The great planet debate: Pluto's redefinition is still controversial 15 years later

It continues to rage 15 years after the IAU attempted to resolve it by defining what a "planet" is.
Matthew S. Williams
Solar system stock photo.
Solar system stock photo.


What is a planet? More importantly, why is it that something that seems so academic is the subject of such scrutiny and controversy? As it turns out, the struggle to define what constitutes a planet is closely tied to our understanding of the Universe, which has evolved considerably in the past twenty years. It also demonstrates that the scientific community is not entirely devoid of politics and divisiveness. But first, a little background!

The word "planet" is derived from the Greek "planētai," which translates to "wanderers." To astronomers in ancient Greece, the name was derived from "planētes asteres" - literally, "wandering stars." This referred to how the planets (which appeared to be particularly bright stars to ancient civilizations) wandered across the night sky in a way that was inconsistent with the "fixed" or "background stars."

Given how Western astronomers trace their traditions to Classical Antiquity (and to the ancient Babylonians before them), the term has remained in use. It was not until the Scientific Revolution - thanks to the contributions of Copernicus, Galileo, Kepler, and others - that our modern understanding of planets began to emerge. Instead of stars revolving around the Earth, astronomers began to think of planets as celestial bodies that (like Earth) orbited the Sun.

The great planet debate: Pluto's redefinition is still controversial 15 years later
A size comparison of 8 large Trans-Neptunian objects and their moons.

By the 18th and 19th centuries, astronomers had observed all eight major planets using telescopes of increasing sophistication. In 1781, German-English astronomer William Herschel discovered Uranus, which had been observed many times, but always mistaken for a star. By 1846, French astronomer Urbain Le Verrier discovered Neptune while working at the Paris Observatory.

The next major discovery did not occur until 1930, when American astronomer Clyde Tombaugh (while working at the Lowell Observatory in Flagstaff, Arizona) discovered Pluto and its largest moon Charon. After Pluto was officially declared the "ninth planet" of the Solar System, few planetary discoveries were made until the turn of the century. But in short order, multiple planets would be discovered within the Solar System and beyond, invariably leading to the "Great Planet Debate."

Of Pluto and TNOs

First, the existence of the Kuiper Belt was confirmed in 1992 by Dr. David Jewitt and graduate student Jane Luu (both from MIT) using telescopes at the Kitt Peak National Observatory in Arizona and the Cerro Tololo Inter-American Observatory in Chile. This massive disk extends from the orbit of Neptune to approximately 50 Astronomical Units (AU) from the Sun. It predominantly consists of icy bodies ("iceteroids") rich in volatile elements like methane, ammonia, and water.

This was confirmed by the discovery of 15760 Albion in 1992, the first Trans-Neptunian Object (TNO) observed since the discovery of Pluto and Charon. This discovery inspired systematic searches of the Trans-Neptunian region for other TNOs, which turned up hundreds of objects over the next few years. In particular, there was the Quasar Equatorial Survey Team (QUEST), which included Caltech Professor Mike Brown, then-postdoctoral researcher Chadwick Trujillo, and Senior Yale Research Scientist David Rabinowitz.

Interesting Engineering spoke to Brown via email and asked him what this discovery process felt like. "The best word to describe it is simply fun," he said. "No matter how many objects you discover, every time a new little ice ball at the edge of the solar system comes into view, you get to be the first person to ever see it. So, it is just fun."

Between 2002 and 2007, the team discovered the large TNOs Quaoar (2002), Sedna (2003), Orcus, Salacia, and Haumea (2004), Eris and Makemake (2005), and Gonggong (2007). The discovery of these and other objects revived a long-running dispute within the scientific community over the classification of TNOs and whether or not they should be included as "planets."

Another consideration was the status of Ceres, the largest object in the Main Asteroid Belt. This body is the only body to have achieved hydrostatic equilibrium and accounts for 39% of the mass of the Main Belt. Discovered in 1801 by Italian astronomer Giuseppe Piazzi at the Palermo Astronomical Observatory, Ceres was originally considered a planet. However, with the discovery of dozens of other bodies in the region throughout the 1850s, it was reclassified as an asteroid. Said Brown:

"The questions had been simmering since objects in the Kuiper belt began being discovered a decade earlier. As increasingly larger objects were discovered, Pluto began looking less likely to be unique. And when Eris was found, it was clear that Pluto was not unique. At that point, SOMETHING had to change – either we needed to add planets or subtract them."

On top of that, many satellites in the Solar System are larger than planets. Notable examples include Jupiter's moon Ganymede (the largest satellite in the Solar System), and Saturn's largest moon Titan. With a radius of 1637 mi (2634 km) and 1600 mi (2574 km), respectively, these satellites are actually larger than Mercury (1,516 mi; 2,440 km) and Pluto (1,476 mi; 2,376 km).

Last but certainly not least, extrasolar planets have been discovered with increasing frequency since the turn of the century. Dozens were detected in 2006 alone, and astronomers knew the numbers would continue to climb, especially with the launch of the Kepler Space Telescope approaching (it was launched on March 7, 2009).

The IAU 2006 decision

Due to these issues, there were concerns that astronomy nomenclature needed to be updated to reflect a changing understanding. As a result, a resolution to establish a new definition for "planet" was tabled for the IAU 2006 General Assembly, which took place from August 15th - 26th in Prague, Czech Republic. Per Resolution 5A, the IAU adopted a definition based on three distinct categories:

  • A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.
  • A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.
  • All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies."

Notably, the General Assembly indicated that by this definition, Pluto was no longer a "Planet" and had been relegated to "Dwarf Planet." While Pluto orbits the Sun and is spherical, its position in the Kuiper Belt (a region filled with smaller bodies and 'icy bodies') meant that it had not "cleared its orbit." By this same definition, TNOs like Eris, Haumea, Makemake, and other bodies discovered in the Kuiper Belt (as well as Ceres in the Asteroid Belt) were also classified as "Dwarf Planets."

For multiple reasons, this definition immediately became the source of controversy. On the one hand, there was the nature of the definitions, which many felt were problematic (or downright unscientific). In addition, the nature of how and when the resolution was passed and its motivation. To break it down, the resolution was seen as being rushed, politically motivated (and executed), and riddled with unscientific reasoning.

A political process?

To break these objections down succinctly, Interesting Engineering spoke with writer, amateur astronomer, and science activist Laurel Kornfeld, who has fiercely advocated for Pluto's "planethood" since the 2006 IAU resolution. As Kornfeld explained to Interesting Engineering via email, the definition's flaws can be divided into two categories. The first is the problematic process through which it was adopted:

"In his book, The Case for Pluto, science writer Alan Boyle delves extensively into the machinations leading up to the IAU vote. IAU bylaws require all General Assembly resolutions to be first vetted by the proper IAU committee before being put to the GA floor for a vote. After its appointed Planet Definition Committee met for months, the committee came up with a definition that included dwarf planets as a subclass of planets.

"That resolution was voted down, at which point the IAU should have put off the issue to its next GA in 2009. Instead, a small group of IAU members at the GA hurriedly threw together a different resolution and - without vetting it by the appropriate committee as required - put it to the floor of the GA for a vote on the last day of the two-week conference."

"At that point, the majority of the original 2100 conference attendees, including Planet Definition Committee Chair Owen Gingerich, had already gone home. Neither he nor any of those who left early knew of the plans to put a different resolution to a vote. Most of those who stayed behind and voted were not planetary scientists but other types of astronomers. Additionally, there was a group of planetary scientists who very strongly advocated a dynamical planet definition that would limit solar system planets to a small number."

The great planet debate: Pluto's redefinition is still controversial 15 years later
The rich color variations of Pluto.

Once the resolution was adopted, many professional planetary scientists signed a formal petition to reject the decision. That group was led by Alan Stern, principal investigator for the New Horizons mission, which had launched seven months earlier (January 19, 2006). This petition was rejected, and all subsequent attempts to open the resolution up for discussion have not been met with success. "The petition was not to the IAU and was never voted on, it was simply a petition showing disgust and objection to the IAU actions of 2006 by hundreds of planetary scientists," said Stern.

Brown, on the other hand, stresses that the IAU decision was indicative of the majority opinion, even if only a minority of the IAU was present to vote on it:

"Based on the IAU vote, it is pretty clear that the large number of objects being found in the Kuiper belt, including those up to the size of Pluto, convinced the overwhelming majority of astronomers that Pluto is more properly classified with the Kuiper belt rather than with the planets, in keeping with the way we classify Ceres – for example – with the asteroids."

An "unscientific" definition

The other major category, said Kornfeld, is the scientific problems arising from the definition. The most problematic is the criterion that states that a body needs to have "cleared the neighborhood around its orbit" to" meet the definition of "planet":

"Most problematic is the third criterion, which requires an object to “clear its orbit" to be a planet. This is inherently biased against worlds further from the Sun or their stars, which have larger orbits to clear. Several scientists determined that if Earth were in Pluto's orbit, it would not clear that orbit either. This means the IAU definition could result in the same object being a planet in one location and not a planet in another.

"The term "clear its orbit" is highly ambiguous. No planet has fully cleared its orbit of asteroids; Jupiter orbits with many Trojans, and Neptune has not cleared its orbit of Pluto. Several exoplanet systems with two giant planets that cross one another's orbit have been discovered. Neither of these clear their orbits, which according to the IAU definition, means they are not planets, so what exactly are they?

"The IAU resolution also states that dwarf planets are not planets at all but another type of object entirely. This is inconsistent with the term "how dwarf" in astronomy, where dwarf stars are a subclass of stars, and dwarf galaxies are a subclass of galaxies. It also contradicts the intent of Alan Stern, the scientist who coined the term in 1991 to designate a new subclass of planets."

These same concerns were cited by Alan Stern, who spoke to Interesting Engineering via Zoom. In addition to being the principal investigator of the New Horizons mission and the former executive director of the Space Science and Engineering Division at the Southwest Research Institute (SwRI), Dr. Stern also served as the Associate Administrator of NASA's Science Mission Directorate (SMD) from 2007 to 2008.

The great planet debate: Pluto's redefinition is still controversial 15 years later
The first mission to the Pluto system and the Kuiper Belt.

As he explained to Interesting Engineering via Zoom, a problem with the definition is the way it prioritizes "gravitational dominance" over intrinsic properties. This is problematic in that it produces scientifically flawed results for what isn't a planet. In short, gravitational dominance means it matters more where the object is than what it is.

"If you put the Earth out in the Kuiper Belt, it would not dominate because there's too much room out there. It's easy for the Earth to dominate here near the Sun in a little zone. But if you put it in a big zone, a thing that everyone agrees is a planet - the Earth - would not be a planet. If you could simply move the Earth there, it would change whether it's a planet or not. That's absolutely ridiculous. Planets have some intrinsic properties, and where you find them should not be how you decide if they're a planet, and that's what the IAU definition does."

Numbers Game

Another common point of contention is the way the IAU resolution was motivated by a desire to keep the total number of planets low. This is easily illustrated by examining all of the bodies under consideration. Were the IAU to stress that Pluto is still a planet (by definition), then Ceres and all the recently-discovered TNOs would also be included. This would bring the total number of planets in the Solar System (as of 2006) to seventeen, whereas the IAU definition resulted in eight.

The question remains, why are fewer planets preferable to more? As Alan Stern characterized it:

"This is more about sociology and human nature, in the sense that a real revolution was taking place in the 20th century. We didn't know about any planets or any of the stars, and the only planets we knew were the ones that were close by in our solar system. And there were nine of them. And it was a small enough number that you could expect schoolchildren to memorize their names. And then suddenly, in the 90s, dwarf planets started popping up in the Kuiper Belt.

"And then extrasolar planets start popping up all over the galaxy. And next thing you know, there are 1000s of planets, and some people go can't cope can't have that. So the only planets, according to the IAU, are the planets in our Solar System. And they just wanted to come up with a reason that they could keep the numbers small. So gravitational dominance became their sieve."

"They wanted to keep a small number of planets that you could arguably believe are influencing the Earth. If you have too many planets, including moons and asteroids, then you can't sort out the [supposedly] astrological influences. So they needed to keep the number of planets small to keep them relevant for astrology. And to believe that the purpose, you know, the idea was these planets may not orbit the Earth, but their purpose is still to serve humans. And so if their purpose is to serve humans, we've got to keep their numbers small and keep it a small system, just a culturally relevant set of objects."

Response from astronomers

According to Dr. Stern, the astronomical community has widely rejected the definition. In short, said Dr. Stern, no one is using the definition some fifteen years after the IAU passed the resolution:

"In papers written ten to fifteen years after the IAU decision, there's [been] plenty of time for it to soak in, and what you find is that the word planet is used with Pluto. The word planet is used with Eris. The word planet is used with the Earth's Moon. And the word planet is used with Sedna and with many other objects. And that's the real data."

"What's brilliant is that scientists are voting with their feet (if you will). When they write a scientific paper, no one is asking them to make a choice, but they make a choice in their head. They call these things planets. And that's really where the mindset of planetary scientists are."

The great planet debate: Pluto's redefinition is still controversial 15 years later
This artist's concept depicts select planetary discoveries made to date by NASA's Kepler space telescope.

To illustrate this point, Dr. Stern cited the work of Dr. Philip Metzger, a physicist with the Planetary Science faculty at the University of Central Florida (UCF). Dr. Metzger has been a vocal advocate for a new definition of "planet" based on geophysical properties rather than astronomical characteristics. In recent years, he performed a survey of 1000 scientific papers using machine learning to determine how often the IAU definition was being used.

For this, he determined that the IAU definition was not being used and that scientists used the term "planet" based on the properties of a celestial object. As Dr. Metzger explained to Interesting Engineering via Zoom:

"[D]efining classes of objects in nature is deep to the heart of what science is. We don't just create arbitrary definitions. In order to have everybody using the same terminology, that's what the IAU seems to think. But no, defining classes of objects is part of science. And so, because of that, we're not going to create one definition and then stick with it forever. We will continue to evolve the definitions to keep up with science because that's what science does."

According to Metzger, the problem with the IAU definition comes down to a simple matter of scientific vs. astrological traditions. From ancient times to Classical Antiquity and the Medieval Era, the perception of planets was of "wandering stars" and "heavenly spheres" that were part of an ordered system. However, since the time of Copernicus and Galileo, the perception has shifted to one where planets became viewed as bodies similar to Earth.

In particular, they have come to be seen as dynamic, evolving places in the cosmos where complexity forms, including life. By the 19th and 20th centuries, scientists learned that no other planets in the Solar System have life on them, nor do they have the necessary conditions to support life (there's no evidence to that effect, at least). This has led to further refinements in how planets are perceived, but as Dr. Metzger stressed, the basic idea has remained the same.

"[T]hey are still these unique objects that sometimes create the conditions for life because of their nature," he said. "We need a definition of a planet that captures that because that's the real essence [going] all the way back to Galileo. That's the essence of what we mean by planets. And that's why we do planetary science. So the geophysical definition is an attempt to encapsulate that idea in a very simple way."

Brown, however, offered a different appraisal. In his estimation, the IAU definition affirmed something that was long treated as established fact:

"The IAU resolution really just reaffirmed what had long ago been established: there are planets, and there are members of small body populations. The fact that we mistakenly classified Pluto as a planet for so long – because we didn't realize it was a member of a small body population – made it seem like we were changing definitions, but we were simply reaffirming the old ones."

What defines a planet?

The geophysical definition that Metzger and other scientists advocate for comes down to what a planet is rather than where it is (i.e., the nature of its orbit). As he explained in a 2018 paper, the term "planet" should apply to any celestial body that is sufficiently massive to have undergone hydrostatic equilibrium. This means that a planet is massive enough to have become rounded by its own gravity but not so large that it triggers nuclear fusion at its core (i.e., a star).

The great planet debate: Pluto's redefinition is still controversial 15 years later
Exoplantes illustration.

As Dr. Metzger explained, this is not an arbitrary definition but a recognition of what takes place in the Universe:

"The reason we use those limits is that they reflect the range of objects. That's the range of sizes of objects where complexity flourishes. So once they're large enough to round themselves by their gravity, you get mantle convection, and you start having active geology at the surface. You start to get outgassing, sometimes retention of atmospheres and oceans, and chemical reprocessing.

"But if it's so large that it's to star and nuclear fusion happens that all that flux of energy tends to break up all the molecules, you don't get complexity anymore... So in my mind, geological complexity is the essence of planets. But the definition is trying to encapsulate geological complexity using geophysical limits, large enough to be grounded by gravity, but not so large that nuclear fusion occurs."

This echoes what Kornfeld said about the IAU's "dynamical definition" of a planet, which gives "primacy to an object's location over its intrinsic properties. In contrast, the geophysical definition, which most planetary scientists prefer, puts an object’s intrinsic properties first, emphasizing that once an object is large enough to be rounded by its own gravity, it develops complex geology and planetary processes not seen on non-spherical objects."

On this point, Brown partly agrees. As he explained, rather than adopting an alternate definition, he suggested a definition is unnecessary. At the same time, he favors keeping the IAU definition for the sake of simplicity and practicality:

"Astronomy does not tend to define objects but rather describe them. There is no definition of galaxy or star or nebula. There are concepts. The planetary concept is that planets are the large gravitationally dominant objects in a planetary system. But if you make a lawyerly three-part definition of what is and is not a planet, you will get lawyerly arguments that are, in fact, ridiculous, like 'the Earth is not a planet because of near-Earth asteroids' and things like that. So I would just keep the concept and mention that we currently know of 8 such objects in our Solar System and are looking for more."

Many questions remain at this point. Will the Great Planet Debate ever be resolved? With the astronomical community ever come together to agree on a standard definition? Is a definition even necessary when many scientists seem perfectly comfortable using it as they see fit? But perhaps these are the wrong questions. Perhaps the most important thing is to recognize how the Debate began.

If there's one thing all sides agree on, it's that this campaign to clear up the nomenclature began as a result of so many discoveries. Today, more TNOs are being discovered with the help of next-generation telescopes, and the census of exoplanets is expected to reach tens of thousands in the near future! With so many more objects available for study, it's entirely possible that another General Assembly will need to be convened.

Hopefully, there will be common ground if and when that day comes. Hopefully, a new definition will be proposed that satisfies the geophysical and astronomical community. Hopefully, but in the meantime, two things appear to be pretty clear. One, the Debate is not over; and two, the IAU Resolution is not the last word on the subject.

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