alt Hacker NewsClassifications are useful. Even the 'proper' planets we split into terrestrials, Jovian giants, and ice giants. Each planet is individually interesting in itself. One might even argue all of the bodies in the Solar System exist on a continuum of mass from the tiniest mote of space dust, to rocks and asteroids, to moons and dwarf planets to terrestrial planets, ice giants, giants, and then the Sun, and then hypergiant stars, neutron stars, black holes, and then supermassive black holes, so let's call all of them 'celestial objects'—which we kind of do. But such a catch-all is pointless. Each object behaves very differently, with wildly different physical laws governing its interior and its neighbourhood.
Classifications impose structure on a collection of celestial objects, and we can derive relationships, commonalities, understand their development, the physics governing these objects, and build useful physical and mathematical models to widen our understanding of the universe.
Most reasonable scientists understand that some of these objects will blur the edges—like many moons being more massive and larger than Mercury. Or like the largest super-Earth exoplanets being nearly as massive as Neptune, or the largest gas giant exoplanets being 30-80 times as massive as Jupiter and hence having a surface temperature of ~2000 K. But that is part of science, and possibly even where the most interesting work is.
Replies
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.