In August 2006, the International Astronomical Union (IAU) voted to reclassify Pluto as a dwarf planet. Now, two decades later, the discussion about maintaining or reviewing this controversial decision is rekindling in the scientific community, driven by new knowledge about planetary systems.

The campaign to remove Pluto from the list of planets in our solar system gained momentum years before the IAU vote. In 2000, Neil DeGrasse Tyson even removed Pluto from the planet exhibition at the Hayden Planetarium in New York. The questioning began in the 1990s, with the discovery of distant objects in the Kuiper Belt, in addition to Neptune, which had orbits similar to Pluto.

The critical point was reached in July 2005, with the discovery of Eris, a body similar in size to Pluto, but even further away from the Sun. The question then arose: would Eris be the tenth planet? If it weren’t, how could Pluto continue to be considered one? A year later, the topic was widely debated at an IAU meeting, where varied proposals were presented, some suggesting increasing the number of planets to up to 14. However, in the end, Pluto was categorized as a dwarf planet.

Although it was officially reclassified, for many, including the public, it felt like a downgrade. This feeling was fueled by scientists opposed to the decision, such as Alan Stern, principal investigator for the New Horizons mission to Pluto. Stern’s probe was launched in 2005 toward a planet, but arrived at a dwarf planet in 2015.

Some defenders argued that Pluto still qualified as a type of planet, just a small one. However, this argument was weakened by the IAU by assigning Pluto the number of a minor planet — 134340 —, a classification generally used for asteroids. There were also nationalist arguments in the United States, which defended Pluto’s permanence as a planet because it was discovered by an American astronomer, Clyde Tombaugh, at the Lowell Observatory, in Arizona, in 1930. Furthermore, when the New Horizons probe reached Pluto, it revealed not a smaller body, but a complex world in its own right.

After all, what defines Pluto in the solar system?

Even with Pluto remaining a dwarf planet for 20 years, the anniversary of the IAU vote raises the question of reviewing the decision. Paul Byrne, a planetary astronomer at Washington University in St. Louis, believes so.

“Yes, Pluto is a planet,” he told Space.com. And not just Pluto, but also the other dwarf planets like Ceres, Eris, Makemake and Haumea, as well as Sedna and Quaoar, which are dwarf planets in all but name.

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“At the end of the day, I think they’re all planets, and I don’t see any advantage in not including Pluto as a planet,” Byrne said.

To some extent, this whole debate is a matter of semantics, a point that Byrne accepts, noting that “Nature doesn’t care what we call Pluto.”

However, public perception is different, and this is where Byrne believes the 2006 IAU decree was harmful. Byrne describes how, when giving public talks about the planets, the audience asks not about Pluto’s active nitrogen glaciers or its gigantic heart-shaped structure, but rather about the reason for its “demotion.” This issue has made scientific communication to the public difficult.

“We do the public a huge disservice from a science communication perspective,” Byrne said. “The message ended up getting in the way of understanding what Pluto is.”

One of the arguments in 2006 was the prediction that models of the formation of the solar system indicated the existence of hundreds of objects such as Pluto and Eris not yet discovered in the Dispersed Disk and the Oort Cloud, a vast unexplored region that could extend up to a light year from the Sun.

“One of the questions raised was: How would children remember the names of 200 planets?” said Byrne. “But just look at how many Pokémon kids know. If, after all, there are 200 planets in the solar system, what’s the problem?”

International Astronomical Union rules for defining a planet

Regardless of semantics, the categorization of celestial objects must be based on scientific thinking. In 2006, the IAU established a three-point plan to define a planet.

First, a planet must be in hydrostatic equilibrium, meaning its gravity has shaped it into a spherical shape. Furthermore, its interior needs to be differentiated, with the heaviest elements concentrated in the core. There appears to be a consensus regarding this part of the definition.

Secondly, the planet must orbit the Sun. Thus, a moon is not a planet because it orbits a planet and not the Sun (although Byrne mentions that, for him, larger moons would also be planets). Strictly this excludes exoplanets, as they orbit their own stars, but common sense would include them. However, what about the rogue planets that roam interstellar space without a host star? Many have been discovered. It can be argued that most formed in orbit around a star and were ejected, retaining planet status. But some appear to have formed directly from the collapse of a gas cloud, without a star, not meeting the IAU definition.

The third point is the one that provokes the most criticism: to be considered a planet, an object must have “cleaned its neighborhood”. In other words, it needs to be dominant in the region of its orbit, with its gravity having swept other bodies. Pluto does not meet this criterion because its orbit intersects with Neptune’s and it is accompanied by other Kuiper Belt objects in nearby orbits. However, we could argue that Neptune would not be a planet either, since Pluto invades its orbit. What about the existence of asteroids close to Earth? Would that mean Earth isn’t a planet because it hasn’t cleaned them up yet? There appears to be some vagueness in this part of the planet definition, perhaps a result of the somewhat hasty nature of the vote 20 years ago.

Understand the geophysical processes that characterize a planet

If the IAU decision were overturned, what would replace it? How would we classify planets? Byrne offers some suggestions.

“To me, the fact that they are spherical and have some degree of differentiation makes a lot of sense,” he said. “Geophysically, if they are large enough to contract into spheres, then they are large enough to have undergone some type of internal chemical processing and be capable of exhibiting some type of active geology, in terms of release of volatiles to form an atmosphere, volcanism, surface geomorphological processes, and even a short-lived magnetic field. That’s why I consider the Moon a planet, because it has undergone these planetary processes.”

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Byrne recognizes that gas giants do not have these properties, as they do not have a solid surface, but no one questions Jupiter’s planetary status. In the case of Uranus and Neptune, we don’t know if there is some kind of super-Earth hidden inside.

The term “dwarf” also doesn’t please Byrne. For him, the word seems to imply a hierarchy of planets, with gas giants at the top and dwarfs at the bottom.

“There is no natural hierarchy; they don’t form according to one,” he said. Although Pluto, Eris and other dwarf planets are made of debris left over from the construction of the solar system, they formed in the same way as the other planets, by joining building blocks together to form larger objects.

However, a counter-argument to the idea of ​​Pluto being a planet is that if it is just one of 200, it will lose its uniqueness; will be forgotten and treated almost as a detail. As a dwarf planet, it is the standard-bearer of its species, the guardian of an entire kingdom of similarly sized worlds. It’s a choice between being a big fish in a small pond or a small fish in a big pond.

Are there still hundreds of other planets like Pluto?

The real challenge will come when astronomers discover some of these supposed 200 planets far from the Sun. As long as they are all about the size of Pluto, there will be no debate. But what will happen when astronomers discover an object the size of Mars, or even as big as Earth? Certainly, we would think of an object as large as a planet. Perhaps this world formed closer to the Sun, but was ejected by gravitational interactions with one of the giant planets. On the other hand, perhaps it formed in the outer solar system, from the same debris as Pluto.

“According to the IAU definition, it would not be considered a planet because it would not have cleaned up its neighborhood,” explained Byrne. “If we found such a planet, it would be another nail in the coffin of trying to distinguish these objects solely based on their location and not their nature.”

Regardless of the outcome, if Pluto is once again considered a planet, it needs to deserve the title from a scientific point of view. Nostalgia, nationalist pride or the desire to have planet names that are easy for children to memorize are not sufficient reasons to revoke the IAU’s original decision. Reconsidering the term “dwarf planet” and finding a more appropriate category could be the way forward.

Perhaps the issue is simply that we don’t have the right words to describe the findings. Concepts like “planets” have historical roots that go back millennia, when there was no understanding of what these wandering “stars” were. So it is perhaps not surprising to encounter difficulties when trying to apply the term “planet” to our modern understanding of different types of worlds. If there is no consistent and satisfactory way for everyone to categorize these objects, perhaps the best option is to worry less about the nomenclature.