Celebrating ‘Pluto Demoted’ Day

Today marks one of the most discussed anniversaries of science - the fourteenth ‘Pluto Demoted Day’. On 24th August, 2006, the International Astronomical Union passed a motion formalising a new definition of ‘planethood’, and introducing a whole new category of object. The ‘dwarf planets’ were born. And across the globe, there was much wailing and gnashing of teeth - and also much rejoicing. 

It’s fair to say that the decision was somewhat controversial - and in the fourteen years since, the debate over Pluto’s planetary nature has raged on, often boiling over into flame wars, anger, and upset. On one side sit those who argue Pluto should never have been considered a planet in the first place, and who are delighted with the new decision (even if they agree that the definitions still need to be tweaked and improved). On the other side sit vocal advocates, arguing either that Pluto should be a planetary exception, or that our Solar system should contain hundreds (or thousands) of planets, and that Pluto should be reinstated.

The whole debate is fascinating - and it reflects one of the great habits of humanity. When we look at the world around us, we naturally try to put things into neat packages. That’s a big part of how we make the world around us comprehensible. Let’s move away from objects in the outer Solar system, for a moment, and think instead of a human lifespan.

We live our lives one day at a time - from second to second, minute to minute, and year to year. Time passes in a continual flood. We go to sleep one night, and wake up the next day essentially unchanged - the same person. Aside from those moments of sleep, we don’t really experience the world in discrete packets - time flows by continually, one second at a time.

But to live in that continual flow of time, we define arbitrary boundaries. One second, one hour, one week. We break the continual flow of time into discrete packets. And we do the same with our lives. Rather than simply being a person, we are a toddler, a child, a teenager, an adult - and so on. We take a continuum of experience (time) and break it into discrete, manageable chunks - each of which we consider to be different to the others. 

We then use those arbitrary chunks to manage our lives. I remember one day, back in the 1990s, going to sleep hugely excited - the next morning, I would wake up, and miraculously, I would be legally able to drive. Between one second and the next, at the stroke of midnight, I went from being too young, to being an adult. Fundamentally, I was the same person one second as the next, but when I crossed that threshold, I was recategorised. I went from ‘non-driving human’ to ‘driving human’ overnight. 

We place these boundaries on our lives without a second thought. And it feels natural and sensible. Of course you should have to wait until you’re a certain age to drive, or to vote, or to drink. But the line we place in the sand is arbitrary - and, indeed, it varies from country to country, and from one generation to the next.

Just as we do these things in our day-to-day lives, we also do them when studying the world around us. To better understand the world, we break it into manageable chunks. We group objects together that are similar, and place objects that are obviously different in different groups. And, typically, we do this as soon as we have enough evidence to feel we can see the different groups clearly. 

In astronomy, we do this all the time. We break galaxies up into classes (elliptical, spiral, irregular), even though there is really a continuum of galaxy shapes, which each reflect the unique history of the galaxies themselves. We break the electromagnetic spectrum into ‘types’ of radiation - from gamma rays through the optical, and all the way out to radio wavelengths.

And so it is with single objects. We have stars (so massive that they undergo fusion in their cores, causing them to shine/be self luminous) and planets (insufficiently massive to undergo fusion, so they tend to be seen by reflected light). A century ago, that definition was sufficient - things were obviously either a planet or a star - and things were simple. 

But, in the late 1980s, astronomers started to find objects that blurred the boundaries - objects that had been predicted for twenty or thirty years, but had previously been too hard to find. These new objects were so massive that, in their youth, they could briefly experience ‘deuterium burning’, but would then sit there, dull and warm, unable to achieve full hydrogen fusion, unable to shine as a star. Enter the ‘brown dwarfs’ - neither star, nor planet, but something in between.

As we found out more about the universe, we had realised that, rather than two discrete groups of objects (‘stars’ and ‘planets’), we had a continuum - and we modified our classification system to adapt for that.

Which brings us back to Pluto…

Now, Pluto is weird - it is highly reflective, much more so than your typical ‘icy Solar system body’. We now know that reflectivity is linked to its here-today, gone-tomorrow atmosphere - when Pluto is closest to the Sun, it grows an atmosphere that then precipitates back out as it cools on receding from the Sun. That means that Pluto’s surface is covered in fresh ice - highly reflective.

Back when Pluto was discovered, in 1930, all we knew about it was how bright it appeared in the sky, and how far it was from the Sun (based on how it moved in the sky). To estimate its size, astronomers had to guess how reflective it was. They assumed that Pluto was similarly reflective to the planets - and, based on that, estimated that it must be about the size of Earth. This fit nicely with the idea that Pluto’s gravity was pulling Neptune around (the whole reason that astronomers had been searching for a missing planet in the first place), and the whole story seemed to make sense. Clearly, the missing planet had been found - making Pluto planet nine. 

Over the decades that followed, however, it became clear that things simply didn’t add up. Over time, our best estimates of Pluto’s size got smaller, and smaller, and smaller still. At the same time, the evidence for Neptune’s perturbed motion disappeared - nothing unseen was pulling the eighth planet around, after all. From a planet the size and mass of Earth, Pluto diminished, being revealed as an object both smaller, and less massive, than the Moon. 

The final nail in Pluto’s planetary coffin came in the 1990s, with the discovery of many other ‘trans-Neptunian objects’. Rather than being a solitary wanderer at the outer edge of the Solar system, Pluto was revealed as just one of many icy bodies, trapped on stable orbits beyond the orbit of Neptune. Whilst it was the largest of those objects, Pluto had far more in common with them than it does with the eight ‘other’ planets.

Based on all this new information, astronomers took the plunge in 2006, and approved (albeit admit argument and controversy) their new definition of a planet. And in the process, they demoted Pluto to become a founder member of a new class of object - the dwarf planets.

Both dwarf and regular planets are massive enough that their gravity can overcome the physical strength of the material they are made from, pulling them into a spherical or near-spherical shape (technically, the definition calls for them to be in hydrostatic equilibrium - so fast rotating planets can be very oblate and still be planets!). Both classes of planet must also orbit the Sun - rather than another body (hence why the Moon, or Jupiter’s Ganymede, which is larger than Mercury, are not considered planets). 

The third criterion for true planethood, under the new definition, is the one that differentiates between the true and dwarf planets - it must have cleared the neighbourhood around its orbit. 

The eight ‘regular’ planets in the Solar system meet this criterion easily - any other objects that are flung onto orbits that cross those of a ‘regular’ planet are rapidly removed from the area as a result of the planet’s gravitational influence. Jupiter, as the most massive planet, is the most efficient at this - regularly taking objects whose orbits come too close and ejecting them from the Solar system entirely. Those objects that remain, sharing the neighbourhood with the regular planets, are corralled by those planets into stable regions - such as the Jovian Trojans, two vast clouds of objects that lead and trail the planet by 60 degrees in its orbit. Whilst those objects share Jupiter’s orbit, they are very much under the control of the giant planet.

The dwarf planets, by contrast, lack sufficient mass to clean up after themselves. Here, Pluto is a fantastic example - it moves in a region littered with other small bodies, and does nothing to clear that debris away. In fact, it turns out that Pluto is itself under the control of Neptune, corralled into a reservoir known as the ‘Plutinos’ with thousands, and potentially millions, of other small bodies. The Plutinos, named for their largest member, are all trapped in ‘mean-motion resonance’ with Neptune - completing two orbits of the Sun for every three completed by the giant planet. 

As a result of being trapped by Neptune in that resonance, Pluto can never come close to the giant planet - despite the fact that their orbits actually cross. At their closest, the two are farther apart than the distance between the Earth and Uranus - more than a billion kilometres distant.

So, with all this in mind, should Pluto be a planet? I’d argue clearly not. To go back to the analogy of a human lifetime - Pluto is like a tall, gangly teenager. From a distance, with no point of comparison, it looks like a grown adult -- it’s only when you look closer that you realise it still hasn’t crossed the magic threshold of being able to drive. And, to top it all off, it can’t even tidy it’s room!

But having said that - this doesn’t diminish Pluto in any way - it is still a magical, beguiling world. The images returned by the amazing New Horizons mission, back in 2015, are truly spectacular, showing an incredible variety of terrains, and providing scientists with data that will keep them busy for decades to come.

So, a wonderful, beautiful world. But it’s still not a planet!

Asteroid Belt video credit: Scott Manley.
Pluto simulation animation video credit: Ethan Siegel.