Pluto’s status in the solar system is once again under scrutiny, thanks to remarks from new NASA Administrator Jared Isaacman during a recent Congressional hearing.

“I’m strongly in favor of reclassifying Pluto as a planet,” he stated, highlighting a growing movement among planetary scientists advocating for Pluto’s reinstatement.

Since astronomers voted to relegate Pluto to ‘dwarf planet’ status in 2006, numerous researchers and members of the public have rallied for a reversal of this decision.

The reasoning is straightforward: Pluto is spherical, showcasing complex geology, including mountains, glaciers, and even a tenuous atmosphere. It certainly appears more planetary than a mere collection of cosmic debris.

Despite Isaacman’s endorsement, many astronomers remain hesitant to promote Pluto, cautioning that this could lead to a cascade of similar claims for other celestial bodies.

If Pluto is reclassified as a planet, it may open the door for numerous other celestial entities, possibly even our moon, to receive similar recognition.

The Divided Planet Debate

The ongoing Pluto debate traces back to the early 2000s. With the advent of more advanced telescopes and thorough exploration of the outer solar system, astronomers have identified a plethora of icy worlds beyond Neptune, many of which resemble Pluto. Notable among them is Eris, discovered in 2005, estimated to be about one-third as massive as Pluto.

A difficult question arose: if Pluto is classified as a planet, shouldn’t Eris be considered one as well? And how far does this classification extend?

This inquiry was presented to the International Astronomical Union (IAU), the body responsible for celestial nomenclature. On August 24, 2006, the IAU established a new definition for a planet, which requires that a celestial body:

• orbits the sun
• is massive enough for its gravity to form it into a round shape
• has cleared the area around its orbit

Pluto did not meet the last criterion and was thus downgraded to ‘dwarf planet’ status, a classification that has sparked debates among planetary experts for nearly two decades.

This ‘clearing the neighborhood’ requirement poses challenges, especially at greater distances from the Sun, where a planet’s orbit expands and necessitates greater mass for effective neighborhood clearing.

This has led critics to argue that the IAU’s definition is inherently biased against Pluto.

“Based on the IAU definition, Earth qualifies as a planet in its current position, but if moved beyond the solar system, it would no longer fit that classification, which seems illogical,” explained Dr. Alan Stern, a leading planetary scientist at the Southwest Research Institute in Texas.

“Classification should not solely depend on location,” he added.

Stern is not alone in this sentiment. Just five days after the IAU’s announcement, a petition with signatures from over 300 professional planetary scientists was raised, which is significant considering only 411 individuals initially voted on the new definition.

The controversy surrounding Pluto persists. A 2018 study determined that even after 12 years, many planetary scientists have yet to adopt this definition.

Additionally, a 2024 paper pinpointed another fundamental flaw: the first requirement, that a planet must orbit the sun, excludes exoplanets revolving around other stars.

“Currently, there is no ‘official’ definition for exoplanets from a legitimate scientific nomenclature authority,” stated Dr. Hannah Wakeford, an astrophysicist at the University of Bristol.

“The IAU definition falls short when applied beyond our solar system. If this definition were utilized in the Proxima system, which has three stars in mutual orbit, the stars themselves could mistakenly be classified as planets.”

A New World Order

So what could be the remedy? Instead of focusing on where a body resides, some scientists, including Stern, argue that planets should be defined by what they are.

“It’s quite simple,” Stern asserted. “A planet is an object in space that is a) large enough to be shaped into a sphere by its own gravity, and b) not large enough to ignite through nuclear fusion, which are stars. That’s the definition.”

This redefinition would not only restore Pluto to its planetary status but also elevate Eris and numerous dwarf planets discovered in recent decades, including Haumea, Makemake, and Sedna, along with Ceres, the only dwarf planet in the inner solar system.

In total, Stern’s proposal could classify over 100 celestial objects as planets in our solar system, including our moon, which satisfies the criteria for roundness, geological complexity, and has six times the mass of Pluto.

“It’s incredible how many planets have been identified in our solar system and beyond,” Stern remarked. “The perspective that there are only a few planets comes from outdated beliefs from the 19th and 20th centuries.”

However, not everyone is onboard with such an expansive classification. Mike Brown, who discovered Eris and many other dwarf planets, fears that this defining method dilutes what it means to be a planet.

“Classification is fundamental for science’s understanding of phenomena,” Brown said. “If we miscategorize, we begin to ask misleading questions.”

He views the push for a modified definition as more of a strategy to restore Pluto as a planet, arguing that this topic was not previously entertained until after Pluto’s demotion.

Americans have a particularly strong attachment to Pluto, which was discovered in 1930 by astronomer Clyde Tombaugh. During his Congressional address, Isaacman emphasized the importance of recognizing Tombaugh’s contributions to astronomy.

Isaacman’s assertion to restore Pluto’s planetary status mirrors political rhetoric, indicating that motivations may intertwine science and politics.

Despite gaining the support of the NASA administrator, Pluto’s future ultimately relies on the IAU, an independent international body, which has shown no signs of revisiting this debate.

Ultimately, whether Pluto is categorized as a planet or not may hold less significance than the ongoing interest it generates. Its status has not diminished its prominence in scientific studies and it has even inspired the naming of an entire class of celestial bodies: dwarf planets within the Kuiper Belt.

“In my view, Pluto has ascended to the throne of dwarf planets, exemplifying its category and showcasing fascinating characteristics. Yet, is that really more prestigious than being labeled an average planet?” Wakeford remarked.

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