bright and reflective, possibly due to the presence of frozen methane.

There is a possibility that Eris has a thin atmosphere that could freeze and fall to the surface as the dwarf planet moves away from the Sun in its orbit. However, more observations and research are needed to confirm the existence and nature of such an atmosphere.

Our understanding of Eris is limited due to its distance and the lack of exploration in the outer solar system. Future observations and potential missions to the region could offer more insight into Eris and its characteristics.


Haumea is a unique and intriguing dwarf planet located in the outer solar system, specifically in the Kuiper Belt. It was discovered in 2004 by a team of astronomers led by Mike Brown, although another team led by José Luis Ortiz Moreno also claimed its discovery independently in 2005. Haumea was officially recognized as a dwarf planet by the International Astronomical Union (IAU) in 2008.

One of Haumea’s most distinctive features is its elongated, ellipsoidal shape, which is believed to be the result of its rapid rotation. The dwarf planet completes one rotation in about 3.9 hours, making it one of the fastest-rotating large objects in our solar system. This rapid rotation causes Haumea to stretch into its unusual shape.


Haumea’s dimensions are estimated to be approximately 1,430 miles (2,300 kilometers) for its longest axis and 620 miles (1,000 kilometers) for its shortest axis. The dwarf planet’s composition is thought to be primarily rock and ice, with a surface that is likely covered in crystalline water ice. This ice gives Haumea a bright, reflective surface.

The dwarf planet has two known moons, Hi’iaka and Namaka, which were discovered in 2005. Both moons are thought to be composed of water ice, similar to Haumea’s surface.

Haumea is also notable for its “family” of objects, which are smaller bodies in the Kuiper Belt that share similar orbits and physical characteristics. These objects are thought to be fragments that resulted from a collision between Haumea and another large body in the distant past.

Our understanding of Haumea is still limited due to its distance and the challenges of observing such distant objects. Further observations and potential future missions to the outer solar system could provide more insight into Haumea and its characteristics.


Makemake is a dwarf planet located in the outer solar system, specifically in the Kuiper Belt. It was discovered in 2005 by a team of astronomers led by Mike Brown, and in 2008, the International Astronomical Union (IAU) officially recognized it as a dwarf planet.

Makemake is the third-largest known dwarf planet in the solar system, after Pluto and Eris. Its diameter is estimated to be around 870 miles (1,400 kilometers). The dwarf planet has an elliptical orbit around the Sun, taking approximately 310 Earth years to complete one full rotation.


The surface of Makemake is primarily covered with frozen methane, which gives it a reddish-brown color and a highly reflective surface. It is believed that the dwarf planet’s interior is composed mainly of rock and ice.

In 2016, astronomers discovered a moon orbiting Makemake named MK 2. The moon is much smaller than Makemake, with an estimated diameter of about 100 miles (160 kilometers). The moon's discovery has allowed scientists to understand Makemake’s mass and density better.

Currently, there is limited information on Makemake’s atmosphere. Observations conducted during an occultation event in 2011 suggested that the dwarf planet might have a thin atmosphere, but more recent observations have not found any clear evidence of an atmosphere. Further study is needed to confirm the presence or absence of an atmosphere around Makemake.

Trans-Neptunian Objects like Pluto

Many TNOs may be considered dwarf planets. They are just so far away that determining their size can prove challenging. What are TNOs, you ask?

Trans-Neptunian objects (TNOs) are celestial bodies in our solar system's outer regions, beyond Neptune's orbit. They predominantly reside in two main areas: the Kuiper Belt and the Oort Cloud. TNOs are composed of various materials, including rock, ice, and frozen gases, and they come in different sizes, ranging from small icy bodies to large dwarf planets.

Kuiper Belt

The Kuiper Belt is a vast, disk-shaped region that extends from about 30 to 50 astronomical units (AU) away from the Sun. It is home to a wide range of objects, including dwarf planets like Pluto, Haumea, and Makemake. Many TNOs in the Kuiper Belt are believed to be remnants from the early stages of the solar system\’s formation, providing important information about its history and evolution.

Oort Cloud

The Oort Cloud is a hypothesized, more distant, spherical region that is thought to extend from about 2,000 to 100,000 AU from the Sun. It is believed to contain a large number of icy objects, some of which may occasionally be perturbed by the gravitational influence of nearby stars, sending them into the inner solar system as long-period comets.

The study of TNOs helps astronomers learn more about the formation and evolution of our solar system and the composition and characteristics of these distant objects. As our observational capabilities improve and more missions are sent to the outer solar system, our understanding of TNOs will continue to expand.

Currently, Eris, Makemake, Haumea, and Pluto are TNOs that are classified as dwarf planets. But there are other contenders that haven’t been classified yet.


Sedna is a distant and intriguing object in our solar system, classified as a trans-Neptunian object (TNO) and a likely dwarf planet. It was discovered in 2003 by a team of astronomers led by Mike Brown. Sedna is located far beyond the Kuiper Belt in the inner Oort Cloud region.


Sedna’s orbit around the Sun is highly elliptical, taking approximately 11,400 Earth years to complete one full orbit. At its closest point, Sedna is about 76 astronomical units (AU) away from the Sun, while at its farthest point, it is around 937 AU away. One AU is the average distance between Earth and the Sun, about 93 million miles (150 million kilometers).


The diameter of Sedna is estimated to be around 620 miles (1,000 kilometers), although there is some uncertainty in its exact size due to the difficulty of observing such a distant object. Sedna’s surface is believed to be composed primarily of rock and ice, with a reddish-brown color, possibly due to the presence of tholins, which are organic compounds formed by the interaction of sunlight with simple molecules like methane and nitrogen.

No moons have been discovered orbiting Sedna, and there is limited information available about its atmosphere and other characteristics. Our understanding of this distant object remains limited due to its extreme distance from Earth and the challenges associated with observing objects so far away.

As astronomers continue to study Sedna and other TNOs, our knowledge of these distant objects will improve. Future observations and potential missions to the outer solar system could provide more insight into Sedna and its characteristics.


Quaoar is a large trans-Neptunian object (TNO) located in the Kuiper Belt, beyond the orbit of Neptune. It was discovered in 2002 by astronomers Chad Trujillo and Michael Brown. Due to its size and shape, Quaoar is considered a likely dwarf planet, although it has not been officially recognized by the International Astronomical Union (IAU).


Quaoar’s diameter is estimated to be around 680 miles (1,100 kilometers), roughly half the size of Pluto. It has an almost circular orbit around the Sun, taking about 285 Earth years to complete one full orbit. The orbit lies in the solar system's plane, similar to the orbits of the major planets.

The surface of Quaoar is thought to be composed primarily of water ice mixed with some rock and other volatile ices, such as methane and ammonia. It has a dark and reddish appearance, possibly due to the presence of complex organic compounds called tholins, which form from the interaction of sunlight with simpler molecules like methane and nitrogen.

Quaoar has one known moon, Weywot, discovered in 2007. The moon is much smaller than Quaoar, with an estimated diameter of about 50 miles (80 kilometers). The discovery of Weywot has helped scientists learn more about Quaoar’s mass and density, providing insights into its composition and internal structure.

There are many more, like Pluto.

Astronomers continue to explore the outer regions of our Solar System using advanced telescopes, such as the James Webb Space Telescope, which launched on Christmas Day 2021. This powerful telescope will aid astronomers in detecting objects beyond Neptune, known as trans-Neptunian objects (TNOs). It is estimated that there may be over 70,000 TNOs, many of which are at least 100 km in diameter. As a result, many of these objects could be classified as dwarf planets.

Some TNOs, like Orcus and Quaoar, are known to have moons. The James Webb Space Telescope’s observations will help identify more TNOs and provide valuable insights into the composition, characteristics, and potential satellite systems of these distant celestial objects.

This is the long explanation of why Pluto should no longer be classified as a planet. It simply isn’t large enough to clear its orbit of debris like the other eight planets.

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