In August of the year 2006, an official statement from the International Astronomical Union caused a stir throughout the astronomical community and the world at large.
Pluto, the IAU declared, was being downgraded in status from ‘Planet’ to ‘Dwarf Planet.’ Our solar system, as we know it, would now consist of 8 planets rather than 9.
The news of Pluto’s demotion prompted extensive and heated debates amongst scientists and laypeople alike, some of which continue to this day.
In this article, we’ll be answering the question ‘Is Pluto a planet?’ once and for all. Starting with the history of this elusive celestial body, we’ll move on to explore the science behind Pluto’s classification as a ‘Dwarf Planet’ as well as the arguments for its planetary status being reinstated.
All About Pluto
Pluto was first discovered in 1930, orbiting the sun Kuiper belt just outside of Neptune’s orbit.
Pluto is a relatively small astral object with a radius of just 1,188.3 kilometers. It’s roughly half the width of the United States, and equal to roughly 18% of the Earth’s diameter.
Despite being so small, Pluto has its own surrounding of natural satellites, otherwise known as moons. Pluto has 5 moons in total: Charon, Hydra, Kerberos, Nix, and Styx.
Pluto’s name, and those of its moons, are derived from Roman mythology. This is also the case for many of the planets in our solar system.
Pluto’s positioning at the edge of the solar system, surrounded by icy celestial bodies, made it a likely contender for the symbolic title of God of the Underworld.
Despite its dark mythological connotations, however, Pluto is quite beautiful in appearance. Its base charcoal color often gives way to striking orange, red, and white patterns. Pluto is actually one of the most contrastive objects in our solar system.
Pluto’s Planetary Status: A History
Pluto may no longer hold planetary status today, but for many years, it was considered a planet like any other - albeit a much smaller one.
The history of Pluto’s planetary status is a complicated and fascinating one that is worth exploring before we delve into the science.
Pluto made its presence known in subtle ways well before its official discovery.
Percival Lowell, another American astronomer, first suggested at the beginning of the 20th century that an undiscovered astronomical body was responsible for irregularities in the orbital trajectories of Neptune and Uranus.
Lowell launched a search effort for the mystery planet in 1906 but died 10 years later, in 1916, before the object could be located.
It was, however, at the eponymous Lowell Observatory (Flagstaff) that Pluto was first officially discovered.
Credit for the discovery goes to the American astronomer Clyde W. Tombaugh, who used a blink microscope and newly developed photographic plates to locate the planet.
Pluto’s mythological name was actually first suggested by English 11-year-old Venetia Burney, whose grandfather passed the suggestion on to Lowell Observatory.
However, not long after Pluto’s discovery, questions as to its status began to arise in the scientific community. The main point of contention at the time was the planet’s size and mass.
It was argued that the planet, as it was then known, did not exert sufficient gravitational dominance to ‘clear the neighborhood’ around its orbit.
But what does it mean, and why did it result in Pluto being demoted to the position of ‘Dwarf Planet’ just 76 years after its discovery?
Why Isn’t Pluto a Planet?
In order to understand why Pluto isn’t currently considered a planet in the same way as Earth or Mars, for example, it’s necessary to consider the prerequisites for planetary status as outlined by NASA and the IAU.
There are 3 primary criteria that a celestial body must meet in order to be considered a planet.
The first criteria is that the body must be in orbit around the sun. This is easy enough to determine, and Pluto clearly meets this criterion.
Secondly, a celestial object must have enough total mass to achieve a state called hydrostatic equilibrium. That means that a planet’s gravitational pull must act in balance with the internal pressure of said planet.
Essentially, a planet cannot be actively expanding or contracting, and it must not be able to collapse on itself. Pluto also ticks all the boxes here.
So, what’s the problem? Well, there is a third criterion, and this one is a little more complicated. In order to be considered a planet, a celestial object must also ‘clear the neighborhood’ around its orbit.
This is another way of saying that the body must be gravitationally dominant over other objects in its path. Other than natural satellites such as moons, a planet should not interact gravitationally with any other planets.
Because Pluto’s orbit intersects with that of Neptune once every 248 years (for a period of 20 years), it does not technically meet this criterion.
Therefore, Pluto only meets 2 out of 3 criteria for planetary status, and it was this discovery that ultimately led to its reclassification as a ‘Dwarf Planet.’
The Argument for Pluto as a Planet
The pushback against the IAU’s decision to demote Pluto from ‘Planet’ to ‘Dwarf Planet’ continues today.
The main argument for reinstating Pluto’s planetary status comes from scientists who consider the third criterion for planetary classification to be misleading and unnecessary.
One example of such a scientist is no other than NASA’s ex-administrator, Jim Bridenstine, who described the definition of the third criterion as ‘sloppy.’
Bridenstine points out that, technically, no planets in our solar system fully fit the definition because they all cross orbits with asteroids.
This argument ties into another relevant discussion about whether asteroids should be considered planets - as they were for a century and a half until the 1950s.
Concerns have also been raised regarding the impact of Pluto’s loss of planetary status on our understanding and appreciation of the celestial body.
An entire ocean lies under Pluto’s surface, shrouded by a complex atmosphere and covered in a myriad of organic compounds.
By considering Pluto as ‘less than’ in comparison to other planets, we risk undervaluing the knowledge and discoveries that it has to offer.
As it stands, Pluto is not currently considered to be a planet because it does not have sufficient gravitational dominance to clear its own orbit around the sun.
However, there is ongoing debate surrounding the current system of planetary classification, with some of the world’s most respected astronomers calling Pluto’s demotion into question.
For now, Pluto remains the mysterious ‘Dwarf Planet’ at the outskirts of our solar system. But who can say what future developments in astronomy will bring?