Something strange happens when people talk about Pluto. This small, distant object probably inspires more widespread debate, and emotion, than anything else in our solar system – and all because it was reclassified from a planet to a dwarf planet.
People were outraged when Pluto was ‘downgraded’, and this feeling has lingered. Even now, when Pluto comes up in conversation, someone will inevitably say, ‘Poor Pluto, it will always be a planet to me.’ It’s strange that such a small, distant object can create such strength of feeling. Or perhaps it isn’t strange at all. I think the emotion surrounding Pluto reflects two important elements of the human condition – elements which often come into conflict with each other, but which are fundamental to us all.
Pluto was discovered in 1930 by American astronomer Clyde Tombaugh, and it was named as the ninth planet in our solar system. It is very small – it has a radius of just 1,188 km, making it less than 0.2 times the size of Earth. It is also very far away – if the average distance from Earth to the Sun is 1 astronomical unit (AU), Pluto is on average 39 AU from the Sun, orbiting in a rock-strewn region beyond Neptune known as the Kuiper Belt.
Pluto remained a planet for 76 years after its discovery, and anybody who went through school in those 76 years learned about it as such. When I was a kid, I was taught that there were nine planets in the solar system. Indeed, the acrostic ‘My Very Easy Method Just Speeds Up Naming Planets’ is sadly incomplete without Pluto.
However, in 2005, a new Pluto-like object was discovered: Eris. Eris has a radius of 1,163 km – about the same as Pluto’s – but it is 27% more massive than our distant neighbour, because it is made up of denser rock, compared to Pluto’s mix of rock and ice. Eris orbits at around 68 AU from the Sun, and this greater distance is probably why it took us so long to find it.
The discovery of Eris presented a problem for Pluto. Although we had known about other large objects orbiting far beyond the orbit of Neptune since the 1800s, Eris was more massive than Pluto. If Pluto was a planet, Eris had to be too – either they were both planets, or neither could be. So why not classify Eris as a tenth planet? It seemed to be just as qualified as Pluto, and living in a solar system with ten planets would be pleasingly neat.
Unfortunately space (and science in general) is rarely pleasingly neat, and more objects were being discovered that made the question of Pluto more pressing. There was frozen Makemake, the bright and icy Haumea, and numerous other, smaller bodies orbiting in the Kuiper Belt. It seemed likely that we would continue to discover more. Should Pluto be reclassified to join its siblings? Or should the new discoveries be named planets too? What acrostic would we come up with to remember all those?
In August 2006, the International Astronomical Union (IAU) – an international organisation of professional astronomers, all PhD level or above – decided on an official definition of a ‘planet’. This consists of three criteria: 1) it must orbit the Sun; 2) it must be massive enough that its gravity has shaped it into a sphere; and 3) it must have cleared the area of its orbit. Pluto does not meet the third criterion, surrounded as it is by other objects, tiny and large, in the Kuiper Belt. It was decided: Pluto was out.
There were (and still are) some objections within the scientific community to the IAU’s definition of a planet, and Pluto’s subsequent reclassification, but what I find more interesting is the response of the average person. These objections aren’t usually based on technical details, but on emotion. When a layperson says ‘Poor Pluto’ or ‘Pluto will always be a planet to me’, they’re not thinking about the science of the thing. More likely they’re imagining a younger version of themselves, sitting in a classroom, reciting the planets and counting them on nine fingers. For this reason, the nostalgia surrounding Pluto probably has an expiry date – the sympathy we feel for this ball of rock and ice, floating out in space, 39 times further away from us than the Sun, will probably only last as long as the people who were taught that Pluto was a planet in school.
These feelings are strange – but also inherently human. Pluto’s reclassification is often spoken of in negative terms: it has been ‘demoted’ or ‘downgraded’. But there is nothing bad about what has happened to Pluto, because there is nothing objectively better or worse about being a planet than a dwarf planet. It’s a label in a system of classification, that’s all. Any feeling that Pluto is somehow being ‘punished’ for not living up to a certain set of criteria is purely our own projection. (And it’s telling, perhaps, that the feelings we associate with Pluto are tied so tightly both to memories of school days and to the idea of not living up to a certain standard.)
Our nostalgia for Pluto isn’t about Pluto. It’s about what happens when new information forces us to rethink our worldview – something that humans are notoriously bad at. There’s even a term for it: the backfire effect. When faced with new evidence that contradicts an existing belief, people will often double down and the original belief will get stronger. This is what has happened with Pluto. People who never cared about space or astronomy or planet classification before suddenly care so deeply that they’re willing to deny high-level scientific consensus in order to keep calling it one. Pluto-the-planet has become personal, through its association with childhood memories and emotional development, and when facts and emotions become bound up together, it’s very difficult to separate them again.
The push and pull going on here is indicative of two things that humans love to do. First, we love learn – to expand our knowledge, discover new things and try to define our surroundings in as much detail as possible. The universe resists this, because it is vast and complicated and chaotic and random, but still we continue to hone our understanding, and – even if we can never truly pin anything down – we at least try to get as close as we can.
Second, we love to hold on to ideas. They are warm, like a comfort blanket. They take us back to the time when we formed them – when we were younger and things felt simpler, perhaps – and once formed, we find it very difficult to let those ideas go. We hold onto them dearly, as if they define us. (And indeed, perhaps they do.) But holding on can be a bad thing when what we’re holding on to is no longer true, or useful. Pluto’s reclassification is not personal, it’s practical. We could keep discovering new objects and calling them planets, but ‘planet’ and ‘dwarf planet’ are scientific terms, invented to help us group certain objects together in order to do useful work. These terms are tools created to help us understand more about the universe and our place in it. And if a tool stops being useful, we may have to adjust it. When holding onto something purely for nostalgia’s sake makes our lives more difficult, we have to let it go.
Pluto is not a planet. From a purely objective point of view, none of our labels apply, because the concept of a ‘planet’ or a ‘dwarf planet’ is human-made, for human purposes. But for those human purposes, right now, ‘dwarf planet’ is the best label we have for Pluto. It wasn’t always this way, but we gather new information, we refine our understanding and we adapt our words to reflect what we have learned. Things change, and that’s OK.
But if you still feel nostalgia for Pluto, just know that it is because you are human. Space is a canvas, and we have been painting our emotions onto it for as long as we have been alive.
1996 photograph by Alan Stern (Southwest Research Institute), Marc Buie (Lowell Observatory), NASA and ESA.
2002/2003 composite photograph by NASA/ESA/SRI (M. Buie) [Public domain].
2015 photograph by NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute [Public domain].