In August of 2006, astronomers at the International Astronomical Union (IAU) democratically voted to reclassify Pluto as a dwarf planet — but 20 years later, with our greater knowledge and understanding of planetary systems, is it time to revisit the controversial decision? Should Pluto be called a planet again?
For several years before the IAU vote, there had been agitators among the astronomical community lobbying for our solar system's nine worlds to become eight, and for Pluto's planetary status to be revoked. In 2000, Neil DeGrasse Tyson even went as far as to remove Pluto from the planets display at the Hayden Planetarium in New York. The questioning of Pluto's status began even earlier, in the 1990s, with the discovery of distant objects in the Kuiper belt beyond Neptune and which had similar orbits to Pluto.
It all came to a head in July of 2005 with the discovery of Eris, which is a Pluto-size body even further from the sun than Pluto is. Was Eris the 10th planet? If it was not a planet, then how could we still call Pluto a planet? A year later, the issue was debated at a meeting of the IAU, where astronomers from all over the world convened to discuss various conundrums. Numerous proposals and counter-proposals were put forward, some of which even increased the number of recognized planets to as many as 14. But in the end, Pluto became a dwarf planet.
Though Pluto was officially "reclassified," to many in the public it seemed as though Pluto had experienced a demotion. This viewpoint was the one promoted by those scientists who were against the decision such as Alan Stern, who was the principal investigator on the New Horizons mission to Pluto. Stern's spacecraft would launch in 2005 on a journey to a planet, but arrive at a dwarf planet in 2015.
Some argued that Pluto was still a kind of planet, just a small one. This argument is undermined somewhat by the IAU giving Pluto a minor planet number — 134340 — which is typically reserved for asteroids. Nationalistic arguments from some in the United States said Pluto should remain a planet because it was discovered by an American astronomer, Clyde Tombaugh, at Arizona's Lowell Observatory in 1930. And when New Horizons reached Pluto, it revealed not a minor body, but rather a complex world in its own right.
What is Pluto?
Nevertheless, for 20 years, Pluto has remained a dwarf planet, but on the anniversary of that fateful IAU vote, is it time to revisit that decision? Paul Byrne, a planetary astronomer at the University of Washington in St Louis, certainly thinks so.
"Yes, Pluto is a planet," he told Space.com. And not just Pluto, but the other dwarf planets too: Ceres, Eris, Makemake and Haumea, not to mention the likes of Sedna and Quaoar that are dwarf planets in all but name.
"Ultimately I think they are all planets and I don't think there's any benefit to not including Pluto as a planet," Byrne said.
To a point this entire debate is semantics, and Byrne accepts this viewpoint, reminding us that "Nature doesn't care what we call Pluto."
However, perception is another matter, which is where Byrne thinks that the IAU's 2006 decree ultimately failed us. Byrne describes how, when he gives public talks about the planets, members of the audience don't ask questions about Pluto's amazing active nitrogen glaciers or what formed its giant heart-shaped feature. Instead, they want to know why it was demoted. The entire issue has got in the way of presenting the science to the public.
"We've done a massive disservice to the public from a science communication perspective," Byrne said. "The messaging has got in the way of trying to understand what Pluto is."
One of the arguments in 2006 was that models of the solar system's formation predict that there could be hundreds of objects like Pluto and Eris that remain undiscovered in the Scattered Disk and the Oort Cloud, which is a vast unexplored region that could extend up to a light-year from the sun.
"One of the issues people brought up was, how would kids remember the names of 200 planets?" said Byrne. "But just look at how many Pokémon there are that kids know the names of. If ultimately there are 200 planets in the solar system, then so what?"
The IAU definition of a planet
Semantics or not, how we categorize celestial objects has to be grounded in some kind of scientific thinking. In 2006 the IAU came up with a three-point plan on what makes a planet.
First, a planet had to be in hydrostatic equilibrium, meaning that its gravity has pulled it into a spherical shape. Furthermore, its interior will have become differentiated, with the heavier elements sinking to the core. Everyone seems to agree with this part of the planetary definition.
Second was that it must be in orbit around the sun, so a moon isn't a planet because a moon doesn't orbit the sun, it orbits a planet instead (though Byrne notes that if he had his way, he'd call the large moons planets too). Strictly speaking, this rules out exoplanets because they don't orbit our sun, but common sense dictates that they're fine because they orbit their own star instead. However, what about rogue planets that wander interstellar space alone? Astronomers are now beginning to discover very many of these free-floating worlds. It could be argued that most of these did indeed form in orbit around a star only to be ejected, so it's okay to still call them planets, but some rogue planets seem to have formed directly from the collapse of a gas cloud without having a parent star. Such planets fall through the gaps in the IAU's definition.
It's the third point that draws the critics' ire the most. It states that to be a planet, an object must have cleared its neighborhood. In other words, it must be dominant in the region around its orbit, its gravity having swept away any other bodies. Pluto fails at this because its orbit actually crosses Neptune's, and it's accompanied by other Kuiper belt objects in neighboring orbits. But then you could say Neptune mustn't be a planet because it has Pluto infringing on its orbital realm. And what about the existence of near-Earth asteroids? Does their existence mean that Earth isn't a planet because it hasn't cleared those away yet? There certainly does seem to be some sloppiness in this part of the definition of a planet, perhaps resulting from the somewhat rushed nature of the vote 20 years ago.
Planetary processes
But if the IAU decision were to be rejected, what should replace it? How should we classify planets? Byrne has some ideas.
"Being round and having some amount of differentiation makes a lot of sense to me," he said. "Geophysically, if they are big enough to pull themselves into balls, then they are big enough to have undergone some kind of internal chemical processing and are capable of having some kind of active geology, in terms of the outgassing of volatiles to form an atmosphere, volcanism, surface geomorphological processes, even a short-lived magnetic field. That's why I count the moon as a planet because it has had these planetary processes."
Byrne acknowledges that gas giants don't have these properties since they have no solid surface, but no one is doubting Jupiter's planetary credentials, and when it comes to Uranus and Neptune we don't actually know whether or not there is some sort of super-Earth itself lurking deep within them.
The moniker "dwarf" doesn't impress Byrne either. To him, it sounds like it implies there is a hierarchy of planets with the gas giants at the top and the dwarf planets at the bottom.
"There is no natural hierarchy; they don't form according to one," he said. And although Pluto, Eris and the other dwarf planets are made from detritus left over from the construction of the solar system, they still coalesced in the same way as the other planets, in the sense that building blocks came together to form these larger objects.
A counterargument to Pluto being a planet, however, is that if it is just one of 200, then it is nothing special; it would become forgotten and ultimately treated almost incidentally. As a dwarf planet, it is the standard bearer of its kind, the guardian of an entire realm of similarly sized worlds. It's a choice between being a big fish in a little pond, or a little fish in a big pond.
The other 200 planets?
The real test will come when astronomers begin to discover some of these purported 200 or so planets far from the sun. As long as they are all roughly Pluto-size, there will be no debate — but what happens when astronomers discover one the size of Mars, or one even as large as Earth? We would surely think of an object so large as a planet. Perhaps this world would have formed closer to the sun but was kicked out through gravitational interactions with one of the giant planets, but on the other hand perhaps it formed out there in the outer solar system from the same detritus as Pluto did.
"By the IAU definition it would fail to be defined as a planet because it wouldn't have cleared its neighborhood," said Byrne. "If we did find such a planet it would be another nail in the coffin of trying to distinguish these things solely on the basis of where they are and not what they are."
Whatever the case, if Pluto is to be a planet again, it must deserve to be so scientifically. Nostalgia, or nationalistic pride, or giving kids names of planets to remember aren;t sufficient reasons to revoke the original IAU decision. Revisiting the name "dwarf planet" and finding a more appropriate category could be the way to go instead.
Or, perhaps we just don't have the words to describe the things that we are discovering. Concepts such as planets are rooted in history going back millennia when there was no understanding what these wandering "stars" really were, so perhaps it is not surprising that we are now running into difficulty when trying to apply the term "planet" to our modern understanding of the different types of worlds. If there is no way of categorizing these objects that is consistent and satisfactory to everyone, then perhaps we ought to stop worrying so much about it.
Whatever we decide to call it, Pluto will always be Pluto.
