Should Pluto be classified as a planet?

After a decade long journey, Earthlings have been able to see the dwarf planet Pluto in more detail than ever before - previously, the most detailed image we had was a few pixels across - now, New Horizons has beamed back high resolution images of the champagne coloured planet and it's iconic heart-shaped feature. But along with these beautiful images, an old debate has been ignited: Should Pluto be classed as a planet? Graihagh Jackson asked David Rothery whether there was a simple answer to this question...

David - There is a simple yes, no answers as to whether Pluto is a planet. The trouble is, the no answer has upset a lot of people.

Graihagh - That's David Rothery, professor of planetary science at the Open University. The reason it's difficult to say is not just because of the politics involved, but also because we seem to have an inordinate fondness for Pluto.

David - Pluto is an icy world. It's smaller than the big icy moons of Jupiter, just over 2,000 kilometres across. Its density is such that the inside must be rocky but the outside is icy. Now at the surface, there is some water ice sticking up, forming mountains, but most of the flat line regions of the surface, are other ices - nitrogen ice, methane ice, and carbon dioxide ice. It takes 248 years to go around the sun, so it's got very long year, very long seasons.

Graihagh - And of course, if I landed, I'd never make it past 1 year old, technically in Pluto's definition either.

David - You'd be forever young.

Graihagh - Forever young. How did Pluto become discovered so to speak?

David - After the planet Uranus was discovered by accident, and it had been tracked for a while, people realised that Uranus's orbit wasn't quite right. There must be something beyond Uranus perturbing its orbit and they looked in the right place and found the planet Neptune. After Neptune had been tracked for a while, people realised that its orbit seem to be not quite right and they thought there must be an unknown planet beyond Neptune. So, they started looking. Percival Lowell started a big campaign which outlasted his life. And a chap called Clyde Tombaugh got the job of photographing the bit of the sky where this mysterious planet X ought to be. Eventually in 1930, he found it.

Graihagh - Hence, Pluto was born. How was it named? I was about to call it she then. How was she named? How was Pluto named?

David - Well, the suggestion came from an English school girl called Venetia Burney. I don't know the background of why she came up with the name, but it seems obvious with hindsight that all the planets are named after gods and goddesses from the Greco Roman pantheon. We've got Saturn, Uranus, Neptune, Pluto - the body in the dark, out of reach was given the name of the god of the underworld. It seems perfectly natural and it was embraced all the more willingly because it meant that the abbreviation for Pluto would be PL which were the initials of Percival Lowell, the guy who started the photographic survey campaign which eventually led to Pluto being discovered.

Graihagh - Like me, you may have noticed that all these astronomers are American. I wondered whether this might have significance when trying to unpick why people were so upset when Pluto was declassified as a planet.

David - Well, other planets had been discovered by Europeans. Well, Pluto was the American planet. So I think, now that Pluto has been declassified as planet, Americans in particular fairly agree that their planet has been taken away from them.

Graihagh - How did this come about? Why was Pluto declassified?

David - When Pluto was first found, it was the only thing beyond Neptune. Pretty soon, it became clear that its orbit would come inside Neptune and that the estimated size of Pluto first of all, thought it was something earth-size or mars-size and it kept shrinking and shrinking, and shrinking as we got better determinations until we realised it's even smaller than our moon. But the real killer for Pluto came in the 1990s when we began to discover other objects in similar orbits. So, there was a whole swarm of objects out there, in the region which we call the Kuiper belt. These are leftover remnants we think of when the planets were forming. So, what we now have is Pluto as the largest of the objects in the Kuiper belt.

Graihagh - But if Pluto isn't a planet then what is it?

David - In my mind, Pluto is just a large Kuiper belt object. But in 2006, when the IAU came up with this definition of what is a planet and therefore what isn't a planet, they inserted an extra category. They said, something which hasn't cleared its orbit, we will call a dwarf planet if that object is sufficiently big, that its mass is strong enough to pull the object into a round shape, into hydrostatic equilibrium.

Graihagh - It would seem there is some clear justifications as to why Pluto was declassified. But there's more to it than that. Pluto is just something that resonates in us. For me, I think of that orange cartoon dog with the floppy ears that David reminded me goes...

David - Yeah. "Hey, Pluto!" is what Mickey Mouse used to say.

Graihagh - And I'm not the only one who has this inordinate fondness for our dwarf planet Pluto. When I asked you on Facebook whether you thought Pluto should be a planet or not, 64 per cent say no. It should stay a dwarf planet, but 37 per cent thought otherwise. Mary argued...

Mary - I like it as a planet. It suits the system.

Graihagh - And I have to agree, the rhyme I made up at school to remember the order of the planets doesn't really work as well - my very excitable monkey junior smelled my Uncle Nathan's pants. Maybe it will just have to be, "smelled my uncle Nathan." And (Fran) raised another interesting conundrum...

Fran - How about redefining planet to include Pluto?

Graihagh - Let's take Fran's comment first. What would be the main objection to redefining planet to include Pluto?

David - I think it would be very hard to construct such a definition except by adding a clause in to say, "But by the way, we'll still count Pluto as a planet" because Pluto shares so many characteristics with lots and lots of other Kuiper belt objects. Unless you say, "on the basis of historical precedent, because we called Pluto a planet for 70 or 80 years, we'll keep Pluto as a planet but we're not admitting any other bodies to the same category."

Graihagh - And I suppose one thing here that we haven't really talked about is that actually, these missions like New Horizon and all of these things are actually publicly funded. So, if that's what the public want then why not?

David - Yes. It's the American public, American taxpayer funding the missions to Pluto, but of course, that doesn't give them ownership over the Solar System. And that's why the international body, the IAU is in my mind the appropriate body to adjudicate on this.

Graihagh - David has a point and that was apparent on Facebook also. Ciudin asked...

Ciudin - Is that really the question that bother us? I think we should use our brain for something more concrete.

Graihagh - And Andrew argued...

Andrew - It doesn't matter in the slightest what you classify it as.

Graihagh - I think I agree with these two comments. Ultimately, it seems a game of semantics and whether Pluto is really a planet or a dwarf planet, it doesn't really matter. Ultimately, it's something fun and interesting to study.

David - I certainly agree with that. Pluto is a superb place. There's so much there that's being discovered. Even if we found flocks of penguins dancing on the ice, that would not make it a planet on the IAU definition. Get on with it. It is what it is. I'm a planetary scientist and I will happily study Pluto. However, it's classified.