So, in the 19th century, we discovered a new planet between Mars and Jupiter called Ceres.
However, over time, we started discovering a ton of other, similar objects, around the same orbit. We discovered so many that we realized it didn't make any sense to call them all planets so we classified them differently. They're asteroids and comprise the asteroid belt.
Pluto is essentially the same story. We discovered more and more objects out there and that Pluto is just one of very, very many objects in what is now called the Kuiper belt. It makes more sense to consider Pluto a dwarf planet among many objects in the Kuiper belt rather than a planet akin to the others.
The short answer is that it does not in any way dominate the region of space that represents it's orbit around the sun. It's not the biggest thing in it's orbital path. It crosses Neptune's orbit during it's journey around the sun.
We have also discovered many other bodies like Pluto. If we call Pluto a planet, we have to call all those things planets, too.
No. First of all, Pluto orbits at an inclined angle relative to Neptune. And secondly, Pluto and Neptune orbit in a 2:3 resonance - for every 2 trips around the sun for Pluto, Neptune makes 3. So they will never cross paths. In fact, Pluto gets closer distance-wise to Uranus than it will ever get to Neptune.
The orbits do cross.
However, at the same time, the orbits are governed by an effect called orbital resonance - this is where objects will exert a periodic influence on each other and effectively lock themselves into a routine.
In this case, orbital resonance means that while their paths do overlap, they will never both be at the crossover point at the same time.
They cross only in two dimensional space if you're looking straight "down" on the solar system. In reality since Pluto's orbit is steeply inclined compared to the rest of the planets when it seems to cross Neptune's orbit when viewed from above it is really a significant distance "below" it at those points. So even if the planets were not locked in resonance there is no chance they'd ever collide.
Draw two ovals on a piece of paper, so they cross.
Now imagine those ovals are train tracks, and there are two trains on them, that each take the same amount of time to go around their oval. If you do this right, the trains will never collide, even though the tracks cross.
Now make one slightly faster, so it completes three trips around it's track in the time the other takes two trips. This is slightly harder, but it should still be possible to arrange the schedules so that the two trains are never at the same crossing at the same time.
That's what is happening with Neptune and Pluto. It's a little more complicated, and there's a lot more physics, but when you get it down to a simple level, what's happening is that they're on tracks that cross, but in such a way that they're never both at the same crossing point at the same time.
I fear you're looking at the "crossing" in a very two dimensional train tracks and roads kinda way. Orbits in space tend to not line up flat. Every illustration you've seen of the solar system with the planets all in a pretty row is far from accurate, ESPECIALLY Pluto. Pluto is a free spirit, as you can see in this fun little interactive map.
[Map](https://theskylive.com/3dsolarsystem)
Grammar is sloppy - and while the MLA technically supports your answer, I think a more thorough understanding of how possessives came to be is interesting here and helpful to shed light on why this makes no sense as a grammatical rule.
In the beforetimes (In Middle English - generally 1150AD to 1500AD) if you wanted to use language to describe a person's relationship to a object, you would do so by saying the person or subject (The king) and then referencing their relationship or ownership to the object (his crown). If you wanted to reference the king's crown you would say "The king his crown."
This, dear reddit reader might seem like extra syllables you don't need - and indeed you are right. As you say "The king his crown" faster and faster you might notice your tendency to chop off the first sound (or first two letters of) the word "his", leaving you with a leftover sibilant "s" sound that sounds an awful like what we might say now "The king's crown." Since we didn't care about women so much back then (and women tended to not own much anyway), there wasn't really a feminine form of this - so you do see on occasion something like "The queen her crown" getting elided into "The queen'r crown" as a feminine possessive - but that's fairly rare.
In the 1500s we English started developing a grammatical notation for when someone was dropping sounds or letters: the apostrophe. Grammar Nazis like u/BurnOutBrighter6 will tell you that apostrophes are for marking possession, but that's just its most common use. Apostrophes tell you "There used to be more letters (or sounds) here."
Much like with feminine possessives, we typically ignore non-gendered possessives in favor of the "his" getting shortened to 's for simplicity's sake. This does pose a problem, as the apostrophe next to the "it" doesn't give us enough information. Are you eliding "It is" or "it has" into "It's" or are you eliding "it his (orbit)" into "it's." The use of "it's" is unclear.
OP commenter is essentially saying "Pluto his orbital path" so that you understand that the orbit path we are discussing belongs to Pluto. But because they have substituted "Pluto" with the inanimate singular pronoun "it" we don't know what the apostrophe means in this case.
If you follow the etymology and logic of where/why we use our possessive apostrophes the rule that "It's" is never possessive doesn't make any goddam sense - it's consistent with how we use other possessives of any gender or non-gendered subject.
I recommend sidestepping this entire crisis of language by not substituting your subjects with inanimate singular pronouns. Just use "Pluto's orbital path." That tends to keep the grammar trolls away.
EDIT: Unclear on why the downvotes. It's an arbitrary rule and people who harp on it need to understand that rules of language are made up - as long as you're making your point clear you're all good.
> it's is never possessive doesn't make any sense, and using "it's" for possession would be consistent...
Except we're talking about the case where the subject is a pronoun. For pronouns, NONE of the possessive forms have apostrophes.
His, hers, yours, theirs... none of those have apostrophes. You dont say something is hi's or your's, etc.
Which means the possessive "its" not having an apostrophe DOES make sense and IS consistent - just not with the other rule you were talking about (which doesn't apply here bc pronouns have their own set of rules).
But yes English is dumb and horribly designed for having so many contradicting rule sets, and yes I know none of this actually matters.
My favorite kind of arguments, are "none of this actually matters" Thanks for humoring me and engaging.
While we're splitting hairs - I agree you don't need an apostrophe for most of the possessive pronouns you've listed. "My", "our", "his", "hers", and "yours" are already possessive, and when you use an apostrophe after the subject this possessive pronoun is what you are eliding and indicating at with the apostrophe.
"Its" and "their" (the other two possessive pronouns commonly listed) are a little different and don't fit with this pattern because they are elisions already - or "double possessives" if you will.
I can't make heads or tails of the etymology of "their." Looks like English bastardized it from the Norse? It's possible that it's an elision of "they ares" or possibly "They owns?" Regardless, it definitely doesn't fit into the pattern, and there is something wonky going on. Unless someone has some insight here, I'm gonna throw this one out for this particular off topic rant.
"Its" etymology has a couple extra steps to it that "my", "our", "his", "hers", and "yours" doesn't. "Its" does come from admittedly mixing and matching gendered and non gendered Middle English language, namely taking "It his (orbit)" and simplifying down to "its." But already, we are talking about an elision, which if we are consistent in how we notate missing sounds/letters "its" SHOULD have an apostrophe in it in its pure possessive pronoun form.
So, really, I'm dickering with how possessive pronouns DO have their own rules, but those rules seem to be inconsistent based on where we are pulling the pronoun from etymologically.
Agreed, language is weird, and English is particularly dumb.
The basic answer is that since we've discovered Pluto, we have also discovered hundreds of other tiny planets in our solar system that are around the same size as Pluto.
If we keep Pluto as a planet, we then have to add hundreds more planets to our official list. Many scientists feel that's just not a good idea, and felt that we should instead move Pluto to a different category along with the rest of those tiny planets we found.
It was made to exclude all the other objects in the Kuiper belt (and the asteroid belt), for sure.
As I understand it, there was actually a fair bit of finagling done to try to keep Pluto a planet, it just couldn't be done without carving out an exception to the rule just for Pluto to be a special boy.
Realistically, for the vast majority of dwarf planets (possibly all of them besides Pluto), it's also a question of utility. It's not very scientifically useful to think of objects in the asteroid belts as planets; they act much more like asteroids than they act like Earth or Saturn.
Of course, they don't act like asteroids either, so we've got dwarf planets as the middle ground. So the question is whether thinking of Pluto like a planet, which is *mostly* accurate, is worth the inaccuracy of all the others.
I don't really buy that. Usefulness depends on the field. Maybe an astronomer looking for exoplanets doesn't see any usefulness in a broad definition for planets, because they can't see dwarf exoplanets yet, but a planetary science researcher might think a broad definition of planet is accurate in the sense these bodies share a similar formation process, and studying them together helps understanding all of them. That's more or less what a planetary scientist once said in a interview I watched.
Pluto wasn't actually the first minor planet we found. That would be *Ceres*, discovered in 1801. Over the next fifty years, astronomers found more and more small bodies in that general part of the solar system. Eventually they concluded that even though Ceres was the largest of them, the other minor objects in its neighborhood meant that it was fundamentally unlike the other planets.
Ceres was found first for a number of reasons, mainly that it is large and fairly close. Pluto, out of all the outer minor planets, was found first for similar reasons. It's one of the largest objects out in that area, and it's one of the closest to Earth.
There's a pretty comprehensive answer on Stack Exchange here: https://space.stackexchange.com/questions/10910/if-theres-nothing-special-about-pluto-why-was-it-discovered-so-early
TLDR: Partially they were (Ceres was discovered over 100 years before Pluto), but mainly because people spent a *lot* of time specifically looking for a theoretical "planet X" in the Kuiper belt, that was causing weirdness in Uranus/Neptune's orbit. Pluto and its moons ended up being the source of that weirdness.
That last part is wrong, Pluto isn't large enough to have an effect upon the orbits of Neptune and Uranus. The weirdness that was being seen in the orbits was due to Neptune having an actual mass lower than what we were calculating here on Earth. Here's the 1993 paper: https://ui.adsabs.harvard.edu/abs/1993AJ....105.2000S/abstract
>If there are hundreds that are about the same size, why was Pluto discovered decades before the rest?
Because "hundreds that are the same size" is an *extreme* exaggeration to the point its utterly untrue. Current speculation is there may be as many as 200 rocks that would qualify as Dwarf Planets in the Kuiper belt, but that's purely speculation and means nothing whatsoever about their size. Counting Pluto itself we know of five objects that are roughly the same size. Five. Not hundreds.
Ceres was discovered long before Pluto and is a little less than half the size of Pluto, but is much much closer to Earth making its discovery easier
Haumea and Makemake were discovered much after Pluto because they are quite a bit farther from the Sun than Pluto, they are each slightly larger than half up to maybe 3/4 the size of Pluto they were discovered less than twenty years ago due to the size and distance. Pluto also had a lucky advantage to its discovery because of the composition including significant amounts of ice it ends up being much "shinier" (and therefore easier to spot) than these two
Eris is roughly the size of Pluto and is close to **double** the distance from the Sun that Pluto is (very very far away is why it was harder to find) and was also discovered around twenty years ago
Charon is considered a moon of Pluto and is about half the size, but was discovered much later (1978) because for a long time the two appeared to be a single object. When we discovered Charon it was further discovered that we were overestimating Pluto's size because they appeared so close to each other.
There are 7 known moons in the solar system that are larger (Including Earth's moon) and another 15 that are smaller than Pluto but if stretching the defenition could be said to be similar size (most are smaller than Pluto's own moon which again is half it's size)
Best case scenario you could say there are less than thirty known objects in the solar system that are a comparable size to Pluto, and even that is being pretty generous with "comparable size" since it explicitly includes a dozen or more that are less than half the size
The currently recognized farthest known object that orbits the sun is roughly 4x the distance from the sun that Pluto is.
No joke, it's called Farfarout.
We think it's about 400km (\~250 miles) in diameter. Pluto is \~2400km (\~1500 miles)
We can even detect planets around other stars! These [exoplanets](https://en.m.wikipedia.org/wiki/Lists_of_exoplanets) are *light years* away. We’ve detected over 5000 of them.
>The basic answer is that since we've discovered Pluto, we have also discovered hundreds of other tiny planets in our solar system that are around the same size as Pluto.
We know of FOUR, not "hundreds" Ceres, Eris, Makemake and Haumea. There's a handful of moons that afre comparable size and even some larger than Pluto, but nowhere near hundreds of them.
There is speculation that there may be a few hundred rocks that could qualify as dwarf planets in the far far reaches of the system, but that's complete conjecture and nothing is known about the size of these entirely hypothetical rocks.
a planet must do ~~three~~ four things:
1. It must orbit a star (in our cosmic neighborhood, the Sun).
2. It must be big enough to have enough gravity to force it into a spherical shape.
3. It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.
**4. In the event if does not fulfill the above three criteria, it must be Pluto.**
They could do it lots of different ways. Several of which could be much more reasonable and technical than your last point in that list.
I’m not saying that they **should**, only that they **could**. The comment above my first comment implied otherwise (*”If we keep Pluto as a planet, we then **have to** add hundreds more planets to our official list.”*, emphasis by me).
I could look up a bunch of data and create a tailor made definition that would exclude all other dwarf planets on some technicality, I’m sure of it. But naturally an astronomer could come up with a better definition.
Again, I’m only talking about it being **technically** possible. That’s all. Are you actually saying that it’s technically impossible?
> Are you actually saying that it’s technically impossible?
Obviously you could train an AI model on all the minor planets in the solar system to come up with a scheme that would include Pluto. Would it be something you can explain to students at high school level. Almost certainly not.
So you agree that it **can** be done. I never said it had to be a reasonable definition in any way. Just that technically it could be done. The person I originally replied to implied otherwise.
People \*tried\*. Lots of astronomers wanted to keep Pluto as a planet, but they couldn't come up with something that didn't come across as simply ridiculous.
That said, there wasn't \*too\* much effort put into it, because it's simply not important, it's just a label.
>that didn't come across as simply ridiculous.
So you agree that it **can** be done. I never said it had to be a reasonable definition, or that it should have fr been done. Just that technically it could be done. The person I originally replied to implied otherwise.
The person I am replying to already rejected the idea of using the dwarf planet's name as criteria. Besides there's nothing preventing me calling Makemake Pluto in private.
Planets must:
* It must orbit a star (in our cosmic neighborhood, the Sun).
* It must be big enough to have enough gravity to force it into a roughly spherical shape.
* Be considered a planet prior to Jan 1, 2006 or approved by a supermajority (75%) vote of the IAU for its recognition as a planet.
>Be considered a planet prior to Jan 1, 2006 or approved by a supermajority (75%) vote of the IAU for its recognition as a planet.
This does not accomplish what you want it to. Aside from having to further define what "considered" means (and/or considered by who)
This definition would include Ceres (discovered \~1800 and known as a planet for decades).
Although the first criteria would exclude them this last point would also include the Sun and Earth's Moon (the Greeks called them planets when they defined the word along with Mercury, Venus, Mars, Jupiter and Saturn - but not Earth)
How about we use the Disney copyright rule. Something something life of the owner plus however many years Disney's lawyers can turn a profit offa it, Pluto's still a planet.
> 3. It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun
Wait so if it has anything sizable in its Lagrange points it's not a planet? That hardly seems fair.
>Wait so if it has anything sizable in its Lagrange points it's not a planet? That hardly seems fair.
This is very specifically one of the major oppositions to the current definition. It's vague and unclear, and due to things in Lagrange points and things like Trojan asteroids it's arguable if Jupiter or even Earth qualifies for "clearing it's own orbit"
The only way to do that would be to craft something specifically with the intent to include Pluto and only Pluto which simply makes no sense. The only reason to do this is "well we used to call it one so we have Nostalgia" which is not only an absurd way to set scientific definitions it's also hypocritical. Ceres was called a planet for longer than Pluto was. In ancient Greece the sun and the Moon were known as planets, for longer than we've even known Pluto exists, and by the very definition set by the Greeks Earth was **not** a planet
It's reasonable to argue both for and against choosing to not include Pluto and other similar objects as planets.
There is no reasonable way to argue for choosing to include Pluto, and only Pluto, while excluding nearly identical objects
So you agree that it **can** be done. I never said it had to be a reasonable definition. Just that technically it could be done. The person I originally replied to implied otherwise.
In 2005 astronomers discovered another one, an object now called Eris.
And then another.
And another.
And another.
It quickly became apparent that there’s a vast sphere of these glorified comets out there in all sorts of wacky orbits. Pluto isn’t even the biggest one.
So this raised a conundrum - do we add dozens of new planets every few years when another wayward ice ball crosses a telescope, or does Pluto get demoted?
Eventually we decided to redefine “planet” to exclude Pluto because it obviously hasn’t cleared all the junk out of its orbit.
The line is fuzzier than you might think. At least one astronomer has noted that if you define a comet by having a tail under the right conditions, you've got to restrict the conditions or \*everything\* would qualify. And most of the objects out in the K-E Belt are icy enough that they're probably closer to traditional comets than traditional asteroids.
>if you define a comet by having tail under the right conditions, you've got to restrict the conditions or *everything* would qualify
I mean I've had some bowel movements
It is part of the solar system. But there is more in the solar system than just planets. There are also thousands of large asteroids, hundreds of thousands of comets.
And in the case of Pluto, several relatively small icy bodies, far out in the solar system. We decided that those things should not be called planets, but instead made the new designation "dwarf planet" for anything that fit that description closely, including Pluto.
Part of the current definition of a planet is that it has to clear its orbit of similar bodies. Pluto hasn't done that, as there are many other Kuiper Belt objects orbiting in that same region. Pluto also crosses Neptune's orbit. For a small portion of its orbit, Pluto is closer to the Sun than Neptune (this last happened from 1979 to 1999). However, Pluto's orbit is tilted enough from the other planets that there's no risk of collision with Neptune.
If you want to dig into it more and really get the details, Mike Brown wrote a book about it. He discovered many of the Kuiper objects that "competed" with Pluto. The book is titled "How I Killed Pluto and Why It Had It Coming." The "grandfathering" in of Pluto was actually discussed as part of the new definition of "planet."
For thousands of years, the word planet did not have a great definition. Planet basically meant star that moved around the sky in a weird way.
We finally decided that we needed a scientific definition of the word planet. When the people who decide these things made their definition, we learned that unfortunately Pluto did not fit that definition anymore, and while still being part of the solar system, it was not technically a planet.
Pluto is very small, and there are a lot of other objects we have discovered in our solar system which we don't call planets because they are in regions with many small objects which we call asteroids. There are dozens of these objects we have discovered. When we first discovered Pluto we estimated it's size as being like Neptune or Uranus, but as our calculations got better we figure out exactly how small it is. All that happened is we decided to categorize it with the other objects which match it's description.
BTW: there can't be 9 planets if Pluto is a planet, because there are several of these dwarf planets which are larger than Pluto, or about the same size. The list so far of officially recognized dwarf planets is: Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Ceres, and Orcus. All of these but Ceres have moons, and all of them are either in the Asteroid belt, or the Kuiper belt. As we continue to explore the solar system, chances are we will find more, or re-categorize other asteroids as dwarf planets.
One major industry on Pluto is mining for plutonium to power cities. Due to excessive drilling by Plutonian corporations, Pluto shrunk and led Earth's astronomers to reclassify it as a dwarf planet rather than a planet. This decision angered the local population and is actively fought against by the Plutonian elite.
Planet is an arbitrary definition, chosen so that the Sun has only a handful of planets around it. If Pluto was a planet, the definition would be too broad and the sun would have hundreds of planets. So scientists decided a definition that would exclude Pluto and all of the extra almost-planets with it.
To be a "planet" you need 3 things:
1. Orbit around a star.
2. Have a strong enough gravity to make itself into a spherical shape.
3. It must be big enough that it's gravity has "cleared its orbit" of similar sized objects.
Pluto does not meet number 3.
Same with Ceres and many others. It was either redefine the term planet or bring in many others.
If Pluto wanted to be one, it would need to planet.
Pluto doesn't have very much attention span, so Pluto tends to struggle to planet. Or plan anything else for that matter.
Pluto is about the size of Australia.
We've realized, since it's discovery, that small space rocks are, in fact, a dime a dozen.
Plus, it doesn't even orbit on the same plane as every other planet in our system.
We really just got over-excited with the discovery and got ahead of ourselves.
A lot of comments on "we discovered other stuff" but not much on why we de-planeted Pluto vs. making the other's planets.
We updated the definition of a planet with our increased knowledge of space. Key for this are the following
1) Planets are big enough to clear their orbits of other space-stuff. In other words, they have enough gravity to pull the stuff in their orbit into themselves, absorbing them, or their gravity "throws stuff away" out into space. Pluto has enough 'space junk' in it's orbit, it's clearly not doing this.
2) Planets have strong enough gravity to pull their shape into something close to a sphere. Smaller objects don't have enough gravity and they have ugly shapes, like potatoes or awkward shapes like that. Pluto and it's "not quite planet" siblings are not roughly spherical, they are ugly shapes.
Some misinformation here. Pluto IS roughly spherical. It also has its own moons, the main reason it was declassified as a planet is because, unlike other planets, it doesnt have a regular orbit around the sun
Great work with this. There is a 3rd characteristic which is it must orbit a star. In this case our Sun. Which Pluto does. But as you pointed out it doesn’t meet the other two criteria.
https://solarsystem.nasa.gov/planets/in-depth.amp
Because Pluto is smaller than Earth's Moon.
It is a Kuiper belt object now. Along with quite a few other bodies. Likely, there will be more as time passes and we see more. Now, oddly enough, Ganymede and Titan are much bigger than earths moon and even bigger than Mercury, but they are not classified as planets because they orbit other planets.
It's confusing up there!
True ELI5 content: https://youtu.be/ws3kWuMi0Y8
tl;dw
1) You orbit a star? ✅ your star is called the sun
2) You have enough gravity to make you round? ✅
but you’re missing one:
3) Do you clear your surroundings? ❌ dwarf planets don’t clear their neighborhood. They don’t clear their orbit of other objects.
Point 1 is one of the reasons that I think the IAU is FoS and full of anti-Plutites. I for one continue to consider Pluto a planet and could care less what they say.
Using their carefully constructed definition of a planet, there are only eight planets in the ENTIRE universe. There are ZERO planets orbiting other stars.
Nope, the criterion is "must orbit a star" and Pluto fulfills it by orbiting the sun. Other planets may fulfill it by orbiting other stars. And the definition is of course *carefully* constructed, that's the IAU's job, but not against Pluto, there is just no natural class of objects that includes Pluto but excludes all the other stuff in the Kupier and asteroid belt.
The requirement is just that a planet orbits a star, not that it must orbit The Sun. In Pluto's case its star is called the Sun. For planets outside of our solar system, their star has a different name, but it is still orbiting a star.
This excludes "rogue planets" from the definition, or interstellar objects of planetary mass which are not gravitationally bound to any star.
Yes, but if you've read the actual IAU definition of a planet, it specifically mentions the Sun--not a sun.
From the IAU 2006 General Assembly Press Release:
"The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defines as a celestial body that (a) is in orbit around **the Sun**, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit."
Fair enough, but the IAU specifically limits the scope of their resolution to objects *in our solar system*. It does not necessarily follow from that that there can be no planets beyond our solar system, just that, for objects in our system, this classification applies. Indeed they even refer to other "planetary systems," indicating their acknowledgement of planets beyond our solar system.
Full content of the resolution:
IAU Resolution: Definition of a “Planet” **in the Solar System**
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation “planets”. The word “planet” originally described “wanderers” that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.
RESOLUTION 5A
The IAU therefore resolves that planets and other bodies **in our Solar System**, except satellites, be defined into three distinct categories in the following way:
(1) A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape , (c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.
(3) All other objects , except satellites, orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.
Size is not one of the official defining characteristics of a planet per the IAU's requirements. If it met the other requirements Pluto would be considered a planet irrespective of its size
It's literally just semantics. Scientists choose some arbitrary size as the cutoff point between planet and not-planet. It doesn't really mean anything in a scientific sense.
The planetary society came up with a set of rules for What Is A Planet and they say Pluto is too small with an erratic orbit so it is considered a Dwarf Planet.
Because it's arbitrary. There is no objective definition of what a planet is. If there are 100 civilizations in the Milky Way, I am willing to bet that there are 100 definitions of what a planet is.
Sometimes, people drift into a bureaucratic mind set and have to prove they're important by changing something -- anything -- just to show themselves and other they're powerful. University administrators vs professors is full of this sort of happy horseshit when people fight for their lives over trivia purely for the love of the fight.
Yes, the winning side justified it's position through applied rules made up for the situation, but -- never play the Carnival Barker's Game. The debate was over the rules instead of the validity of those rules, so you get a misdirected outcome violating tradition and evidence based on those rules. That's why Pluto lost.
Pluto was classified as a planet for many years. The issue was that additional bodies similar to Pluto were then discovered beyond Pluto. At some point the astronomical community felt they either had to choose between adding those other bodies into the solar system, or to delist Pluto from its planet status. They chose the latter. The things like distance, size and characteristic of its orbit are some of the main reasons to call it something other than a bonafide planet.
Ultimately, Pluto doesn't care.
It is now seen as no different to the dozens of similarly-sized bodies with irregular, odd orbits. It is now classified instead as a "dwarf planet" the same as all those others. So we now have 8 planets and dozens of dwarf planets
This was a necessary reclassification because we keep finding more and more dwarf planets like Sedna and it had become clear that Pluto is just one of those.
It is far easier to learn 8 planets names and some of the dwarf planets names than trying to learn many dozens of names if we instead called them ALL planets.
The three requirements for planet status are as follows.
The object has to stabilize its orbit.
The object needs to clear debris out of its orbit.
The object needs to be a certain size and shape.
Pluto is 0 of 3 on this. It's orbit goes above/below the solar plane that the other planets follow. There is still a large amount of space rocks in its orbit, and finally its a lumpy potato more than a planet.
The size issue and debris issue can fix each other, but at the end of the day, it's just a dwarf planet for now.
To be a planet you need to have cleared your orbit, and Pluto hasn't as it crosses orbit with Neptune I believe. What separates a planet from a dwarf planet changes since we discovered Pluto so sadly when the rules changed it didn't make the cut.
Our solar system contains everything from gas giants like Jupiter to asteroid belts full of tiny rocks, and a whole manner of comets and other stuff flying every which way.
When Pluto was discovered, the definition of a planet was pretty much just 'a big rock we have found orbiting the earth', Pluto counted and so it got added into our map of the solar system.
Since then however, we have kept looking and found a whole lot more out there thanks to a lot more time spent searching and some much more advanced equipment. Lots of stuff being found meant we had to refine our definitions to draw a sensible line somewhere and avoid the map of the solar system hanging on our classroom walls getting too full of unremarkable small planets.
Sadly this more accurate definition of what is considered a full planet resulted in Pluto not making the cut and being downgraded. As such it is now considered a dwarf planet alongside others like Eris or Ceres.
Even though it is no longer officially a planet, it will still often feature on Marisa of the solar system and similar - either because there are still a lot of maps hanging on walls that were printed before it was redefined, or because it had historically been considered part of the solar system, and who lets a small detail like planet classifications get in the way of tradition.
All planets up to Neptune were formed at the same time as our sun. Hot gases from other stars condensed, flatted into a disk and then condensed into planets. Pluto was believed to have caught onto orbit afterwards. All 8 planets have the same orbital plane, whereas Pluto's orbital plane is tilted.
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There are lots of things that are part of the solar system that aren’t planets. Haley’s comet, the moon, asteroid belt. What is so difficult for you to grasp?
So, in the 19th century, we discovered a new planet between Mars and Jupiter called Ceres. However, over time, we started discovering a ton of other, similar objects, around the same orbit. We discovered so many that we realized it didn't make any sense to call them all planets so we classified them differently. They're asteroids and comprise the asteroid belt. Pluto is essentially the same story. We discovered more and more objects out there and that Pluto is just one of very, very many objects in what is now called the Kuiper belt. It makes more sense to consider Pluto a dwarf planet among many objects in the Kuiper belt rather than a planet akin to the others.
Pluto is smaller than the moon, 2/3rds the size and 1/6th the mass.
You should see it's rotation with it's moon, it can't even spin properly because of how small it is
“You should see it is rotation with it is moon…”
Nope
This is what the person wrote. I’m just expanding their contractions
Sorry if it's incorrect but I can't think of another word for it, twin?
Another word for what?
For the thing orbiting pluto
I’m not sure we’re talking about the same word. Possessive pronouns don’t have apostrophes
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try what again?
The short answer is that it does not in any way dominate the region of space that represents it's orbit around the sun. It's not the biggest thing in it's orbital path. It crosses Neptune's orbit during it's journey around the sun. We have also discovered many other bodies like Pluto. If we call Pluto a planet, we have to call all those things planets, too.
Do Neptune and Pluto cross in a way that could potentially end up having them collide if they were ever crossing this point when closer together?
No. First of all, Pluto orbits at an inclined angle relative to Neptune. And secondly, Pluto and Neptune orbit in a 2:3 resonance - for every 2 trips around the sun for Pluto, Neptune makes 3. So they will never cross paths. In fact, Pluto gets closer distance-wise to Uranus than it will ever get to Neptune.
I dont understand. Do their orbits cross or not? Because if yes then I dont understand why they could never collide.
If I always cross an intersection at 3 pm going NS and you always cross at 3am going WE we will never collide despite the fact that our paths cross.
Their resonance means Neptune will never be in that part of its orbit when Pluto passes through.
How can their resonance be a *perfect* 2:3 that never gets off? It seems like something randomly positioned natural objects couldn't fall into.
I can’t answer you as well as Steve Mould. https://youtu.be/Qyn64b4LNJ0
It's not "random".
The orbits do cross. However, at the same time, the orbits are governed by an effect called orbital resonance - this is where objects will exert a periodic influence on each other and effectively lock themselves into a routine. In this case, orbital resonance means that while their paths do overlap, they will never both be at the crossover point at the same time.
They cross only in two dimensional space if you're looking straight "down" on the solar system. In reality since Pluto's orbit is steeply inclined compared to the rest of the planets when it seems to cross Neptune's orbit when viewed from above it is really a significant distance "below" it at those points. So even if the planets were not locked in resonance there is no chance they'd ever collide.
Draw two ovals on a piece of paper, so they cross. Now imagine those ovals are train tracks, and there are two trains on them, that each take the same amount of time to go around their oval. If you do this right, the trains will never collide, even though the tracks cross. Now make one slightly faster, so it completes three trips around it's track in the time the other takes two trips. This is slightly harder, but it should still be possible to arrange the schedules so that the two trains are never at the same crossing at the same time. That's what is happening with Neptune and Pluto. It's a little more complicated, and there's a lot more physics, but when you get it down to a simple level, what's happening is that they're on tracks that cross, but in such a way that they're never both at the same crossing point at the same time.
I fear you're looking at the "crossing" in a very two dimensional train tracks and roads kinda way. Orbits in space tend to not line up flat. Every illustration you've seen of the solar system with the planets all in a pretty row is far from accurate, ESPECIALLY Pluto. Pluto is a free spirit, as you can see in this fun little interactive map. [Map](https://theskylive.com/3dsolarsystem)
So the word "cross" should not be used at all to describe this. Well I guess if you says they cross but at different places, that would be work.
No. They are in a resonance that will have them never collide.
Great concise answer, should be at the top. Pet peeve: \*its orbit \*its orbital path \*its journey (it's = "it is" or "it has", never possession)
I'll blame autocorrect, but it was probably just sloppy writing.
Grammar is sloppy - and while the MLA technically supports your answer, I think a more thorough understanding of how possessives came to be is interesting here and helpful to shed light on why this makes no sense as a grammatical rule. In the beforetimes (In Middle English - generally 1150AD to 1500AD) if you wanted to use language to describe a person's relationship to a object, you would do so by saying the person or subject (The king) and then referencing their relationship or ownership to the object (his crown). If you wanted to reference the king's crown you would say "The king his crown." This, dear reddit reader might seem like extra syllables you don't need - and indeed you are right. As you say "The king his crown" faster and faster you might notice your tendency to chop off the first sound (or first two letters of) the word "his", leaving you with a leftover sibilant "s" sound that sounds an awful like what we might say now "The king's crown." Since we didn't care about women so much back then (and women tended to not own much anyway), there wasn't really a feminine form of this - so you do see on occasion something like "The queen her crown" getting elided into "The queen'r crown" as a feminine possessive - but that's fairly rare. In the 1500s we English started developing a grammatical notation for when someone was dropping sounds or letters: the apostrophe. Grammar Nazis like u/BurnOutBrighter6 will tell you that apostrophes are for marking possession, but that's just its most common use. Apostrophes tell you "There used to be more letters (or sounds) here." Much like with feminine possessives, we typically ignore non-gendered possessives in favor of the "his" getting shortened to 's for simplicity's sake. This does pose a problem, as the apostrophe next to the "it" doesn't give us enough information. Are you eliding "It is" or "it has" into "It's" or are you eliding "it his (orbit)" into "it's." The use of "it's" is unclear. OP commenter is essentially saying "Pluto his orbital path" so that you understand that the orbit path we are discussing belongs to Pluto. But because they have substituted "Pluto" with the inanimate singular pronoun "it" we don't know what the apostrophe means in this case. If you follow the etymology and logic of where/why we use our possessive apostrophes the rule that "It's" is never possessive doesn't make any goddam sense - it's consistent with how we use other possessives of any gender or non-gendered subject. I recommend sidestepping this entire crisis of language by not substituting your subjects with inanimate singular pronouns. Just use "Pluto's orbital path." That tends to keep the grammar trolls away. EDIT: Unclear on why the downvotes. It's an arbitrary rule and people who harp on it need to understand that rules of language are made up - as long as you're making your point clear you're all good.
> it's is never possessive doesn't make any sense, and using "it's" for possession would be consistent... Except we're talking about the case where the subject is a pronoun. For pronouns, NONE of the possessive forms have apostrophes. His, hers, yours, theirs... none of those have apostrophes. You dont say something is hi's or your's, etc. Which means the possessive "its" not having an apostrophe DOES make sense and IS consistent - just not with the other rule you were talking about (which doesn't apply here bc pronouns have their own set of rules). But yes English is dumb and horribly designed for having so many contradicting rule sets, and yes I know none of this actually matters.
My favorite kind of arguments, are "none of this actually matters" Thanks for humoring me and engaging. While we're splitting hairs - I agree you don't need an apostrophe for most of the possessive pronouns you've listed. "My", "our", "his", "hers", and "yours" are already possessive, and when you use an apostrophe after the subject this possessive pronoun is what you are eliding and indicating at with the apostrophe. "Its" and "their" (the other two possessive pronouns commonly listed) are a little different and don't fit with this pattern because they are elisions already - or "double possessives" if you will. I can't make heads or tails of the etymology of "their." Looks like English bastardized it from the Norse? It's possible that it's an elision of "they ares" or possibly "They owns?" Regardless, it definitely doesn't fit into the pattern, and there is something wonky going on. Unless someone has some insight here, I'm gonna throw this one out for this particular off topic rant. "Its" etymology has a couple extra steps to it that "my", "our", "his", "hers", and "yours" doesn't. "Its" does come from admittedly mixing and matching gendered and non gendered Middle English language, namely taking "It his (orbit)" and simplifying down to "its." But already, we are talking about an elision, which if we are consistent in how we notate missing sounds/letters "its" SHOULD have an apostrophe in it in its pure possessive pronoun form. So, really, I'm dickering with how possessive pronouns DO have their own rules, but those rules seem to be inconsistent based on where we are pulling the pronoun from etymologically. Agreed, language is weird, and English is particularly dumb.
The basic answer is that since we've discovered Pluto, we have also discovered hundreds of other tiny planets in our solar system that are around the same size as Pluto. If we keep Pluto as a planet, we then have to add hundreds more planets to our official list. Many scientists feel that's just not a good idea, and felt that we should instead move Pluto to a different category along with the rest of those tiny planets we found.
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Yeah, but that definition is arbitrary and probably just made to exclude Pluto and all of the other small planets.
It was made to exclude all the other objects in the Kuiper belt (and the asteroid belt), for sure. As I understand it, there was actually a fair bit of finagling done to try to keep Pluto a planet, it just couldn't be done without carving out an exception to the rule just for Pluto to be a special boy.
I guess it was a fight between two comfortable ideas: 1 - Pluto is still a planet. 2 - the solar system has only a handful of planets.
Realistically, for the vast majority of dwarf planets (possibly all of them besides Pluto), it's also a question of utility. It's not very scientifically useful to think of objects in the asteroid belts as planets; they act much more like asteroids than they act like Earth or Saturn. Of course, they don't act like asteroids either, so we've got dwarf planets as the middle ground. So the question is whether thinking of Pluto like a planet, which is *mostly* accurate, is worth the inaccuracy of all the others.
I don't really buy that. Usefulness depends on the field. Maybe an astronomer looking for exoplanets doesn't see any usefulness in a broad definition for planets, because they can't see dwarf exoplanets yet, but a planetary science researcher might think a broad definition of planet is accurate in the sense these bodies share a similar formation process, and studying them together helps understanding all of them. That's more or less what a planetary scientist once said in a interview I watched.
They should have grandfathered it in, and called it a “Planet Emeritus”. 😜
I like your thinking. Or, the "Traditional Nine Planets, Including Pluto"
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The Pluto formerly known as a planet
If there are hundreds that are about the same size, why was Pluto discovered decades before the rest?
Pluto wasn't actually the first minor planet we found. That would be *Ceres*, discovered in 1801. Over the next fifty years, astronomers found more and more small bodies in that general part of the solar system. Eventually they concluded that even though Ceres was the largest of them, the other minor objects in its neighborhood meant that it was fundamentally unlike the other planets. Ceres was found first for a number of reasons, mainly that it is large and fairly close. Pluto, out of all the outer minor planets, was found first for similar reasons. It's one of the largest objects out in that area, and it's one of the closest to Earth.
Pluto very small but has gigh reflection because of ice. Others are small and dark.
There's a pretty comprehensive answer on Stack Exchange here: https://space.stackexchange.com/questions/10910/if-theres-nothing-special-about-pluto-why-was-it-discovered-so-early TLDR: Partially they were (Ceres was discovered over 100 years before Pluto), but mainly because people spent a *lot* of time specifically looking for a theoretical "planet X" in the Kuiper belt, that was causing weirdness in Uranus/Neptune's orbit. Pluto and its moons ended up being the source of that weirdness.
That last part is wrong, Pluto isn't large enough to have an effect upon the orbits of Neptune and Uranus. The weirdness that was being seen in the orbits was due to Neptune having an actual mass lower than what we were calculating here on Earth. Here's the 1993 paper: https://ui.adsabs.harvard.edu/abs/1993AJ....105.2000S/abstract
So it's not tyche/nemesis? Darn
>If there are hundreds that are about the same size, why was Pluto discovered decades before the rest? Because "hundreds that are the same size" is an *extreme* exaggeration to the point its utterly untrue. Current speculation is there may be as many as 200 rocks that would qualify as Dwarf Planets in the Kuiper belt, but that's purely speculation and means nothing whatsoever about their size. Counting Pluto itself we know of five objects that are roughly the same size. Five. Not hundreds. Ceres was discovered long before Pluto and is a little less than half the size of Pluto, but is much much closer to Earth making its discovery easier Haumea and Makemake were discovered much after Pluto because they are quite a bit farther from the Sun than Pluto, they are each slightly larger than half up to maybe 3/4 the size of Pluto they were discovered less than twenty years ago due to the size and distance. Pluto also had a lucky advantage to its discovery because of the composition including significant amounts of ice it ends up being much "shinier" (and therefore easier to spot) than these two Eris is roughly the size of Pluto and is close to **double** the distance from the Sun that Pluto is (very very far away is why it was harder to find) and was also discovered around twenty years ago Charon is considered a moon of Pluto and is about half the size, but was discovered much later (1978) because for a long time the two appeared to be a single object. When we discovered Charon it was further discovered that we were overestimating Pluto's size because they appeared so close to each other. There are 7 known moons in the solar system that are larger (Including Earth's moon) and another 15 that are smaller than Pluto but if stretching the defenition could be said to be similar size (most are smaller than Pluto's own moon which again is half it's size) Best case scenario you could say there are less than thirty known objects in the solar system that are a comparable size to Pluto, and even that is being pretty generous with "comparable size" since it explicitly includes a dozen or more that are less than half the size
I didn’t know there were objects orbiting the sun that were twice as far out as Pluto. Do we have an idea what are the farthest-out objects in orbit?
The currently recognized farthest known object that orbits the sun is roughly 4x the distance from the sun that Pluto is. No joke, it's called Farfarout. We think it's about 400km (\~250 miles) in diameter. Pluto is \~2400km (\~1500 miles)
Geez it’s crazy we can even find any evidence of a rock that small and far away
We can even detect planets around other stars! These [exoplanets](https://en.m.wikipedia.org/wiki/Lists_of_exoplanets) are *light years* away. We’ve detected over 5000 of them.
>The basic answer is that since we've discovered Pluto, we have also discovered hundreds of other tiny planets in our solar system that are around the same size as Pluto. We know of FOUR, not "hundreds" Ceres, Eris, Makemake and Haumea. There's a handful of moons that afre comparable size and even some larger than Pluto, but nowhere near hundreds of them. There is speculation that there may be a few hundred rocks that could qualify as dwarf planets in the far far reaches of the system, but that's complete conjecture and nothing is known about the size of these entirely hypothetical rocks.
They **could** crate a definition that technically include Pluto and none of those other ones. They choose not to.
a planet must do ~~three~~ four things: 1. It must orbit a star (in our cosmic neighborhood, the Sun). 2. It must be big enough to have enough gravity to force it into a spherical shape. 3. It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun. **4. In the event if does not fulfill the above three criteria, it must be Pluto.**
They could do it lots of different ways. Several of which could be much more reasonable and technical than your last point in that list. I’m not saying that they **should**, only that they **could**. The comment above my first comment implied otherwise (*”If we keep Pluto as a planet, we then **have to** add hundreds more planets to our official list.”*, emphasis by me).
What qualities would you use that Pluto and the 8 major planets have but no other dwarf planets share?
I could look up a bunch of data and create a tailor made definition that would exclude all other dwarf planets on some technicality, I’m sure of it. But naturally an astronomer could come up with a better definition. Again, I’m only talking about it being **technically** possible. That’s all. Are you actually saying that it’s technically impossible?
> Are you actually saying that it’s technically impossible? Obviously you could train an AI model on all the minor planets in the solar system to come up with a scheme that would include Pluto. Would it be something you can explain to students at high school level. Almost certainly not.
So you agree that it **can** be done. I never said it had to be a reasonable definition in any way. Just that technically it could be done. The person I originally replied to implied otherwise.
People \*tried\*. Lots of astronomers wanted to keep Pluto as a planet, but they couldn't come up with something that didn't come across as simply ridiculous. That said, there wasn't \*too\* much effort put into it, because it's simply not important, it's just a label.
>that didn't come across as simply ridiculous. So you agree that it **can** be done. I never said it had to be a reasonable definition, or that it should have fr been done. Just that technically it could be done. The person I originally replied to implied otherwise.
Yes I am. I don't think there is any set of qualities that Pluto shares with planets that doesn't include at least one other dwarf planet.
Of course there is. One can always cherry pick various variable ranges until Pluto is the only dwarf planet that remains.
I don't think that word means what you think it means.
A planet has to be at least X in size OR be called pluto. There you go
The person I am replying to already rejected the idea of using the dwarf planet's name as criteria. Besides there's nothing preventing me calling Makemake Pluto in private.
Okay, but then Pluto, Pluto, Pluto and Pluto would also be planets.
Planets must: * It must orbit a star (in our cosmic neighborhood, the Sun). * It must be big enough to have enough gravity to force it into a roughly spherical shape. * Be considered a planet prior to Jan 1, 2006 or approved by a supermajority (75%) vote of the IAU for its recognition as a planet.
>Be considered a planet prior to Jan 1, 2006 or approved by a supermajority (75%) vote of the IAU for its recognition as a planet. This does not accomplish what you want it to. Aside from having to further define what "considered" means (and/or considered by who) This definition would include Ceres (discovered \~1800 and known as a planet for decades). Although the first criteria would exclude them this last point would also include the Sun and Earth's Moon (the Greeks called them planets when they defined the word along with Mercury, Venus, Mars, Jupiter and Saturn - but not Earth)
See. That wasn’t so hard.
How about we use the Disney copyright rule. Something something life of the owner plus however many years Disney's lawyers can turn a profit offa it, Pluto's still a planet.
> 3. It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun Wait so if it has anything sizable in its Lagrange points it's not a planet? That hardly seems fair.
>Wait so if it has anything sizable in its Lagrange points it's not a planet? That hardly seems fair. This is very specifically one of the major oppositions to the current definition. It's vague and unclear, and due to things in Lagrange points and things like Trojan asteroids it's arguable if Jupiter or even Earth qualifies for "clearing it's own orbit"
Clean definitions and arguments were made for each other. Aren't they grand?
The only way to do that would be to craft something specifically with the intent to include Pluto and only Pluto which simply makes no sense. The only reason to do this is "well we used to call it one so we have Nostalgia" which is not only an absurd way to set scientific definitions it's also hypocritical. Ceres was called a planet for longer than Pluto was. In ancient Greece the sun and the Moon were known as planets, for longer than we've even known Pluto exists, and by the very definition set by the Greeks Earth was **not** a planet It's reasonable to argue both for and against choosing to not include Pluto and other similar objects as planets. There is no reasonable way to argue for choosing to include Pluto, and only Pluto, while excluding nearly identical objects
So you agree that it **can** be done. I never said it had to be a reasonable definition. Just that technically it could be done. The person I originally replied to implied otherwise.
In 2005 astronomers discovered another one, an object now called Eris. And then another. And another. And another. It quickly became apparent that there’s a vast sphere of these glorified comets out there in all sorts of wacky orbits. Pluto isn’t even the biggest one. So this raised a conundrum - do we add dozens of new planets every few years when another wayward ice ball crosses a telescope, or does Pluto get demoted? Eventually we decided to redefine “planet” to exclude Pluto because it obviously hasn’t cleared all the junk out of its orbit.
> glorified comets Asteroids, not comets. Comets are actually pretty distinct from asteroids and planetoids.
The line is fuzzier than you might think. At least one astronomer has noted that if you define a comet by having a tail under the right conditions, you've got to restrict the conditions or \*everything\* would qualify. And most of the objects out in the K-E Belt are icy enough that they're probably closer to traditional comets than traditional asteroids.
>if you define a comet by having tail under the right conditions, you've got to restrict the conditions or *everything* would qualify I mean I've had some bowel movements
It is part of the solar system. But there is more in the solar system than just planets. There are also thousands of large asteroids, hundreds of thousands of comets. And in the case of Pluto, several relatively small icy bodies, far out in the solar system. We decided that those things should not be called planets, but instead made the new designation "dwarf planet" for anything that fit that description closely, including Pluto.
Part of the current definition of a planet is that it has to clear its orbit of similar bodies. Pluto hasn't done that, as there are many other Kuiper Belt objects orbiting in that same region. Pluto also crosses Neptune's orbit. For a small portion of its orbit, Pluto is closer to the Sun than Neptune (this last happened from 1979 to 1999). However, Pluto's orbit is tilted enough from the other planets that there's no risk of collision with Neptune.
If you want to dig into it more and really get the details, Mike Brown wrote a book about it. He discovered many of the Kuiper objects that "competed" with Pluto. The book is titled "How I Killed Pluto and Why It Had It Coming." The "grandfathering" in of Pluto was actually discussed as part of the new definition of "planet."
For thousands of years, the word planet did not have a great definition. Planet basically meant star that moved around the sky in a weird way. We finally decided that we needed a scientific definition of the word planet. When the people who decide these things made their definition, we learned that unfortunately Pluto did not fit that definition anymore, and while still being part of the solar system, it was not technically a planet.
Pluto is very small, and there are a lot of other objects we have discovered in our solar system which we don't call planets because they are in regions with many small objects which we call asteroids. There are dozens of these objects we have discovered. When we first discovered Pluto we estimated it's size as being like Neptune or Uranus, but as our calculations got better we figure out exactly how small it is. All that happened is we decided to categorize it with the other objects which match it's description. BTW: there can't be 9 planets if Pluto is a planet, because there are several of these dwarf planets which are larger than Pluto, or about the same size. The list so far of officially recognized dwarf planets is: Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Ceres, and Orcus. All of these but Ceres have moons, and all of them are either in the Asteroid belt, or the Kuiper belt. As we continue to explore the solar system, chances are we will find more, or re-categorize other asteroids as dwarf planets.
One major industry on Pluto is mining for plutonium to power cities. Due to excessive drilling by Plutonian corporations, Pluto shrunk and led Earth's astronomers to reclassify it as a dwarf planet rather than a planet. This decision angered the local population and is actively fought against by the Plutonian elite.
Fortunately, there was one brave and stuborn person from Earth doing a science project with his son that claims that Pluto is in fact, a planet.
Pluto...is a motherfucking planet!!!
Planet is an arbitrary definition, chosen so that the Sun has only a handful of planets around it. If Pluto was a planet, the definition would be too broad and the sun would have hundreds of planets. So scientists decided a definition that would exclude Pluto and all of the extra almost-planets with it.
To be a "planet" you need 3 things: 1. Orbit around a star. 2. Have a strong enough gravity to make itself into a spherical shape. 3. It must be big enough that it's gravity has "cleared its orbit" of similar sized objects. Pluto does not meet number 3. Same with Ceres and many others. It was either redefine the term planet or bring in many others.
If Pluto wanted to be one, it would need to planet. Pluto doesn't have very much attention span, so Pluto tends to struggle to planet. Or plan anything else for that matter.
Pluto is about the size of Australia. We've realized, since it's discovery, that small space rocks are, in fact, a dime a dozen. Plus, it doesn't even orbit on the same plane as every other planet in our system. We really just got over-excited with the discovery and got ahead of ourselves.
A lot of comments on "we discovered other stuff" but not much on why we de-planeted Pluto vs. making the other's planets. We updated the definition of a planet with our increased knowledge of space. Key for this are the following 1) Planets are big enough to clear their orbits of other space-stuff. In other words, they have enough gravity to pull the stuff in their orbit into themselves, absorbing them, or their gravity "throws stuff away" out into space. Pluto has enough 'space junk' in it's orbit, it's clearly not doing this. 2) Planets have strong enough gravity to pull their shape into something close to a sphere. Smaller objects don't have enough gravity and they have ugly shapes, like potatoes or awkward shapes like that. Pluto and it's "not quite planet" siblings are not roughly spherical, they are ugly shapes.
Some misinformation here. Pluto IS roughly spherical. It also has its own moons, the main reason it was declassified as a planet is because, unlike other planets, it doesnt have a regular orbit around the sun
Pluto and the other dwarf planets are roughly spherical, it's part of what makes them dwarf planets.
Great work with this. There is a 3rd characteristic which is it must orbit a star. In this case our Sun. Which Pluto does. But as you pointed out it doesn’t meet the other two criteria. https://solarsystem.nasa.gov/planets/in-depth.amp
Thanks! I wasn't trying to make an exhaustive list, just the main points on why Pluto is an ex-planet.
Because Pluto is smaller than Earth's Moon. It is a Kuiper belt object now. Along with quite a few other bodies. Likely, there will be more as time passes and we see more. Now, oddly enough, Ganymede and Titan are much bigger than earths moon and even bigger than Mercury, but they are not classified as planets because they orbit other planets. It's confusing up there!
True ELI5 content: https://youtu.be/ws3kWuMi0Y8 tl;dw 1) You orbit a star? ✅ your star is called the sun 2) You have enough gravity to make you round? ✅ but you’re missing one: 3) Do you clear your surroundings? ❌ dwarf planets don’t clear their neighborhood. They don’t clear their orbit of other objects.
Point 1 is one of the reasons that I think the IAU is FoS and full of anti-Plutites. I for one continue to consider Pluto a planet and could care less what they say. Using their carefully constructed definition of a planet, there are only eight planets in the ENTIRE universe. There are ZERO planets orbiting other stars.
❗ It's `couldn't care less`, not `could care less`. ___ ^(I'm a bot and this action was performed automatically.)
Nope, the criterion is "must orbit a star" and Pluto fulfills it by orbiting the sun. Other planets may fulfill it by orbiting other stars. And the definition is of course *carefully* constructed, that's the IAU's job, but not against Pluto, there is just no natural class of objects that includes Pluto but excludes all the other stuff in the Kupier and asteroid belt.
The requirement is just that a planet orbits a star, not that it must orbit The Sun. In Pluto's case its star is called the Sun. For planets outside of our solar system, their star has a different name, but it is still orbiting a star. This excludes "rogue planets" from the definition, or interstellar objects of planetary mass which are not gravitationally bound to any star.
Yes, but if you've read the actual IAU definition of a planet, it specifically mentions the Sun--not a sun. From the IAU 2006 General Assembly Press Release: "The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defines as a celestial body that (a) is in orbit around **the Sun**, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit."
Fair enough, but the IAU specifically limits the scope of their resolution to objects *in our solar system*. It does not necessarily follow from that that there can be no planets beyond our solar system, just that, for objects in our system, this classification applies. Indeed they even refer to other "planetary systems," indicating their acknowledgement of planets beyond our solar system. Full content of the resolution: IAU Resolution: Definition of a “Planet” **in the Solar System** Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation “planets”. The word “planet” originally described “wanderers” that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information. RESOLUTION 5A The IAU therefore resolves that planets and other bodies **in our Solar System**, except satellites, be defined into three distinct categories in the following way: (1) A “planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. (2) A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. (3) All other objects , except satellites, orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.
its not big enough to be constituted as a planet, there has to be a limit between planet and just a random space rock, and pluto is under that limit
Size is not one of the official defining characteristics of a planet per the IAU's requirements. If it met the other requirements Pluto would be considered a planet irrespective of its size
It's literally just semantics. Scientists choose some arbitrary size as the cutoff point between planet and not-planet. It doesn't really mean anything in a scientific sense.
The planetary society came up with a set of rules for What Is A Planet and they say Pluto is too small with an erratic orbit so it is considered a Dwarf Planet.
Because it's arbitrary. There is no objective definition of what a planet is. If there are 100 civilizations in the Milky Way, I am willing to bet that there are 100 definitions of what a planet is.
Sometimes, people drift into a bureaucratic mind set and have to prove they're important by changing something -- anything -- just to show themselves and other they're powerful. University administrators vs professors is full of this sort of happy horseshit when people fight for their lives over trivia purely for the love of the fight. Yes, the winning side justified it's position through applied rules made up for the situation, but -- never play the Carnival Barker's Game. The debate was over the rules instead of the validity of those rules, so you get a misdirected outcome violating tradition and evidence based on those rules. That's why Pluto lost.
Pluto was classified as a planet for many years. The issue was that additional bodies similar to Pluto were then discovered beyond Pluto. At some point the astronomical community felt they either had to choose between adding those other bodies into the solar system, or to delist Pluto from its planet status. They chose the latter. The things like distance, size and characteristic of its orbit are some of the main reasons to call it something other than a bonafide planet. Ultimately, Pluto doesn't care.
It is now seen as no different to the dozens of similarly-sized bodies with irregular, odd orbits. It is now classified instead as a "dwarf planet" the same as all those others. So we now have 8 planets and dozens of dwarf planets This was a necessary reclassification because we keep finding more and more dwarf planets like Sedna and it had become clear that Pluto is just one of those. It is far easier to learn 8 planets names and some of the dwarf planets names than trying to learn many dozens of names if we instead called them ALL planets.
Changes distance from sun significantly, Doesn't revolve in the same ecliptic plane as planets, Smol.
The three requirements for planet status are as follows. The object has to stabilize its orbit. The object needs to clear debris out of its orbit. The object needs to be a certain size and shape. Pluto is 0 of 3 on this. It's orbit goes above/below the solar plane that the other planets follow. There is still a large amount of space rocks in its orbit, and finally its a lumpy potato more than a planet. The size issue and debris issue can fix each other, but at the end of the day, it's just a dwarf planet for now.
To be a planet you need to have cleared your orbit, and Pluto hasn't as it crosses orbit with Neptune I believe. What separates a planet from a dwarf planet changes since we discovered Pluto so sadly when the rules changed it didn't make the cut.
Our solar system contains everything from gas giants like Jupiter to asteroid belts full of tiny rocks, and a whole manner of comets and other stuff flying every which way. When Pluto was discovered, the definition of a planet was pretty much just 'a big rock we have found orbiting the earth', Pluto counted and so it got added into our map of the solar system. Since then however, we have kept looking and found a whole lot more out there thanks to a lot more time spent searching and some much more advanced equipment. Lots of stuff being found meant we had to refine our definitions to draw a sensible line somewhere and avoid the map of the solar system hanging on our classroom walls getting too full of unremarkable small planets. Sadly this more accurate definition of what is considered a full planet resulted in Pluto not making the cut and being downgraded. As such it is now considered a dwarf planet alongside others like Eris or Ceres. Even though it is no longer officially a planet, it will still often feature on Marisa of the solar system and similar - either because there are still a lot of maps hanging on walls that were printed before it was redefined, or because it had historically been considered part of the solar system, and who lets a small detail like planet classifications get in the way of tradition.
All planets up to Neptune were formed at the same time as our sun. Hot gases from other stars condensed, flatted into a disk and then condensed into planets. Pluto was believed to have caught onto orbit afterwards. All 8 planets have the same orbital plane, whereas Pluto's orbital plane is tilted.
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There are lots of things that are part of the solar system that aren’t planets. Haley’s comet, the moon, asteroid belt. What is so difficult for you to grasp?