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Comment by parenthephobia | Hacker News Reader

> The classification should be based on its size, shape, and make up. Instead they classified it based on the number of objects in its orbit, which is so arbitrary and weird.

Why should it be one rather than the other? What harm or benefit comes from Pluto being called, or not being called, a planet?

> If hypothetically we cleared up the objects and dumped them in Earths orbit, Pluto would suddenly become a planet and Earth would lose its classification.

No, because the definition requires that a planet clears its own neighbourhood. Having a nearby technological civilization clean up its neighbourhood for it doesn't cut it.

> Just bite the bullet and accept there are a lot of small planets in our solar system

Dwarf planets, you might say.

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>Why should it be one rather than the other? What harm or benefit comes from Pluto being called, or not being called, a planet?

It violates everyone's internal model of the word "planet". Everyone knows what a planet is, and the formal definition should try to match our internal models as closely as possible. To do otherwise feels wrong and arbitrary.

>No, because the definition requires that a planet clears its own neighbourhood. Having a nearby technological civilization clean up its neighbourhood for it doesn't cut it.

So now the definition is based on history? We can't determine what is a planet based on just observing it, we have to know how it came to exist in that state? If we ever travel to other solar systems, we will never be able to know whether they are true planets or not. For all we know aliens or other events cleared the orbits before we observed them.

>Dwarf planets, you might say.

Highly misleading. Dwarf implies it's just small, and if it was bigger it would be a regular planet. But the definition isn't based on size.

If it were bigger, weren't it clear its own neighborhood? Is there an example of a sufficiently large body that doesn't?

(I'm by no means an expert, that's why I'm asking.)

> If we ever travel to other solar systems, we will never be able to know whether they are true planets or not. For all we know aliens or other events cleared the orbits before we observed them.

I don't even.

I mean, I'm not unfamiliar with lame arguments, having presented many in the past. But this one...wow.

I've got one better. Planets orbit their star. What if the aliens have antigravity devices so a planetary-mass object doesn't orbit its star or some other planet. Then what will we call it?

("That's Not A Moon...It's A Space Station!" tells us that those people long ago and far away didn't think that moon had to orbit a planet.)

To add more fun to the pile, is the "rogue planet" PSO J318.5-22 a planet? https://en.wikipedia.org/wiki/PSO_J318.5-22 . And yes, the IAU has already considered this case.

I think the reason we are having this disagreement is our basic vocabulary was developed when we knew of the existence of far fewer types of objects orbiting the Sun than we do now, and we are trying to fit the new objects into our old vocabulary.

What aren't you understanding? Of course I don't seriously believe aliens move objects around the solar system. It's just a silly thought experiment.

The point is it seems absurd that the past history of an object should matter, for this definition. Like if it's a big round object orbiting a star, it's a planet. It shouldn't matter how it got there. It seems quite possible there could be cases where another object clears the orbit of another object. Perhaps not aliens, but a rogue planet.

I was trying to recreate the "star trek test" argument that I heard a real astronomer use. He was upset with the new definition, and made an analogy to start trek. On the show they don't wait around for 12 months to observe the planet and any objects in its orbit, let alone it's past history. He argued that any sensible definition should be possible to determine immediately.

Of course we are arguing about the definition of a word, and all arguments are subjective and appeal to intuition. But still there can be bad definitions.

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