Pluto, it’s not you, it’s us

Did you know we offer a Winter School course in Communicating Science? If you’re interested in science writing for both specialist and non-specialist audiences, presenting to communicate science and the use of emerging online social media in science communication, this is a great course for you. If you’re interested, check out “Communicating Science” in course outlines for more details.

Over the coming weeks we’ll be showcasing blog posts written by Communicating Science students during their course last year. The next blog in the series is “Pluto, it’s not you, it’s us” by By Rohan Hudson


It’s no secret that astronomers on Earth have a tumultuous relationship with the planet/dwarf planet/plutoid/whatever you want to call it named Pluto. Most of us would have been taught in school that our solar system had nine planets, with Pluto being the smallest and furthest from us. However, in recent years, there’s been a huge uproar as to what exactly Pluto should be classified as, and this debate has been inflamed in the last week or so as the New Horizons space probe completed the first ever fly-past of this distant object.

An accurate depiction of most office conversations this last week. (Credit: Rob DenBleyker)

An accurate depiction of most office conversations this last week. (Credit: Rob DenBleyker)

The Planet ‘Percival’?

The first direct observation of Pluto is credited to astronomer Clyde Tombaugh in 1930, however its existence was first proposed 25 years earlier, to help explain the ‘wobbles’ observed in the orbits of Neptune and Uranus. And it was very nearly not named ‘Pluto'; other names were suggested, such as ‘Zeus’ (Yeah, pretty awesome, right?), or ‘Percival’ or ‘Constance’ (…..slightly less awesome). ‘Pluto’ was one of the Ancient Greek names for the God of the Underworld – a fitting name, it was thought, given how dark and cold the planet must have been. This new planet was quickly ingrained into our scientific and popular cultures (much like Mars; see my previous post); a new element discovered in 1940 was named after this new planet, giving us the ‘plutonium’ that we know today (and to think, it was nearly ‘percivilium’!). It’s also thought that Walt Disney named Mickey Mouse’s pet dog after the new planet, although there are many conflicting accounts of this.

Eris and the Kuiper Belt

For the next half-century, Pluto was widely accepted as the ninth and furthest planet from the sun. This all changed in the 1990’s, though, with the discovery of many, many relatively large bodies orbiting the sun beyond the orbit of Neptune. These objects were classed together in what’s called the Kuiper Belt, the region of which roughly intersected with the orbit of Pluto. The video below shows a brief outline of how these were detected and what significance Kuiper Belt objects have for astronomers.

(Credit: YouTube, Discovery TV)

More and more of these were discovered over the next decade, until in 2005, an object was discovered which was actually more massive than Pluto. A trio of hilarious astronomers named this object ‘Eris‘ after the Greek goddess of strife and jealousy, anticipating the inevitable furore within the astronomical community which would follow.

And indeed, there was a furore.

Pluto Demoted

Many astronomers and astrophysicists were concerned that Eris and any other large Kuiper Belt objects discovered would have to be classified as planets (as they were comparable to Pluto), and so a whole new host of planets would have to be named and added to textbooks. To deal with this, the International Astronomical Union (IAU) passed a resolution in 2006 giving a formal definition of a ‘planet’. To be a planet, an object had to satisfy three criteria:

The object must orbit the sun.

The object must have a roughly spherical shape.

The object must clear (remove) all other bodies from its orbit.

Pluto and Eris passed the first two criteria easily; however, with all the Kuiper Belt objects floating about around them, they didn’t make the third. And this meant for Pluto:

Poor Pluto....  (Credit: Kards Unlimited)

Poor Pluto…. (Credit: Kards Unlimited)

Yes, this sadly meant that on August 24, 2006, Pluto was demoted to what’s called a ‘dwarf planet‘. It was unceremoniously thrown out of the planet club, told that it would have to hang out with its Kuiper Belt neighbours instead of the ‘in’ crowd. (Get it? The other planets are closer ‘in’ to the sun?……….)

Yeah, sorry.

To some astronomers and astrophysicists, this was a demotion which Pluto had had coming for a while. Being so small, so icy and surounded by so many other bodies, Pluto appeared to have more in common with comets than with other planets. As prominent astrophysicist Neil Degrasse Tyson put it:

“Pluto is my favourite comet”. Brutal. (Credit: YouTube, Business Insider)

The Great Planet Debate

To others, though, having a separate category for dwarf planets made absolutely no sense. There was so much opposition to this, in fact, that a three-day ‘Great Planet Debate‘ was convened in 2008 at John Hopkins University, Maryland. On one side of the debate was Neil Degrasse Tyson, supporting the new definition of a ‘planet’ and advocating for Pluto to be kept as a dwarf planet. The opposition, led by Alan Sternfrom NASA and Mark Sykes – director of the Planetary Science Institute – argued that this new definition of ‘planet’ relied too heavily on where the body in question was, rather than what it was. For instance, if Earth was moved into the Kuiper Belt, it too would be reclassified as a dwarf planet! They proposed an alternative definition of a planet, based solely upon the ’roundness’ of the body of interest.

New Horizons

The ‘Great Planet Debate’ ended in a stalemate; while both sides of the argument made concessions, an agreement couldn’t be reached, and so the IAU definition of Pluto being a dwarf planet remains to this day. The debate had died down slightly over the last seven years, until just this last week, when NASA’s New Horizons probe collected the most detailed images and analysis of Pluto ever completed.

And boy, were there some surprises for us!

Firstly, while Eris remains more massive than Pluto, it turns out Pluto is actually slightly larger in size – making it still the largest object in the Kuiper Belt found so far. Also, imaging of both Pluto and its largest moon ‘Charon’ has indicated that there are relatively young surface features of less than 100 million years old; younger than some features of Earth’s geology! This suggests that there’s definitely some form of geological or volcanic activity happening on Pluto, and so while it may be covered in frozen methane and nitrogen, it’s certainly not as dead as its Greek namesake would suggest.

For the moment, none of this changes the fact that Pluto currently doesn’t fit into the IAU definition of a planet. However, as data from New Horizons tells us more about Pluto and other Kuiper Belt worlds, how we define a ‘planet’ may well soon change. I guess it’s really just a case of ‘watch this space’.

Get it? The probe is in space?

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