Many people grew up learning that Pluto was the ninth planet in our solar system. It was a familiar part of our cosmic neighborhood, often depicted in colorful diagrams and models. However, in 2006, the International Astronomical Union (IAU) reclassified Pluto as a “dwarf planet,” sparking considerable debate and confusion. So, why is Pluto no longer considered a planet?
In essence, Pluto lost its planet status due to its size, shape, orbital path, and, crucially, the discovery of numerous other celestial bodies similar to it in the outer solar system. To truly understand why Pluto is not a planet anymore, we need to delve into the history of its discovery, the evolving definition of a planet, and the scientific reasons behind its reclassification.
From Planet to Dwarf Planet: The Reclassification of Pluto
On August 24, 2006, the International Astronomical Union (IAU), the internationally recognized authority for naming celestial bodies, officially redefined the term “planet.” This decision, formalized in IAU Resolution 5A, led to Pluto’s reclassification as a “dwarf planet.” This wasn’t a sudden decision but rather the culmination of years of scientific discussion and new discoveries that challenged our traditional understanding of planetary status.
The journey of Pluto, from its initial discovery as the ninth planet to its current classification as a dwarf planet and a Kuiper Belt Object, is a compelling narrative of scientific progress. It highlights how our understanding of the solar system has evolved and how the very definition of a “planet” has been refined over time.
To fully grasp the reasons behind Pluto’s reclassification, we must journey back to its discovery and the astronomical context of the early 20th century.
The Serendipitous Discovery of Pluto
The story of Pluto’s discovery begins not with direct observation, but with anomalies in the orbit of Uranus. By the 19th century, astronomers had meticulously charted the orbits of known planets, guided by the laws of physics established by Johannes Kepler and Isaac Newton. However, Uranus, discovered in 1781, wasn’t behaving exactly as predicted.
French astronomer Alexis Bouvard, in 1821, published detailed tables of Uranus’s position, noting discrepancies between predicted and observed locations. He hypothesized that the gravitational influence of an unseen planet beyond Uranus was causing these deviations.
Independently, John Couch Adams in England and Urbain Le Verrier in France calculated the potential position of this hypothetical planet. Le Verrier’s calculations were instrumental in the discovery of Neptune in 1846 by Johann Galle and Heinrich d’Arrest. However, even after Neptune’s discovery, irregularities in Uranus’s orbit persisted, suggesting yet another celestial body might be at play.
Percival Lowell, a wealthy American astronomer, became fascinated by this mystery. He founded the Lowell Observatory in Flagstaff, Arizona, in 1894, specifically to search for “Planet X,” the hypothetical planet perturbing Uranus and Neptune.
Lowell’s search continued until his death in 1916, and the observatory’s new director, Vesto Slipher, maintained the quest. In 1929, the task was assigned to Clyde Tombaugh, a young astronomer who meticulously compared photographic plates of the sky taken weeks apart using a blink comparator. This device allowed him to spot objects that had shifted position against the background stars.
On February 18, 1930, Tombaugh’s persistence paid off. He identified a faint object moving against the backdrop of stars in the constellation Gemini. Planet X, later named Pluto, had been found.
Doubts Emerge: Pluto’s Peculiarities
Despite the initial excitement of discovering a ninth planet, doubts about Pluto’s planetary status soon began to surface. Early observations revealed Pluto to be surprisingly faint, indicating a small size. Even with powerful telescopes, its disc remained unresolved, suggesting it was considerably smaller than the other gas giants and even terrestrial planets. Initial estimates suggested it might be similar in size to Earth or Mars, but further observations revised this downwards, eventually showing it to be smaller than Earth’s Moon.
This diminutive size cast doubt on whether Pluto could indeed be the Planet X that was supposedly perturbing Uranus and Neptune. Its faintness and small estimated mass suggested it was unlikely to exert a significant gravitational influence on these giant planets.
Further complicating matters was Pluto’s unusual orbit. Unlike the other planets that orbit the Sun in relatively circular paths within a common plane (the ecliptic), Pluto’s orbit is highly elliptical and significantly inclined to the ecliptic. This peculiar orbit further distanced Pluto from the characteristics of the established planets.
In 1978, James Christy’s discovery of Charon, Pluto’s largest moon, proved pivotal. Charon’s existence allowed astronomers to accurately calculate Pluto’s mass for the first time. The results were startling: Pluto’s mass was far less than anticipated, much smaller than even the Moon.
The combination of its small size, low mass, and unusual orbit led many astronomers to question whether Pluto truly belonged with the other planets. The stage was set for a re-evaluation of what defines a planet.
The Kuiper Belt and the Planet Definition Debate
The 1990s brought advancements in astronomical technology, particularly CCD cameras, which were far more sensitive to light than traditional photographic film. These new tools allowed astronomers to peer deeper into the outer solar system and detect fainter objects than ever before.
These deeper surveys revealed a vast population of icy bodies beyond Neptune’s orbit, in a region now known as the Kuiper Belt. This belt is a reservoir of icy objects, remnants from the solar system’s formation, similar to the asteroid belt but far larger and more distant. Crucially, many of these Kuiper Belt Objects (KBOs) were found to be similar in size to Pluto, and some even larger.
The discovery of Quaoar, Sedna, Makemake, and Eris, among others, forced astronomers to confront a critical question: if Pluto is a planet, should these other similar-sized or larger KBOs also be classified as planets? The prospect of adding dozens, or even hundreds, of new planets to the solar system challenged the existing planetary paradigm.
This led the International Astronomical Union (IAU) to formally address the definition of a planet. In 2006, the IAU established a new definition, leading to Pluto’s reclassification.
The IAU Definition: What Makes a Planet?
According to the IAU’s definition, a “planet” is a celestial body that:
- Orbits the Sun.
- Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
- Has “cleared the neighborhood” around its orbit.
This third criterion is the key factor in Pluto’s reclassification. “Clearing the neighborhood” means that a planet has become gravitationally dominant in its orbit, either by accreting or scattering away other objects in its orbital zone.
Pluto, while meeting the first two criteria, fails to meet the third. It resides within the Kuiper Belt and shares its orbital space with numerous other KBOs. It has not gravitationally dominated its orbital zone.
Therefore, Pluto was reclassified as a “dwarf planet,” a category specifically created for objects that are planet-like in being round and orbiting the Sun, but have not cleared their orbit. Other dwarf planets include Eris, Makemake, Haumea, and Ceres (located in the asteroid belt).
Arguments and Sentiments: The Pluto Debate Continues
The IAU’s decision was not without controversy. Many people, scientists and the public alike, expressed disappointment and even outrage at Pluto’s “demotion.” Arguments for retaining Pluto’s planetary status often invoke historical sentiment and point to perceived flaws in the IAU definition.
Sentimental Value and Historical Context
A common argument is that Pluto was historically recognized as a planet for over 70 years. For many, Pluto was the ninth planet they learned about in school, and its reclassification felt like a loss of a familiar member of the solar system. This argument emphasizes the sentimental and cultural significance of Pluto’s planetary status.
Issues with “Clearing the Neighborhood”
Some astronomers argue that the “clearing the neighborhood” criterion is ambiguous and even flawed. They point out that Earth’s orbit is crossed by thousands of near-Earth asteroids, and Jupiter’s orbit contains over 100,000 Trojan asteroids. By a strict interpretation, these planets also haven’t fully “cleared” their orbits.
Size Matters: Redefining Planetary Status by Size
Another proposed alternative definition focuses on size. Some argue that any object orbiting the Sun that is large enough to be rounded by its own gravity (meeting the hydrostatic equilibrium criterion) should be considered a planet. A specific size threshold, such as a surface area greater than 1,000 km, has been suggested. Under such a definition, both Pluto and Eris, and potentially other large KBOs, would regain planetary status.
Conclusion: Pluto’s Enduring Significance
While Pluto may no longer be officially classified as a planet, its scientific importance remains undiminished. Pluto’s reclassification reflects the dynamic nature of scientific understanding and the ongoing process of refining our knowledge of the universe.
Pluto, as the largest known Kuiper Belt Object, provides invaluable insights into the formation and evolution of the outer solar system. Missions like NASA’s New Horizons have revealed Pluto to be a complex and geologically active world, with mountains, glaciers, and a surprisingly dynamic atmosphere.
Whether planet or dwarf planet, Pluto remains a fascinating and vital object of study, continuing to enrich our understanding of the solar system and the diverse range of celestial bodies it contains. The debate surrounding Pluto’s planetary status highlights the evolving nature of scientific classification and the enduring human fascination with our cosmic neighborhood.
This article is inspired by content originally published in BBC Sky at Night Magazine.
