You may have grown up learning that Pluto was the ninth planet in our solar system. However, in 2006, the International Astronomical Union (IAU) reclassified Pluto as a “dwarf planet.” This decision sparked considerable debate and may leave you wondering: why is Pluto not a planet anymore?
The short answer involves Pluto’s size, shape, orbit, and, crucially, the discovery of many celestial bodies similar to it in the outer solar system. Let’s delve into the fascinating story of Pluto, its journey from planet to dwarf planet, and the scientific reasoning behind this reclassification.
From Planet to Dwarf Planet: The Reclassification of Pluto
On August 24, 2006, a significant shift in our understanding of the solar system occurred. The International Astronomical Union (IAU), the globally recognized authority for naming celestial bodies, officially redefined the term “planet.” This redefinition, formalized in IAU Resolution 5A, led to Pluto being reclassified from a planet to a dwarf planet.
This wasn’t an arbitrary decision but the culmination of years of scientific discoveries and discussions about what truly constitutes a planet. Pluto’s story, from its initial discovery as the supposed ninth planet to its current status as a Kuiper Belt Object, is a compelling narrative of evolving astronomical knowledge and our ever-refining understanding of the cosmos.
To understand why Pluto lost its planetary status, we need to journey back to its discovery and the events that unfolded in the decades that followed.
The Discovery of Pluto: A Quest for “Planet X”
The story of Pluto’s discovery begins in the 19th century, amidst the backdrop of a scientific revolution. Astronomers, armed with Johannes Kepler’s laws of planetary motion and Isaac Newton’s law of gravity, possessed unprecedented tools for understanding the celestial mechanics of our solar system. The prevailing view was of a predictable, clockwork-like universe.
However, by the early 1800s, anomalies began to surface. Uranus, recently discovered, wasn’t precisely following its predicted orbital path. French astronomer Alexis Bouvard noted these discrepancies and proposed that the gravitational influence of an unseen, distant body was tugging Uranus off course.
Both John Couch Adams and Urbain Le Verrier independently calculated the potential location of this hypothetical planet. While Le Verrier also famously (and mistakenly) theorized the existence of a planet named Vulcan to explain Mercury’s orbital oddities, it was his calculations for a planet beyond Uranus that proved fruitful. Johann Galle and Heinrich d’Arrest successfully located Neptune in 1846, based on Le Verrier’s predictions.
Despite the discovery of Neptune, the orbital irregularities of Uranus persisted. Something was still influencing Uranus’s path. Percival Lowell, a prominent astronomer, became deeply interested in this problem. In 1894, he established the Lowell Observatory in Flagstaff, Arizona, specifically to search for “Planet X,” the body he believed was responsible for these gravitational disturbances.
Lowell dedicated over a decade to the search, but “Planet X” remained elusive before his death in 1916. Vesto Slipher, the new observatory director, continued the quest. In 1929, the task fell to Clyde Tombaugh, a young astronomer who had recently joined the Lowell Observatory.
Tombaugh’s method involved painstakingly comparing pairs of photographic plates of the night sky, taken days apart, using a blink comparator. This device allowed him to rapidly switch between the two images. Stars would appear stationary, but a planet, moving against the background stars, would appear to jump back and forth.
After months of meticulous searching, on February 18, 1930, Tombaugh made a momentous discovery. He identified a faint object moving against the backdrop of stars in the constellation Gemini. “Planet X” had finally been found.
Doubts Emerge: Pluto’s Size and the Kuiper Belt
The newly discovered “Planet X” was named Pluto, after the Roman god of the underworld. However, almost immediately, doubts arose about whether Pluto was indeed the Planet X Lowell had envisioned, and even whether it deserved the title of “planet” at all.
Pluto appeared exceptionally faint, even in powerful telescopes. Its disc remained unresolved, indicating a small size, possibly comparable to Earth or Mars, significantly smaller than the gas giants of the outer solar system. This diminutive size cast doubt on whether Pluto could exert the gravitational influence necessary to cause the observed perturbations in Uranus’s orbit.
Despite the observational challenges, dedicated astronomers continued to study Pluto. Gradually, with the construction of larger telescopes, they learned more. Observations suggested Pluto’s surface was likely covered in methane ice.
A crucial breakthrough came in 1978 when astronomer James Christy discovered Charon, Pluto’s largest moon. Charon’s discovery enabled astronomers to accurately measure Pluto’s mass, which turned out to be far less than previously estimated – even smaller than Earth’s Moon.
Pluto’s small size and its unusual, highly inclined, and eccentric orbit, further fueled the debate about its planetary status. However, it was discoveries in the 1990s that truly revolutionized our understanding of the outer solar system and ultimately led to Pluto’s reclassification.
The advent of Charge-Coupled Device (CCD) cameras in the 1990s, significantly more sensitive to light than photographic film, allowed astronomers to detect fainter objects and probe deeper into the solar system’s outer reaches. These new instruments revealed that Pluto was not alone. Numerous Pluto-sized and even larger objects were discovered beyond Neptune’s orbit.
These objects were found to be part of the Kuiper Belt, a vast, donut-shaped region beyond Neptune populated by icy bodies, remnants from the solar system’s formation. Pluto, it turned out, was not a unique planet but the largest known member of the Kuiper Belt.
Among the significant Kuiper Belt discoveries were Quaoar (2002), Sedna (2003), Haumea (2004), Makemake (2005), and Eris (2005). Eris, in particular, was initially estimated to be larger than Pluto, further intensifying the debate. If Pluto was a planet, should Eris and these other large Kuiper Belt Objects also be considered planets?
This question led the International Astronomical Union (IAU) to address the definition of a planet directly.
Defining a Planet: The IAU Criteria
In 2006, the International Astronomical Union (IAU) formally defined what constitutes a “planet” in our solar system. According to the IAU resolution, a planet is a celestial body that:
- Orbits the Sun: It must be in direct orbit around the Sun, not orbiting another planet (like a moon).
- Is Round Due to Its Own Gravity: It must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape. In simpler terms, its own gravity pulls it into a sphere.
- Has “Cleared the Neighborhood” Around Its Orbit: This is the crucial criterion that Pluto fails. A planet must have gravitationally dominated its orbital zone, meaning it has either cleared away other objects of comparable size or drawn them in as satellites.
Pluto fulfills the first two criteria; it orbits the Sun and is round. However, Pluto has not cleared its neighborhood. It resides within the Kuiper Belt and shares its orbital space with numerous other Kuiper Belt Objects. Due to this, Pluto was reclassified as a “dwarf planet.”
While the term “dwarf planet” might sound like a demotion, it simply reflects a more accurate scientific classification based on our expanded knowledge of the solar system. Dwarf planets, like Pluto, are fascinating and important objects in their own right, offering valuable insights into the early solar system.
Arguments and Sentiments: The Pluto Planet Debate
The IAU’s decision was not without controversy. There are ongoing arguments and sentimental reasons why some believe Pluto should still be considered a planet.
Sentimental Value and Historical Context
One argument rests on sentimentality. For many decades, Pluto was taught as the ninth planet. For some, reclassifying it feels like diminishing its historical significance and their childhood understanding of the solar system.
Issues with the “Cleared Neighborhood” Criterion
Some astronomers argue that the “cleared neighborhood” criterion is vaguely defined and inconsistently applied. They point out that even Earth and Jupiter, considered planets, have not fully cleared their orbits. Earth orbits with thousands of near-Earth asteroids, and Jupiter has tens of thousands of Trojan asteroids within its orbit. If “clearing the neighborhood” is strictly interpreted, some argue that even Earth and Jupiter might technically fail the definition.
Size Matters: A Surface Area Proposal
Another viewpoint suggests that size should be the primary factor. Proponents of this idea propose defining a planet as any object orbiting the Sun with a surface area greater than 1,000 kilometers. By this definition, both Pluto and Eris would qualify as planets.
Despite these arguments, the IAU’s definition remains the internationally accepted standard. While Pluto may have lost its status as a “planet,” it remains a captivating and scientifically valuable world. Its reclassification reflects the dynamic nature of science, where our understanding evolves with new discoveries and refined definitions. Pluto’s story teaches us that classification is a tool for understanding, and the cosmos is far more diverse and fascinating than we could have imagined when Pluto was first discovered.
This article is inspired by content originally published in BBC Sky at Night Magazine.
