Dwarf planet candidate appears which takes roughly 280 years to orbit our sun (smithsonianmag.com)

I mean, why not?

Yes. I wouldn't call a dwarf person any less of a person, nor would i refer to a dwarf planet as being any less a planet. Between the terrestrial planets, the gas giants, the ice giants, and arguably a couple of rounder moons, we have far more than 50 planets in our solar system.

By some extreme definitions, Earth might not be a planet. Some say the moon is too big to qualify the earth "cleared out its orbit" criteria, making Earth-moon some sort of double planet... like Pluto-Charon.

And a spattering of domain subject experts being offended that they don't have veto power over colloquial terminology

Classifications are useful! It is poor form to take a widely used and historic term and add on a whole different (dynamic!!) category of attribute as a filter on the definition. There is quite a bit of absurdity in the 2006 IAU's definition of a planet. If a massive object came into the solar system and perturbed orbits, caused chaos and collisions, then any "planet" with debris within its neighborhood would cease to be a planet. What do we gain from adding a whole different category of attribute (relationship to its neighborhood) as a criterion? Does it realistically outweigh decades or centuries of peoples intuition of the existing definition? Its not just a matter of science, but language and communication. If you want greater specificity, come up with a new term or use compounded terms.

It very much seems like the definition was just shoe-horned to stem any more planets needing to be added. It would be interesting to know if astronomers walk the walk when it comes the definition. If I research when planets were formed would I get the date when their neighborhoods were cleared? Do discussions around large events that send debris into the neighborhood of planets correctly reflect that they cease to be planets until it is cleared?

Alan Stern said it best

>"We're going to call it a cow, except when it's in a herd." A river is a river, independent of whether there are other rivers nearby. In science, we call things what they are based on their attributes, not what they're next to.

>And the IAU definition is even worse, because it produces different categorizations for identical objects, depending on where they are. Get this — Earth at the same distance from the sun [as Pluto] would not be a planet by the IAU’s measure, because Earth can’t clear that zone either. I would say any definition that produces a result where Earth is not a planet under any circumstance is immediately indicted as ridiculous, because one thing we all agree is a planet is planet Earth!

>That's crazy. No wonder teachers and students are frustrated, confused, unhappy. It reflects poorly on astronomy, to have something so illogical.

https://www.space.com/12710-pluto-defender-alan-stern-dwarf-...

Yes, regardless of the specific labels, there are clearly interesting and meaningful differences between these things, and it’s worth having vocabulary capable of distinguishing them. The whole universe is a continuum of stuff but it helps to have more than just the word “stuff”.

> Classifications are useful.

I wasn't arguing against classification, just that I remain unconvinced that the classification which IAU adopted is superior to the alternatives.

One problem with the IAU definition: it only works for our planetary system, not for exoplanets – with the end result that we have two different definitions of "planet", one for our system, one for everywhere else.

Now, of course, that in part reflects that we have vastly more information about our own system than any other – but, a few centuries or millennia from now, I expect we'll know a lot more about some of those other systems – we will probably have sent probes to explore the nearer ones by then – at which point people (or AIs?) will probably push to unify the two definitions. My own bet is they'll be more likely to unify them by dropping "clear the neighbourhood" from the our-system definition than extending it to apply to other systems. Splitting the common category of "planet" into two subcategories based on "does it clear its neighbourhood" works much better than insisting on two disjoint categories based on that criterion when you have very limited information on where that criterion is met.

I think the backlash was fed by insistence dwarf planets are not just a subtype of planets, it's such a "well ackchyually" type thing to choose that name for it regardless of how valid it was to break out the category that I'm not sure it'll ever stop being talked about.

Even something as simple as "planets" contains "major planets" and "minor planets" would have avoided 20 years of people arguing about the names instead of caring to learn what's different about them. But you never hear people explaining why the IAU insists "dwarf planets" aren't "planets" even when "dwarf planets" has several subcategories which don't share the "planet" moniker, you just hear people argue about missing Pluto or call other people ignorant of the technical details of the categories.

Ganymede and Titan are both larger than Mercury.

The problem is not experts having precise terminology in technical settings; the problem is the army of “experts” who will descend upon you if you use the incorrect arbitrary term in colloquial settings. The same sort that will descend on you if you dare refer to a planet orbiting a different star as a planet, not an exo-planet.

> So, a factor of 14.4 is enough to put the line between Ceres and Pluto, but a factor of 23.0 is not enough to put it between Pluto and Mercury?

Why not? It's an arbitrary line, and there has to be a lower limit somewhere.

In the IAU definition, hydrostatic equilibrium loosely defines a lower limit for all planets including dwarf planets, but at the limits that's a fuzzy property that gets debated for small bodies. In fact Ceres is less massive than what some studies have found to be required for hydrostatic equilibrium, e.g.: https://iopscience.iop.org/article/10.3847/PSJ/ad55f3 . The abstract of that paper starts out with, "The current IAU definition of 'planet' is problematic because it is vague and excludes exoplanets."

If we had more small planetary candidates in the solar system, they would have had to come up with some additional constraint to separate planets from non-planets.

Following this logic, a black hole with earth's mass would be a planet.

It's easy to exclude the moon, like the IAU does, by requiring that planets orbit the Sun. (Although you might want to avoid the IAU's parochialism, which excludes exoplanets, and say "a star" instead.)

> By that logic, if we want to consider pluto a planet then the earth's moon also has to qualify. I don't really care one way or the other but that just doesn't seem sensible to me.

Ganymede is larger than Mercury, so a definition that doesn't specify orbiting a star would have to include some moons, or exclude Mercury.

This exposes another problem with this kind of definition - it seems to make more sense to define a body in terms of its intrinsic properties rather than whatever location it happens to be in.

For example, a rogue planet is not a planet by the IAU definition. You'd have to call it something like a free-floating planetary-mass object. ("Free-floating" is not the right term either, but I don't have the research funds to come up with a better one.)

> I have something of a personal hatred of poorly executed retconning

Totally agree. Wanna join me outside George Lucas' house to protest the Kessel Run?

Their claim is that it fails (a) because the Sun and Jupiter orbit a common barycenter, and hence Jupiter does not orbit the Sun (or any star).

It's too small to affect other orbits as much as the hypothetical planet X.