Pluto and the definition of a planet

[Terabithya]

February 18 marked the 90th birthday of the discovery of Pluto, the ninth planet from the Sun. For 76 years it counted as a planet for everyone until a part of the International Astronomical Union (IAU) in 2006 set up a definition of 'planet' and decided that Pluto is no longer one. The reason for that was the discovery of another planet, the tenth planet Eris, called 'Xena' back then. Instead of accepting Eris as the tenth planet, the IAU started to debate on what a planet actually is because they feared that there'd be plenty of new planets, a fear that didn't come true since no other bodies are as big and as massive as Pluto and Eris. There might be an 11th planet far beyond the Kuiper belt so if it exists there would be 11 planets.

The IAU could have easily set up a definition that would include Eris as the tenth planet, but anyway it wasn't necessary to create a definition for 'planet'. There is no definition for 'continent' either. They should just have accepted Eris as the tenth planet and name it accordingly.

If one accepts Pluto and Eris as planets, then perhaps Haumea is a planet too because new research suggests that its equatorial diameter might be larger than that of Eris.

[Dogmeat]

Meanwhile Ceres is quietly depressed that no one ever even remembers when it got demoted in exactly the same way, and for the same reason.. Poor Ceres.

[Farnhamia]

February 18 marked the 90th birthday of the discovery of Pluto, the ninth planet from the Sun. For 76 years it counted as a planet for everyone until a part of the International Astronomical Union (IAU) in 2006 set up a definition of 'planet' and decided that Pluto is no longer one. The reason for that was the discovery of another planet, the tenth planet Eris, called 'Xena' back then. Instead of accepting Eris as the tenth planet, the IAU started to debate on what a planet actually is because they feared that there'd be plenty of new planets, a fear that didn't come true since no other bodies are as big and as massive as Pluto and Eris. There might be an 11th planet far beyond the Kuiper belt so if it exists there would be 11 planets.

The IAU could have easily set up a definition that would include Eris as the tenth planet, but anyway it wasn't necessary to create a definition for 'planet'. There is no definition for 'continent' either. They should just have accepted Eris as the tenth planet and name it accordingly.

Pluto and Eris are planets. I can't describe how wrong, vague and unscientific the 2006 definition is for that would take multiple pages. Please tell me what you think is a planet. Do you consider Pluto a planet and why or why not?

Pluto is a dwarf planet because it hasn't cleared its orbit to the extent required by the IAU (and there are only three requirements). You could very easily look this stuff up and include some of it in the OP. The excuse that you "can't describe how wrong, vague and unscientific the 2006 definition is for that would take multiple pages" is just that, an excuse. But hey, you know better than professional astronomers and astrophysicists and telescope people, so ... sure. I'll stick with the IAU definition. Sorry, Pluto.

[Terabithya]

Pluto is a dwarf planet because it hasn't cleared its orbit to the extent required by the IAU (and there are only three requirements). You could very easily look this stuff up and include some of it in the OP. The excuse that you "can't describe how wrong, vague and unscientific the 2006 definition is for that would take multiple pages" is just that, an excuse. But hey, you know better than professional astronomers and astrophysicists and telescope people, so ... sure. I'll stick with the IAU definition. Sorry, Pluto.

And how would you know if I am an astronomer or not? And what about yourself? How do you define "clearing the orbit"? The IAU makes no concrete requirements. All "dwarf planets" and dwarf planet candidates can be considered to have cleared their hill sphere. On the other hand, no planet, not even Jupiter, "cleared their orbit" so there would actually be no planets at all. But it's not really the IAU's definition because about 4.2% of the IAU voted for the 2006 definition while other IAU astronomers were gone.

[Northwest Slobovia]

the International Astronomical Union (IAU) in 2006 set up a definition of 'planet' and decided that Pluto is no longer one. The reason for that was the discovery of another planet, the tenth planet Eris, called 'Xena' back then.

No. By then, Quaoar and Sedna had also been discovered. Wikipedia isn't a perfect source, but it's good for checking obvious stuff, like "Why did the IAU change the definition of 'planet'."

So...

The IAU could have easily set up a definition that would include Eris as the tenth planet, but anyway it wasn't necessary to create a definition for 'planet'. There is no definition for 'continent' either. They should just have accepted Eris as the tenth planet and name it accordingly.

No. They would have to have set up a definition that likely would make dozens to hundreds of "planets" in the solar system, since the estimates for the Kuiper Belt's mass suggest there probably are that many more round objects of roughly Pluto's size/mass... and as Dogmeat mentioned, asteroids that large as well.

And, um, pro tip: we're not going to discover any more big landmasses on Earth, so it really doesn't matter if we define "continent" formally. Once we start getting images of terrestrial exoplanets with oceans, it might start to matter.

I can't describe how wrong, vague and unscientific the 2006 definition is for that would take multiple pages.

Then you don't understand the problem. Please read the Wikipedia page until you do, then add a summary of what you think is wrong with the IAU definition to the OP.

[Terabithya]

Meanwhile Ceres is quietly depressed that no one ever even remembers when it got demoted in exactly the same way, and for the same reason.. Poor Ceres.

You can vote for number 5: "The 2006 definition is not good and we should accept all dwarf planets and most-likely-dwarf-planets as planets." It includes Ceres.

[Terabithya]

...add a summary of what you think is wrong with the IAU definition to the OP.

Quaoar and Sedna are not as big and massive as Pluto and Eris. You don't understand it (or don't want to). You could have checked WP for yourself. There probably won't be found bodies as large and massive as Pluto and Eris. But you're free to vote for another option.

[Northwest Slobovia]

Pluto is a dwarf planet because it hasn't cleared its orbit to the extent required by the IAU (and there are only three requirements). You could very easily look this stuff up and include some of it in the OP. The excuse that you "can't describe how wrong, vague and unscientific the 2006 definition is for that would take multiple pages" is just that, an excuse. But hey, you know better than professional astronomers and astrophysicists and telescope people, so ... sure. I'll stick with the IAU definition. Sorry, Pluto.

And how would you know if I am an astronomer or not?

Oh, that's easy. If you were, you'd be arguing with them, at their conferences, by email, and in all other ways scientists communicate with each other. Same way I argue with other biochemists about technical stuff.

How do you define "clearing the orbit"? The IAU makes no concrete requirements. All "dwarf planets" and dwarf planet candidates can be considered to have cleared their hill sphere. On the other hand, no planet, not even Jupiter, "cleared their orbit" so there would actually be no planets at all.

See? That didn't take "multiple pages", did it? Please add that to the OP.

[Northwest Slobovia]

...add a summary of what you think is wrong with the IAU definition to the OP.

Quaoar and Sedna are not as big and massive as Pluto and Eris. You don't understand it (or don't want to). You could have checked WP for yourself. There probably won't be found bodies as large and massive as Pluto and Eris. But you're free to vote for another option.

I think the IAU definition will do, thanks. And you got a citation for that claim?

[Earth Orbit]

If the IAU didn't set very strict requirements for the definition of planet, there'd be too many planets in the Solar System.

Under the current rules, bodies like Pluto, Sedna, Eris and Ceres do not meet all qualifications - their orbit isn't clear. We know there are likely thousands of bodies we can't see that meet the other two requirements - a Solar orbit and a mostly-spherical shape. (Size, mass, composition and gravity are not considered.) It's much easier to classify these bodies as dwarf planets or planetoids rather than considering them planets.

[Terabithya]

Quaoar and Sedna are not as big and massive as Pluto and Eris. You don't understand it (or don't want to). You could have checked WP for yourself. There probably won't be found bodies as large and massive as Pluto and Eris. But you're free to vote for another option.

I think the IAU definition will do, thanks. And you got a citation for that claim?

Read the sentence before. There you have one citation.

[Lord Dominator]

I don't get the big deal with Pluto being a dwarf planet. It isn't any less special now, we've just realized we need more specific standards for planetness (of course, this same thing happened with Ceres some time ago...)

You're correct that there's no universal definition of continent, but why mention that?

[Terabithya]

You're correct that there's no universal definition of continent, but why mention that?

Because there was no such definition of planet and it wasn't needed either.

[Estanglia]

I don't see why it matters in the first place. It's not like Pluto changes based on whether we call it a planet or a dwarf planet.

If the definitions were actually used for something important, sure. But nothing changes if Pluto's a planet or not other than the number of planets.

[Lord Dominator]

You're correct that there's no universal definition of continent, but why mention that?

Because there was no such definition of planet and it wasn't needed either.

Ah. Well in that case, I'll just note that there's no actual agreement on what a continent is (I can make arguments for anywhere from 3 to 7 of them, or more), and that's not an ideal situation when it comes to objects being actively studied (in our Solar System and elsewhere), particularly when there is potentially an actual lot of them.

[Catsfern]

If dwarf people are still people then dwarf planets are still planets, all im gonna say on this matter.

[Earth Orbit]

You're correct that there's no universal definition of continent, but why mention that?

Because there was no such definition of planet and it wasn't needed either.

Comparing the concept of a continent to the concept of a celestial body is like comparing apples to oranges. Also, we do have a generally accepted definition of continents - one of Earth's main continuous expanses of land, separated by water or tectonic faults.

The definition of a planet, meanwhile, is harder to pin down. The current consensus is that any spherical object in a clear orbit around the Sun is a planet; dwarf planets and planetoids have not cleared their orbits and therefore are not planets. And besides, calling Ceres or Pluto a dwarf planet changes nothing about its qualities - if it's useful, humans will explore and eventually colonize it regardless of its classification.

[Estanglia]

If dwarf people are still people then dwarf planets are still planets, all im gonna say on this matter.

How does this make sense?

[Alvecia]

Pluto is a dwarf planet because it hasn't cleared its orbit to the extent required by the IAU (and there are only three requirements). You could very easily look this stuff up and include some of it in the OP. The excuse that you "can't describe how wrong, vague and unscientific the 2006 definition is for that would take multiple pages" is just that, an excuse. But hey, you know better than professional astronomers and astrophysicists and telescope people, so ... sure. I'll stick with the IAU definition. Sorry, Pluto.

And how would you know if I am an astronomer or not? And what about yourself? How do you define "clearing the orbit"? The IAU makes no concrete requirements. All "dwarf planets" and dwarf planet candidates can be considered to have cleared their hill sphere. On the other hand, no planet, not even Jupiter, "cleared their orbit" so there would actually be no planets at all. But it's not really the IAU's definition because about 4.2% of the IAU voted for the 2006 definition while other IAU astronomers were gone.

Again, this is stuff that can be easily googled
See here: https://en.m.wikipedia.org/wiki/Clearin ... ghbourhood

In the end stages of planet formation, a planet (as so defined) will have "cleared the neighbourhood" of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence. A large body that meets the other criteria for a planet but has not cleared its neighbourhood is classified as a dwarf planet. This includes Pluto, which is constrained in its orbit by the gravity of Neptune and shares its orbital neighbourhood with many Kuiper belt objects. The IAU's definition does not attach specific numbers or equations to this term, but all the IAU-recognised planets have cleared their neighbourhoods to a much greater extent (by orders of magnitude) than any dwarf planet, or any candidate for dwarf planet.

[Terabithya]

In the end stages of planet formation, a planet (as so defined) will have "cleared the neighbourhood" of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence. A large body that meets the other criteria for a planet but has not cleared its neighbourhood is classified as a dwarf planet. This includes Pluto, which is constrained in its orbit by the gravity of Neptune and shares its orbital neighbourhood with many Kuiper belt objects. The IAU's definition does not attach specific numbers or equations to this term, but all the IAU-recognised planets have cleared their neighbourhoods to a much greater extent (by orders of magnitude) than any dwarf planet, or any candidate for dwarf planet.

The 2006 definition doesn't get any concrete. And what you described in your citation would have to ban Pluto, but not Eris, from the planets.

[Catsfern]

If dwarf people are still people then dwarf planets are still planets, all im gonna say on this matter.

How does this make sense?

It means if dwarf people are still considered people then dwarf planets are still planets.

[Estanglia]

How does this make sense?

It means if dwarf people are still considered people then dwarf planets are still planets.

Yes.

How does that make sense? How does 'dwarf people are people' mean 'dwarf planets are planets'?

[Catsfern]

It means if dwarf people are still considered people then dwarf planets are still planets.

Yes.

How does that make sense? How does 'dwarf people are people' mean 'dwarf planets are planets'?

Is the grammar incorrect? because as far as i can tell its perfectly clear English.

[Estanglia]

Yes.

How does that make sense? How does 'dwarf people are people' mean 'dwarf planets are planets'?

Is the grammar incorrect? because as far as i can tell its perfectly clear English.

Your grammar's correct.
Your logic? Not so much.

[Alvecia]

In the end stages of planet formation, a planet (as so defined) will have "cleared the neighbourhood" of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence. A large body that meets the other criteria for a planet but has not cleared its neighbourhood is classified as a dwarf planet. This includes Pluto, which is constrained in its orbit by the gravity of Neptune and shares its orbital neighbourhood with many Kuiper belt objects. The IAU's definition does not attach specific numbers or equations to this term, but all the IAU-recognised planets have cleared their neighbourhoods to a much greater extent (by orders of magnitude) than any dwarf planet, or any candidate for dwarf planet.

The 2006 definition doesn't get any concrete. And what you described in your citation would have to ban Pluto, but not Eris, from the planets.

No, Eris fails for the same reasons.