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Column: Why is Pluto not a planet?

Most of us grew up learning that there were nine planets in the Solar System. But why did Pluto get demoted? Conor Farrell explains how our understanding of the icy body changed.

Conor Farrell

MANY OF US have grown up learning about the nine planets of the Solar System. For those of us who can’t quite recall, they were:

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto

Closest to the Sun is Mercury; third one out is Earth, and the last one, far away, was that icy cold planet Pluto.

Yes, I said “was”. As some of you may already know, Pluto is not a planet in the normal sense. And if you ask me, it never was.

One of the first photographs I saw of Pluto was simply a blob with another blob stuck to the side of it. This was the result of what was high-tech imaging from the 1970s and 80s. The image depicted Pluto and its known moon Charon. However, we now know that Pluto is home to a number of other moons, including Nix, Hydra, and two other moons that have yet to be named.

But there was always something about Pluto that bugged me. I’ve been watching the stars for a long time now, and the motions of the planets were something I learned along the way: all the planets orbit around the Sun in an almost-circle (an ellipse, if you want to get technical), and they all do so on the same plane. If you drew a dot in the middle of sheet of paper, and simply scribbled circles on the sheet with that dot in the centre of the circles, you would have a not completely inaccurate depiction of our planetary system. All the planets orbit the Sun as thought they were all rolling around on the same, single sheet of paper; all in a flat plane.

Except for Pluto.

It was clear the icy world was different

For nearly 20 years it bugged me. Eight out of the nine planets followed the same simple orbital idea: on a plane, and in an almost-circle. But on learning about the orbit of Pluto it was clear that the icy world was different. First of all, Pluto does not travel along the same plane as the rest of the planets. It travels around the Sun at a very significant tilt (17 degrees to the plane on which all other planets orbit). Secondly, instead of an almost circular orbit, it has a highly elliptical orbit (a very flattened oval shape), again unlike any of the other planets. In fact, its orbit is so elliptical that it sometimes is closer to the Sun than Neptune, the so-called 8th planet.

There was something not quite right here, and in 2006 a group of astronomers known as the International Astronomical Union held a meeting to sort this out once and for all. Chairing this meeting was the Professor Dame Jocelyn Bell Burnell, who I had the pleasure of talking to about Pluto for the Scibernia radio show. During our chat she told me about the meeting and the reasons why Pluto is no longer considered a planet, but a dwarf planet instead:

  1. The object must be in orbit around the Sun. – Okay, Pluto does that…
  2. The object must be massive enough to be a sphere by its own gravitational force. — Yup, Pluto is also a sphere…
  3. It must have cleared the neighbourhood around its orbit. — A-ha. Pluto is too small to do that.

Clearing the neighbourhood

“Clearing the neighbourhood” means getting rid of any other debris and bits of rock and stuff that share the same orbit as you. Realistically, this only happens when you’re large enough to have sufficient gravitational force to pull in minor asteroids early on, and leave your path clear in the later life of the planet. All planets – including Earth – have done this. But Pluto hasn’t.

You might think that this is unfair on Pluto, and it should be considered a planet, regardless. Okay, but if we call Pluto a planet, then we have to consider all the other similar dwarf planets.

In the early 1800s the object Ceres was considered to be a planet, and even appeared on charts in between the orbits of Mars and Jupiter. Ceres is an asteroid, not much smaller than Pluto, but just like the distant ice-world, it does not satisfy criterion number 3. So, if Pluto is allowed to only satisfy criteria 1 and 2, then Ceres should too, right? Suddenly, we now have 10 planets.

But it doesn’t stop there. Pluto and Ceres are dwarf planets, but there are a number of other such objects. The biggest dwarf planet – more than twice the diameter of Pluto – is Eris. It has a highly elliptical orbit, just like Pluto, and it also has one known moon, known as Dysnomia. Now we have 11 planets, right?

And it still goes on. The dwarf planets Makemake and Haumea are other bodies similar to Pluto, but if we consider Pluto to be a planet, then surely we need to consider these objects as planets, too? There are even more unconfirmed dwarf planets, including Quaoar, Sedna, Orcus, and the object designated 2007 OR10.

The Solar System is not a simple, straightforward thing

So that’s 17 “planets”. To date, there are over 1,200 known objects beyond the orbit of Neptune (the 8th planet), with around 200 of them given official designated names and/or numbers. Theories that model the structure of the whoel Solar System point to billions of objects, including asteroids and comets. Are all of these planets, too?

I think the point is clear: there is a relatively small number of large objects in orbit around the Sun, that can ‘sweep’ away debris in their orbits (this is why we don’t get many asteroid impacts on Earth anymore). There are other objects that can’t do this, but are spherical and orbit the Sun. Finally, there are even more – a lot more – objects that orbit the Sun, but are too small to clear their orbits, or to even form a spherical shape because their gravitational strength is too low.

The Solar System is not a simple and straightforward thing made up of one star and a handful of planets. There are literally millions – if not billions – of objects that orbit the Sun, just like Earth, that we need to take into account. We cannot consider every single fragment or piece of debris to be a planet, and as we learned more and more about the nature of our Solar System we realised that Pluto falls outside of what could be considered a definition for a planet.

Since I was young, I never considered Pluto to be a ‘real’ planet; but, as I grew up with it, it will always hold a special place in my heart.

Conor Farrell is an avid science enthusiast and studied physics with astronomy at Dublin City University. He now works with Astronomy Ireland to promote all things space-related to a wider audience. In his spare time he writes about science and current affairs, and can be followed on Twitter at @conorsthoughts. Read more of Conor’s columns here.

This article first appeared on

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