Pluto, planets and you

SpaP_2_01-600x900Engines to power! Turbines to speed!

Action Science Theatre is back, everyone!

I’m only allowed, by law, to use one more exclamation point in this blog, so I’m going to save it for later.

Now that we’ve returned, let’s tackle one of the big problems, something which has dogged humanity since at least 2006.

Is Pluto a planet or not?

To figure this out, we have to know what a planet is. And the answer to that is pretty simple – a planet is whatever we all agree to call a planet. Pluto has no opinion on the topic, the important thing is that we all know exactly what it means when we say ‘planet’.

When Pluto was discovered in 1930, it was thought that it was about the size of the Earth. As we learned more and more about it, and imaged it more and more exactly, that size estimate kept getting smaller and smaller. And smaller. And smaller. Now we reckon it’s about 0.00218 the mass of the Earth – so you’d need to put together 459 Plutos to make 1 Earth. It is tiny.

And then we found other objects in our solar system that were about the same size as Pluto. So the question became: do all of these tiny objects get called planets too? Or do we need another definition of what a planet is?

The International Astronomical Union’s big meeting rolled around in 2006, and this is where the collected intellectual might of the world’s astronomers would decide exactly what we mean when we call something a planet. Or, at least, the collected intellectual might of the astronomers who were there would decide.

Now there were plenty of astronomers who wanted to keep Pluto as a planet, and just add these new objects to the roster. But they were outvoted – a new, strict, set of definitions were drawn up that would define what a planet is.

A planet must:
1) Orbit the Sun
2) Be big enough to be spherical (if an object is big enough, the gravitation force of all that mass tends to pull it into spherical shapes)
3) Clear its neighbourhood.

It’s number 3 – clearing its neghbourhood – that Pluto fails. To clear your neighbourhood, astronomically speaking, means that you are the biggest thing in your orbital zone by a long way. Pluto has lots of friends of about that same size in its orbital zone – called, adorably, Plutinos – which means it loses its status as a planet.

So, at the end of it all, Pluto (and objects like it) have been reclassified as ‘Dwarf-planets’. And everything is a little bit tidier. Unfortunately for the scientists in our latest episode, everything is about to get a lot less tidy, and a lot more confusing

We’re back! That’s worth breaking the law for an extra exclamation point! Woo!

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