The Pluto debate: planet or dwarf planet?

The debate over Pluto's planetary status has been re-ignited...
18 September 2018


Graphic of planets in the solar system


Are you a Pluto sympathiser who’s been made despondent by this miniature marble’s demotion to a “dwarf planet” a little while ago? A paper has recently been published arguing that Pluto should in fact be reclassified as a planet again. But why? And why was it “dwarfed” in the first place? Georgia Mills and Adam Murphy have been doing a bit of star gazing to find out...

Georgia - So back when you and I were kids, it was very easy to remember the order of the planets in our solar system. My very easy method just speeds up naming planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

Adam - Yeah. Except your very easy method got it wrong. In 2006, the International Astronomical Union voted to reclassify Pluto as a dwarf planet.

Georgia - #justiceforpluto. So why did they feel the need to debate this?

Adam - Back in 1930 when Pluto was discovered it was the only celestial body in that part of the sky that we knew about. But there are actually a lot of bodies out there on the fringes of our solar system.

Georgia - Ah, yes. This is the Kuiper Belt. This is the massive doughnut on the outer solar system which is full of icy and gassy bodies.

Adam - Exactly. And that’s the problem. Pluto is just one of many objects inhabiting the same orbital zone and we’ve been finding things there almost as big as Pluto. One called Quaoar was found found in 2002, Sedna in 2003, and Eris in 2005, which was actually bigger than Pluto.

Georgia - Ah, 2005. The year before the big debate!

Adam - Yeah, exactly. Did we have shed loads of new planets all of a sudden?

Georgia - You’d need a very long mnemonic.

Adam - Something had to be done so a committee sat down in Prague to decide what constitutes a planet:

After a lot of debating they had a vote on the following definition: a planet is a celestial body which a) is in orbit around the Sun...

Georgia - Check.

Adam - b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape...

Georgia - Uhh!

Adam - Is it round?

Georgia - Check.

Adam - and c) has it cleared the neighbourhood around its orbit? That is to say, is it dominant enough in its orbit that anything else around has either been absorbed or booted out into space?

Georgia - Ah, failed on the last hurdle. The Kuiper Belt is a very busy neighbourhood.

Adam - Exactly. So under that definition Pluto gets booted from the ‘exclusive planetary club’ into the ‘dwarf planet economy lounge.’

Georgia - Hang on. Are we sure that the other planets fulfill that last definition? There’s a heck of a lot of junk flying around the rest of our solar system.

Adam - Well, some have argued that under the new definition, Earth and Jupiter fail to meet the IAUs definition but they’re in the minority.

Georgia - I hope Earth gets to stay a planet. I don’t know how I’d feel about living on a non-planet. What about this new paper? They’re definitely team “Pluto is a planet”.

Adam - Very much so. Lots have argued that Pluto should be reinstated since the decision for a bunch of different reasons. This time they’ve gone through the literature and look for examples about the third part of the definition actually being used.

Georgia - Ah. This is the one about being the most dominant in the orbit?

Adam - That’s the one. They checked in papers in the past 200 years.

Georgia - They really care about Pluto.

Adam - And they found it’s barely been historically - only once in the the 19th century. They argue that it’s an arbitrary definition.

Georgia - Isn’t that the point of a definition?

Adam - Well, it’s hardly the first or the strongest challenge to the ruling and it won’t be the last. But the IAU are reportedly happy to debate the topic again so we’ll have to watch this… Space.

Georgia - I’m demoting you from ‘human’ for that!



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