Astronomical Answers! – Poor Pluto…

This marks the start of a new series of posts, Astronomical Answers; I’ve been asking people for awhile now about the kinds of questions they have about planetary science and space in general. I’ve got a decent collection of questions now and over the next few posts I’m going to attempt to answer them to the best of my ability- so here are a few of the questions I’ve been asked recently:

Why can’t Pluto be in ‘the club’?

Similar to Pluto, Eris is one of many dwarf planets found in our Solar System Image courtesy of NASA

Similar to Pluto, Eris is one of many dwarf planets found in our Solar System
Image courtesy of NASA

I assume by ‘in the club’, you mean why was Pluto demoted from a planet to a planetoid? There are a few parameters that an object must achieve to be considered a planet as defined by the IAU, (the order of these rules is also important!):

1) The object is in orbit around the Sun (or a star); Seems like an obvious one doesn’t it? But the classification system has a tiered naming system, meaning that each criterion a celestial body successfully achieves gives the object a different name – objects fulfilling just the first rule are called small Solar System bodies.

2) The object has sufficient mass to assume hydrostatic equilibrium – This is where things start getting complicated… Hydrostatic equilibrium is the reason planets have a roughly spherical shape. It’s the name given to the balance of gravity and the pressure gradient force, each which act in opposition to each other.

The gravitational force is what holds everything together, it causes matter to be drawn together and compact down into a smaller area . The pressure gradient force acts in the opposite way- The pressure in the middle of a body is greater than on the surface. This creates a pressure gradient that the system will want to equalize. In essence, the pressure gradient force is pushing everything in the object further away from each other. When these forces are in equilibrium, the shape with the lowest surface area is formed- a sphere. Up until this point, celestial bodies are almost always odd shapes. If the celestial body meets the first and the second criteria but not the final criterion, it is termed a dwarf planet.

3) The object must have cleared its orbital path of similarly dominant objects. This is a process commonly referred to as ‘clearing the neighbourhood’. This means that the object has become gravitationally dominant in the orbital region it occupies and has removed all other bodies that could compete for dominance over the region.

This final rule has been a topic of hot debate since it was first proposed. The disagreement stems from the many smaller bodies that still inhabit almost every orbital distance around the Sun.

It has been proposed that rather than being the ‘only’ body in the orbital region, it would be better defined as the sole dominant object. This means that the body controls the orbit of these smaller objects in the same region as it. Objects fulfilling this final criterion as considered planets.

So why isn’t Pluto a planet anymore? Well, it all went wrong when we started finding other similarly sized objects near Pluto’s orbit. It rapidly became apparent that Pluto wasn’t the only object at that orbital distance and more importantly, it became apparent that Pluto was only 0.07 times the total mass of material in that region. In comparison, Earth is 1.7 million times more mass than the remaining mass in the region. Thus, Pluto isn’t really a planet- although to its credit, it went down fighting; not only did it become a dwarf planet but also created its own classification of plutoids (trans-Neptunian objects).

Stay tuned for the next installment where I’ll be finding answers to more of the questions you have been asking me!

Orbits of the outer planets, showing Pluto's trans-Neptunian nature  Image courtesy of NASA

Orbits of the outer planets, showing Pluto’s trans-Neptunian nature
Image courtesy of NASA

Naturally, this is the internet, so you shouldn’t 100% trust everything I say- but I try my best to ensure what I write is accurate and valid. That being said, everyone makes mistakes from time to time! If you spot one, leave me a comment and I’ll be sure to amend it!

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About Mark Nottingham

Mark is a PhD student in the Isotope Geochemistry and Cosmochemistry group at UoM. Primarily working on the RIMSKI and RELAX noble gas instruments.
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One Response to Astronomical Answers! – Poor Pluto…

  1. Duncan says:

    This is really fascinating stuff. You’re bringing Cosmology (and other) theory to the masses in a clear way that can be understood by all 🙂 Keep this going x

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