Arizona announces its official state planet, but it isn't actually a planet

I mean, you can call it whatever you want, but orbit clearing actually is a really sharp divide – our solar system has eight massive objects with values well above one and thousands of less massive objects with values well below it. The same is expected for other solar systems too, although on very rare occasions you do find ones with intermediate cases. Nonetheless, as real world measurements go, it is a remarkably non-fuzzy one.




Oh for sure, the IAU definition is clear and defensible. That’s why they picked it, ultimately - there’s no parsimonious non-historical definition that rules in exactly nine planets in the Solar System.

But, I see no reason to expect legislators to think that’s the definition that’s relevant to them. I do think this particular choice by AZ is silly for other reasons, but in general I don’t think the form of argument implied by this post is valid.

Well, if it doesn’t have to be an actual planet…


This chart is my gender

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As far as I am concerned, Pluto will always be a planet to me, IAU be damned.

Which would make our solar system way cooler. (Well, it already is pretty cool, but having a half dozen additional planets is awesome!)

Party Soccer GIF by Coca-Cola

@RickMycroft I love Planet Claire!


… this kind of “argument” :roll_eyes: says more about the party presenting it than about the person or group it supposedly describes


Neptune hasn’t cleared its orbit of Pluto, so Neptune clearly is not a planet.


(That dude totally deserved it)


… yeah, that’s exactly what it means

It’s like when creationists think of a very clever reason that evolution is wrong — I bet they’ll really have to change those science books now :open_mouth:

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It amazes me how many people are happy to opine on this subject without ever bothering to check how orbit clearing is actually defined. Spoiler: the eight biggest masses have, the other smaller masses orbiting them or in belts are nowhere close, and the IAU are not actually stupid.

But heaven forbid they should try to describe the solar system in a way that reflects its structure, when it doesn’t place America’s favorite belt object in with the eight that formed differently. Who cares about the science behind that anyway? :face_exhaling:


My comment (and I suspect @smulder 's also) was meant in jest, but as long as you’re being all serious about it, where can we find the IAU’s definition of orbit clearing? Is there something better and more official than the Wikipedia entry?

I dunno. I work in government contracting, including USAID. Sometimes an Administrator’s only contribution to the Agency is to change job titles from “Contract Officer Representative” to “Contract Officer Technical Representative” or back again.

I can totally see someone at the IAU (a multilateral scientific organization) trying to justify their appointment by “demoting” Pluto and similar bodies. Given the New Horizon flyby showed evidence of an atmosphere, geological activity, and a possible liquid sub-surface ocean, Pluto starts looking more planet-like than Mercury, and the 2006 demotion a little hasty, at least to me.


When something you were genuinely interested in still can’t be brought up a decade later without people who don’t really care about the science mocking the scientists who do, the jests stop being funny, sorry.

I don’t there’s anything wrong with the discussion on wikipedia. I already linked to it – in particular the table that shows that for multiple ways of calculating it, Neptune is well above while objects like Pluto are well below. Because it depends on relative mass.

Jesus, this is unfair. Pluto is part of a belt of other small objects, and probably formed similar to them. Mercury is way more massive and is not, and probably formed similar to the other massive ones. I can’t see how anyone actually interested in our solar system could miss that distinction.

Yes, Pluto has all sorts of interesting features. But why do you assume that means it belongs with the bigger objects, instead of that lots of little objects might? That’s what categorizations are for, you know, to let you try and generalize…and yet you’re not even considering that possibility.

Instead you’re inventing the idea that it was just bureaucracy as if the IAU likes changing things for the sake of it. They never have, there’s a reason all but a few astronomers have followed them, and it’s infuriating to see people just assume they all must not have put thought into it because you didn’t like their choice.


And so much more!

(Note the “debate hole” where we’ll all end up if we aren’t careful…)


This is as good an excuse as any to post Interplanet Janet, regardless as to how Pluto is now categorized:


Well according to that same Wikipedia entry

which is a pretty unsatisfying answer. But whatever.

It goes on to give multiple ways of measuring it that all give the same dichotomy. The IAU doesn’t pick a specific one because they all agreenone say that Pluto has or Neptune hasn’t cleared its orbit. I would think that makes it a lot more satisfying than a single equation that gives different answers than others. It’s a real, non-arbitrary distinction in a world that doesn’t have many, and I hate that nobody is ever interested because they aren’t concerned about understanding, just whether this one object gets its title. (As if being an ice dwarf or KBO is somehow insulting.)


Team “Pluto is a planet” and i vote.

besides, the mnemonic rhyme doesn’t make sense without it. so i vote Pluto is (great-) grandfathered in, and draw the line after Pluto.


Pluto? Mercury seems like the obvious choice. Or if it doesn’t really have to be a planet, the Sun.

As I often say: biology does not give a shit about the human need for nice neat categories.

Some people apparently don’t consider hagfish to be vertebrates (because they have, well, no vertebrae). “Fish” is a category born out tradition more than anything. Cladistically it doesn’t begin to hold water.

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