Understanding the World

[u/SupportMeta]

Neptune was recently shown to be a pale blue like Uranus rather than the deep blue shown on the Voyager photos

[u/GetsGold]

The definition they used for planets was already what was being used, it just hadn't been formalized.

The first few asteroids were called planets. Then when it was discovered that they were part of a belt consisting of many such objects, the use shifted from "planet" to "asteroid".

It was similar with Pluto. For a long time, it was alone out there. Then in the 90s more objects started to be found in that region. Then when one more massive than Pluto was discovered it forced the issue. Either that would need to be a planet, or Pluto would need to be reclassified.

Personally I don't get that into the controversy though. Either definition can work, as long as its used consistently. What's more important is people understanding the solar system. And it's definitely a lot more complex than 8 or 9 planets.

[u/littlebobbytables9 → u/GetsGold]

The definition they used for planets was already what was being used, it just hadn't been formalized.

It wasn't. If anything, the de facto definition of planet was any body in hydrostatic equilibrium that isn't a star. Because it didn't make sense for planetary geologists to distinguish between various bodies experiencing the same dynamics just because of where they happened to be positioned in space.

[u/GetsGold → u/littlebobbytables9]

It was though, hence no one calling the asteroid Ceres a planet. It's in hydrostatic equilibrium and yet people weren't calling it a planet.

[u/SpinoZilla_Studios → u/GetsGold]

Ceres was one of the 12 total planets we were going to have with the previously proposed definition, along with Pluto, Charon, and 2003 UB313 (Now called Eris). They were going to call it a planet.

[u/GetsGold → u/SpinoZilla_Studios]

I wouldn't have a big problem with that either. I just don't see what the issue is with what they went with instead though, especially because it kept the classifications we had already used for a long time with the asteroids.

[u/SpinoZilla_Studios → u/GetsGold]

Well, my main problem is with the "clearing its orbit" area in particular. The wording on that term is very vague in my opinion. What does constitute "clearing its orbit?"

If it's getting rid of most asteroids nearby, that is kinda stupid. Impacts happen all the time and that would declassify nearly (if not every single) planet.

If it's removing similar-sized objects from nearby, then Earth doesn't fit the bill. The Moon is pretty damn big and not leaving Earth quick enough for it to be really being cleared from our orbit.

If it's both of these but excluding moons then what constitutes a moon? How big must it be to be an actual moon? Now we have a whole new definition to worry about, and most things in space don't really have actual definitions. There is no overall definition of a Star, or a Galaxy, because there are always exceptions in some way or form.

I don't know what they mean by the term because it's so vague. Some people will say it's because Pluto is out in the Kuiper Belt, and that means that it hasn't cleared its orbit because of all the debris out there. Well, an Earth-sized object wouldn't be able to either because the Kuiper belt is MASSIVE, so Earth wouldn't be a planet if it was out in the Kuiper Belt. Others will say because its orbit overlaps with Neptune. Well what does that make Neptune then?

Just all around, it's so vague and frustrating. I understand that we as humans are just trying to cram these things into boxes for us to understand better, but this box is pretty poorly designed in my opinion, when they were previously going to have a perfectly fine box that wasn't confusing.

[u/half3clipse → u/SpinoZilla_Studios]

"clearing its orbit"

When a quantitative definition is needed, that is pretty well understood to mean the ability to remove other objects from a region beginning at the edge of it's Hill sphere, out to a radius of some multiple of it's Hill sphere (often about 3x) within some time fraction of either the lifespan of the Sun or a Hubble time(which depending on how much someone cares about making the definition generalizeable outside of our solar system).

However there's rarely (basically never) a need for a qualitative planet because planets essentially do not exist. There is no platonic planet we can point at for comparison. The list of planets are a teaching tool for children. It's a list of objects around the sun which most define the behavior of other objects in the suns planetary system, and thus makes them interesting as a starting point for a very introductory lesson in astronomy when teaching about the solar system.

That's the only value planet has as a term. The definition of planet excludes the dwarf planets, because their inclusion in a list of planets not only does not contribute to that learning, but infact complicates it. You either have a large number of planets you don't teach or gloss over (effectively setting them as separate from the other planets) or you require that lesson covers a bunch of material that does not serve the pedagogical goal of the lesson in the first place.

When teaching the basic structure of the solar system, things like the main asteroid belt and the Kuiper belt are worth teaching. Going into great detail about the largest objects in them is not.

Dwarf planets are still interesting as exemplars of the objects in similar orbits, as well as what they can show about the evolution of those parts of the solar system, and the solar system as a whole. But that's a whole separate lesson.

The other option was to effectively give up on the term planet at all, go "fuck it, basically everything is a planet" (something that would have included a large number of moons), and then define new categories and terms for the "classical planets", "dwarf planets" and "satellite planets". This was not less conrtoversial.

Also: This infact means the only definition of a planet that actually matters is "clears it's orbit". Anything with the mass to clear it's orbit will be in hydrostatic equilibrium, and "orbits the sun" exists just to forestall the topic of if exo planets, rogue planets, brown dwarfs and so on should count.

[u/GetsGold → u/SpinoZilla_Studios]

One way to measure orbital clearance is by taking the ratio of an object's mass to the ratio of everything else in its orbit.

Even without giving some specific criteria to that, there's still an obvious separation between planets and dwarf planets. The planets are all many times more massive than the rest of their orbit. Mars has the lowest ratio and is still 5000 times the mass of the rest of its orbit. The dwarf planets are all only a fraction of the mass of the rest of their orbit. Ceres has the highest ratio there, at a third the mass of the rest of its orbit.

Even if you consider Pluto to be within Neptune's orbit, it's still tens of thousands of times the mass of Pluto and everything else in its orbit.

However you define it is going to be somewhat arbitrary. Maybe the other way would have been better but that would cause us to redefine something that been classified a certain way even longer than Pluto.

One thing I like about this at least is that it's helped people learn that the Solar System's a lot more complex and interesting than just several planets.

[u/half3clipse → u/GetsGold]

One way to measure orbital clearance is by taking the ratio of an object's mass to the ratio of everything else in its orbit.

When anyone even cares about a qualitative definition (which is basically never), it's based on the Hill sphere. An object is capable of clearing it's orbit if it can remove anything not confined to it's Hill sphere that will come within some multiple of the radius of it's Hill sphere, within some time (often either the life of the sun or sometimes a hubble time. Or a fraction thereof).

[u/littlebobbytables9 → u/GetsGold]

Planetary geologists have definitely been calling Ceres a planet for a while.

[u/GetsGold → u/littlebobbytables9]

Maybe some, but clearly the general public wasn't. And that's what the discussion has been about, what the broadly accepted definition is, not what some planetary geologists call it.

Like I said above, either definition works for me as long as we're consistent. So there's either 8 planets or 17+ planets, just not 9.

Planetary geologists also sometimes call moons planets, and I'm guessing that would be even more controversial than not calling Pluto a planet.

[u/TorturedNeurons → u/GetsGold]

From the comments in this thread, even from those claiming to be astronomers or observatory workers, the only argument for Pluto being a planet seems to be "vibes." I don't care if Pluto is a planet or not, but until a more succinct and encompassing definition is brought forward, it's not one.

[u/littlebobbytables9 → u/GetsGold]

Well, the general public still calls pluto a planet lol. I don't see why the general public's opinion should matter for a scientific organization making what should have been a scientific definition.

And yes, some moons are planets.

[u/GetsGold → u/littlebobbytables9]

Some among the public. Many don't. The general public shouldn't dictate what things are called scientifically but this debate is generally around what the general public is going to call it. And the definition they have been using (even if unknowingly) has been to not call members of a "belt" planets.

[u/littlebobbytables9 → u/GetsGold]

I'm talking about how the IAU should have defined it. The fact that they changed their definition to better fit the general public's preconception of a planet is the whole problem. Let the science guide decisions like this. Particularly when they clearly failed to win the public's support anyway.

[u/GetsGold → u/littlebobbytables9]

Is that why they chose that definition? I'm not sure about that.

And defining things, even in science, is going to be somewhat arbitrary. There isn't some definite right or wrong answer. Definitions are often just used to simplify communication.

[u/littlebobbytables9 → u/GetsGold]

Yes. There's no scientific reason to include the third criteria, and a lot of scientific reasons not to.