The Great Pluto Debate
For nearly 76 years, Pluto enjoyed its reign as the ninth planet of our solar system; a lonely outpost orbiting far beyond Neptune. Schoolchildren memorized its name, and it became a symbol of the outermost reaches of our cosmic neighborhood.
But in 2006, the astronomical world was shaken to its core when Pluto was stripped of its planetary status. How did a beloved little world go from being a planet to a mere “dwarf planet”? The story of Pluto’s demotion is one of scientific debate, cosmic discovery, and an evolving understanding of our place in the universe.
A Clash of Criteria
Pluto’s troubles began long before it was officially cast out of the planetary club. Ever since its discovery in 1930 by American astronomer Clyde Tombaugh, scientists questioned whether this icy body truly belonged among the likes of Mercury, Venus, and Earth.
It was smaller than our Moon, and as telescopes improved over the decades, researchers realized Pluto wasn’t alone in its icy domain. It turned out to be just one of many objects in a vast region beyond Neptune known as the Kuiper Belt; a swath of the solar system teeming with frozen bodies, comets, and dwarf planets.
In 2006, the International Astronomical Union (IAU) finally tackled the thorny question: What exactly makes a planet a planet? The answer was not as straightforward as one might think. After much debate, the IAU settled on three key criteria:
- A planet must orbit the Sun.
- It must be massive enough for its gravity to shape it into a roughly spherical form.
- It must have “cleared its orbit” of other debris, meaning it has enough gravitational influence to dominate its orbital neighborhood.
Pluto met the first two criteria; it orbits the Sun and its gravity is strong enough to pull it into a spherical shape. But it faltered on the third. Pluto’s orbit is a messy place. It shares its path with numerous objects in the Kuiper Belt, and its gravitational pull isn’t powerful enough to clear out these neighboring bodies. As a result, the IAU reclassified Pluto as a “dwarf planet.”
The Aftermath of Demotion
For many, the decision was a shock. Pluto had been a planet for so long, and to suddenly say it wasn’t felt almost sacrilegious. Protests erupted; not just among scientists, but in classrooms and living rooms around the world. People felt a sense of betrayal, as if an old friend had been wronged. Yet, for astronomers, the demotion made sense. It wasn’t about downgrading Pluto; it was about refining the way we categorize objects in our solar system.
But why not just keep Pluto as a planet, given its history? The problem is that if Pluto were allowed to stay, the floodgates would open. There are currently five officially recognized dwarf planets; Pluto, Eris, Haumea, Makemake, and Ceres. Four of these, including Pluto, reside in the Kuiper Belt, and scientists believe there could be dozens or even hundreds more dwarf planets waiting to be discovered in that distant region.
Some estimates suggest that there could be as many as 70,000 objects similar in size to Pluto lurking in the Kuiper Belt alone. If we classified all of these as planets, our solar system would become impossibly crowded with planets, diluting the very definition of what a planet should be.
Understanding Pluto’s Place in the Cosmos
So what is Pluto now, if not a planet? Officially, it’s a member of a unique group known as the “dwarf planets.” These objects share many characteristics with planets, but their inability to clear their orbits sets them apart. They are a fascinating mix of planet-like qualities and comet-like characteristics, orbiting in the dim recesses of the solar system where sunlight is but a distant memory.
The reclassification of Pluto and the introduction of the dwarf planet category have helped clarify our understanding of the solar system’s structure. Beyond Neptune lies the Kuiper Belt; a region of space that is still largely uncharted and filled with countless icy bodies that could rewrite what we know about the formation and evolution of the solar system. By acknowledging Pluto as a dwarf planet, scientists have opened the door to a deeper understanding of these mysterious outer regions.
The dwarf planets
Pluto
A Dwarf Planet by Any Other Name…
Even though Pluto lost its planetary status, it hasn’t lost its charm. It remains one of the most studied and celebrated objects in the solar system. In 2015, NASA’s New Horizons spacecraft flew by Pluto, capturing breathtaking images of its icy mountains, smooth plains, and complex surface features. The data collected during this historic encounter revealed Pluto to be a dynamic world, with a thin atmosphere and signs of geological activity; features that make it every bit as fascinating as any of the “true” planets.
The story of Pluto’s demotion is far from over. In some ways, it has ignited more interest and debate than ever before. Some astronomers continue to push for Pluto’s reinstatement as a planet, arguing that the IAU’s definition is too restrictive and doesn’t reflect the diversity of celestial bodies in our solar system. Others suggest that we should adopt a more fluid classification system, one that embraces the uniqueness of objects like Pluto without adhering strictly to rigid definitions.
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
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.
Surface
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
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
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.
Size
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
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.
Surface
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
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.
Orbit
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).
Size
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
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).
Composition
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.
