I was out and I am sorry I did not get a post in yesterday, but this one is interesting and is a good read. The following is from interestingengineering.com and written by Matthew S. Williams:

"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."

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 'iceteroids') 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 Kornfield, who has fiercely advocated for Pluto's "planethood" since the 2006 IAU resolution. As Kornfield 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."

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 19th, 2006). This petition was rejected, and all subsequent attempts to open the resolution up for discussion have not been met with success.

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 Kornfield, 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.

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."

"[T]hey 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 suppose that 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 Eros. 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."

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:
"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).

As Dr. Metzger explained:
"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 object in our Solar System and are looking for more."