In 2006, Pluto was relegated from being the ninth planet in the solar system after the International Astronomical Union (IAU) demoted it to a "dwarf planet". Officially, Pluto is now known quite unceremoniously as “minor planet 134340 Pluto.” How rude.
Many astronomy buffs were disappointed by the new assignment, seeing the criteria for Pluto's demotion as rather nitpicky or even arbitrary.
But don't scratch Pluto off as a planet just yet. There are still many authoritative voices in science who argue that the frozen world on the outer rim of the solar system should be reinstated as a planet in its own right.
Why was Pluto demoted in the first place?
The existence of Pluto was first proposed in the early 20th century by Percival Lowell, whose calculations showed that wobbles in the orbits of Uranus and Neptune must be caused by the gravitational pull of an unknown ninth planet. Pluto was confirmed a decade later, on March 13, 1930, at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh.
Since then Pluto's status as the solar system's ninth planet remained unchallenged until astronomers discovered 2003 UB313, also known as Eris, which is nearly as large as Pluto, but with a lot more mass. Other similarly sized bodies have been found across the years beyond Neptune's orbit,
The discovery of Eris triggered a fiery debate among astronomers. If Pluto is a planet, then so should Eris. Eventually, both were classed as dwarf planets after the IAU issued new criteria for what constitutes a planet. Essentially, the IAU says that a planet must:
- orbit around the sun;
- have sufficient mass to assume hydrostatic equilibrium (gets squished into a sphere-like object due to gravity);
- must clear the neighborhood around its orbit.
In August 2006, the IAU ruled that Pluto would no longer be considered a planet because it doesn't “clear the neighborhood around its orbit.” Pluto’s oblong orbit overlaps that of Neptune, so it was disqualified.
Instead, the IAU classed Pluto as a dwarf planet, which is a celestial body that meets only the first two criteria. Besides Pluto and Eris, there are three other known dwarf planets in the solar system: Ceres in the asteroid belt between Mars and Jupiter; Haumea, which is located beyond Neptune's orbit, and Makemake, the second-largest Kuiper Belt object.
Why Pluto is not just some dwarf planet
Bearing this in mind, it's clear that had the IAU not stuck to its new rules, we would now have had at least 13 planets in the solar system. But why should that be a problem?
Many have criticized IAU's definition of a planet, claiming the criteria are arbitrary. Among these critics is former NASA administrator Jim Bridenstine, who during a keynote at the 2019 International Astronautical Congress said:
“I am here to tell you, as the NASA administrator, I believe Pluto is a planet,” before adding that “Some people have argued that in order to be a planet you have to clear your orbit around the sun. If that’s the definition we’re going to use then you could undercut all the planets—they’re all dwarf planets—because there isn’t a planet that clears its entire orbit around the sun.”
The NASA chief is referring to asteroids, which regularly whizz past all the planets in the solar system.
“I think it’s a sloppy definition,” said Bridenstine. “I think the way you should define a planet is based on its intrinsic value, not values that constantly change like orbital dynamics.”
Even if Ceres and other bodies like it don't deserve to be viewed as a 'planet', Pluto has many features that make it more than just some big asteroid. Pluto's surface is incredibly dynamic and constantly changing. The dwarf planet's atmosphere is made up primarily of nitrogen, methane, and carbon monoxide, which freeze out onto the surface when temperatures drop. This creates a layer of frozen gas that insulates the planet's interior, causing it to warm up and release gas back into the atmosphere. This process creates a never-ending cycle that shapes the planet's surface and atmosphere.
In addition to its complex surface and atmosphere, Pluto is also unique in its place in our solar system. It is the only known dwarf planet that crosses Neptune's orbit, and it has a highly elliptical orbit that takes it from inside the orbit of Neptune to nearly 50 astronomical units (AU) from the sun. This means that Pluto spends most of its time in a region of the solar system that is vastly different from the inner, rocky planets and the outer gas giants.
Pluto's five moons, Charon, Styx, Nix, Kerberos, and Hydra, are another aspect of the dwarf planet that sets it apart from other dwarf planets. These moons are a system in miniature, with each moon having its own unique characteristics and contributing to the overall complexity of the Pluto system.
Charon, the largest of Pluto's moons, is particularly fascinating. It is nearly half the size of Pluto and is thought to have formed from a massive impact that created a debris ring that eventually coalesced into a moon. Charon and Pluto are also unique in their binary system, as they orbit a common center of mass that lies outside the surface of either body.
Classifying planets has become bureaucracy
Bridenstine's stance on Pluto is supported by the authors of a study published in a 2019 edition of the journal Icarus by a team of researchers led by Philip Metzger, a planetary physicist with the Planetary Science faculty at the University of Central Florida. Although the paper doesn't focus on Pluto specifically, the authors explain how even asteroids were recognized as planets until the 1950s.
"In the 1950s, developments in planet formation theory found it no longer useful to maintain taxonomic identification between asteroids and planets, Ceres being the primary exception. At approximately the same time, there was a flood of publications on the geophysical nature of asteroids showing them to be geophysically different than the large planets. This is when the terminology in asteroid publications calling them planets abruptly plunged from a high level of usage where it had hovered during the period 1801–1957 to a low level that held constant thereafter," the scientists wrote.
In effect, asteroids were reclassified as non-planets based on their geophysical characteristics. By extension, all cosmic bodies should also be classified by their geophysical characteristics and not arbitrarily through voting by a panel.
"I believe the IAU made several deep mistakes and so the definition is not valid and not scientifically useful and should be rejected. First, definitions should never be voted on for taxonomical concepts like “planet” because taxonomy is supposed to evolve and develop as an integral part of the science. By voting on a particular taxonomical choice they shut down that portion of the scientific process. So the entire vote was anti-science to begin with," Metzger told ZME Science.
Metzger adds that the IAU actually violated their principles, "including their explicit statutes and bylaws, which require the actual language of a proposed vote to be reviewed by all the members for four months before the assembly."
Since the IAU was in a hurry to force a decision, the organization violated its by-laws by not sharing the proposal text until the 2006 assembly.
"That was the first violation, the first mistake. Then the proposal was rejected at the convention, and since they had already set the precedent of breaking the rules they continued blindly breaking the rules even more. So they made up the definition during the assembly, giving nobody anywhere else in the world any time to digest it, giving nobody the choice to come to the meeting to present a well reasoned and well-researched case to sway the outcome, and they forced a vote. They got a deep split in the votes, proving there really is no consensus, but one side won so they declared that the deed was done. That became highly politicized and caused people to take sides and get emotionally involved, so now most people have an attitude about the whole thing and it has poisoned the ability to readdress the question," Metzger said.
Perhaps you've noticed a pattern: the main objection that Pluto planetary advocates have against the IAU decision is that it didn't follow a scientific process. Instead, it was based on a subjective, bureaucratic process.
"Nobody is creating a theory about how objects that fail to clear their orbits are fundamentally different than those that do clear their orbits or those that are satellites of another planet. Nobody has ever proposed differences in geology, geochemistry, atmospheres, oceans, the emergence of life, mineralogy, etc., that are consistent one way in bodies that clear their orbits versus another way in bodies that do not clear their orbits. On the other hand, the term “planet” actually is being used to compare different planets across different dynamical states. So a paper might discuss Pluto, Triton, and Mars, calling them all “planets”, even though one cleared an orbit, one did not, and one was captured by Neptune to become a satellite," Metzger said.
"The IAU definition does not match how scientists actually use the planet concept in doing real, reductionist science. The definition they created was actually designed to keep the number of planets small so school kids could memorize the planets, but having a small number is not the concern of science. The IAU abandoned science in order to create a cultural definition instead," he added.
Wait, does that mean that the Moon and other satellites are technically planets too? Exactly.
"The idea that satellites are a distinct category with no overlap in the “planet” category is actually a recent invention. For almost all scientific history, moons were planets. What scientists used to say before the 1920s is that planets orbiting the sun are “primary planets” while planets that orbit another planet are “secondary planets” or just “satellite” or “moon” for short (but they were still known to be planets)," Metzger told me.
"Being a satellite was just a dynamic relationship that a planet could have with another planet. When we decided that most asteroids are too small to be planets, in the 1960s, then we should have decided at the same time that the smaller satellites are also too small to be planets for the exact same reason, while the larger, round satellites are still planets. Unfortunately, astronomers had already become confused about satellites by the 1920s and forgot the meaning of the word “planet”, switching over to a cultural “folk taxonomy” that actually has its roots in astrology and geocentrism. The public was abandoning geocentrism in the late 1700s and early 1800s, and astrology was still very influential, and this led them to develop the idea that satellites are not planets. It was similar to the general public’s idea that green beans are a vegetable rather than a fruit, although biologists say a green bean is a fruit. The public develops a “folk taxonomy” that is human-centric and not scientifically reductionist, in contrast to the scientific taxonomy that is designed to align along the natural divisions of reductionist theory, thus providing deep explanatory power about nature," he added.
"So although the public is still unaware of this fact, it is true that planetary scientists are referring to moons as “planet”. Many of us do not even realize we are doing this. We say Titan has a planetary core, a planetary crust, a planetary radius, etc. They are “planetary” because they are characteristic of planets. Being a satellite has nothing to do with it. But if having these things are characteristic of planets, and bodies that are in both primary and secondary orbits have them equally, then every time we say “planetary” we are acknowledging that the type of orbit a body has is irrelevant to whether it is a planet or not."
If Pluto is reinstated as a full-fledged planet, then the other four dwarf planets should join it too, bringing the total number of planets in the solar system to 13. However, by Metzger's account of what constitutes a planet by scientific taxonomy, there ought to be at least 150 planets in the solar system.
"Most of them are in the Kuiper Belt. Ceres, which is in the asteroid belt, is also a planet. But also, large moons (satellites) are planets," he said.
Another vocal supporter of Pluto's planetary status is Alan Stern, principal investigator of NASA's New Horizons mission, which flew by Pluto in 2015, revealing the icy body in unprecedented details, including a stunning heart-shaped nitrogen-ice plain.
Last year, Stern hosted an online debate for the Philosophical Society of Washington, in which he asked people to vote on whether or not Pluto should be reinstated as a planet. Before voting was opened, Stern explained their argument that a planet should be defined by its geophysical properties -- if a body is massive enough to assume a nearly round shape but not massive enough to trigger nuclear fusion in its interior like a star, then that's a planet. The poll closed with 130 votes in favor of making Pluto a planet again and 30 against it.
During his presentation, Stern went on to describe New Horizons' findings, which show that Pluto has mountains, glaciers, avalanches, a liquid ocean beneath its icy crust, and a complex atmosphere. These are all hallmarks of planetary processes.
What's more, the third criterion for planetary eligibility -- that a planet has to clear its orbit of other bodies like asteroids -- is not fair for bodies orbiting so far away from the sun. The more you move away from the sun, the harder it is to clear away small objects because the orbit is much slower than that of objects closer to the sun. This means that a planet needs to be increasingly massive in the outer reaches of the solar system in order to clear objects. In fact, Stern makes the assertion that not even Earth would qualify as a planet if it was in Pluto’s orbit.
"The IAU definition is not aligned with useful reductionism. They voted to maintain the folk taxonomy of the public. Not only did they vote that moons are not planets, but they added an “orbit clearing” requirement to keep the number of planets small, so the set of planets would continue to be similar to the set that was known from geocentrism and astrology. Galileo rejected that set. He called the moons of Jupiter “planets”. He did not think dynamics should have anything to do with the definition of a planet. He did not think the number should be kept low to match the ancient geocentrism and astrology. It is really too bad that astronomers rejected Galileo and restored some of the ideas he fought to reject," Metzger said.
"The basic problem is that modern astronomers have failed to realize that taxonomy, and evolving concepts like “planet”, are crucial to the program of science. They thought there was no harm in adopting a folk taxonomy from culture. They did not see any reason not to. This is where the most work needs to be done to repair the damage. The astronomers who believe the IAU’s planet definition is good need to get a much broader view of the functional role of taxonomy in science, to understand how important and beneficial taxonomy can be. If they think the IAU's definition is good and beneficial, it is only because they have such a low concept of how much better things could be. Biologists understand this very well, so they demand that scientists retain “taxonomical freedom” and that taxonomy should never be a matter of regulation or restraint. Astronomers need some growth in this area," he added.
More than 17 years after Pluto's historic demotion to a dwarf planet, this debate is far from over. Perhaps Pluto might rejoin the ranks of the solar system's roster of planets, but until then spirits remain high. When it comes down to it, the argument for why Pluto should be considered a planet should come down to objective arguments based on reductionist science, such as geophysical properties, rather than arbitrary criteria that are designed to preserve a 'folk science' narrative.
If you call Pluto a planet, you're not wrong, and don't let anyone else tell you otherwise.