<p>You are correct that the IAU definition of "clearing the orbit" has the problem of being not explicitly quantified. And a complete clearing was obviously never the intention behind the definition. I like <a href="https://ui.adsabs.harvard.edu/abs/2008DPS....40.0701S/abstract" rel="noreferrer">this statement</a> by Steven Soter:</p>

<blockquote>
  <p>The IAU definition of a planet as a heliocentric body that "has cleared the neighborhood around its orbit” is problematic. Clearance is never complete because the asteroid and comet reservoirs are leaky, and resonant planet-crossing orbits can be stable. A more accurate criterion for planetary status is "dynamical dominance". An object is dynamically dominant if it sweeps up or scatters other objects from its orbital zone in a time much less than the age of the system (theoretical criterion), and/or if its mass is much greater than the total mass of all other objects in its orbital zone (observational criterion), where "orbital zone” can also be simply quantified.</p>
</blockquote>

<p>However, there really are substantial difference in between Eris/Pluto and the regular planets concerning their dominance in their respective orbits. The Wikipedia article on "<a href="https://en.wikipedia.org/wiki/Clearing_the_neighbourhood" rel="noreferrer">Clearing the neighbourhood</a>" has a neat overview over a few of the proposed ways to make this difference more quantifiable. </p>

<p>Probably the most straight forward definition is the fraction <span class="math-container">$\mu = M/m$</span> between the mass of the object in question <span class="math-container">$M$</span> and the combined mass of other objects in its orbits zone <span class="math-container">$m$</span>. But there are other options as well:</p>

<p><span class="math-container">$$
\begin{array} { r   |r   |r   |r   |r   |r   |r   |r   }
   \mathrm{ Name}  &amp; \mathrm{Margot's  \,\,} \Pi &amp; \mathrm{Soter's \,\,  } \mu  &amp; \mathrm{Stern–Levison\, parameter \,\, } \Lambda   &amp; \mathrm{Mass} (kg)   \\
\hline
    Jupiter    &amp;   4.0  \times  10^4        &amp;   6.25   \times 10^5        &amp;   1.30  \times  10^9        &amp;   1.8986   \times 10^{27}      \\ 
    Saturn      &amp;   6.1  \times 10^3        &amp;   1.9  \times  10^5       &amp;   4.68   \times 10^7        &amp;   5.6846   \times 10^{26}        \\
     Venus      &amp;   9.5  \times 10^2        &amp;   1.3  \times 10^6        &amp;   1.66   \times 10^5        &amp;   4.8685  \times 10^{24}         \\
       Earth      &amp;   8.1  \times 10^2        &amp;   1.7  \times 10^6        &amp;   1.53  \times 10^5        &amp;   5.9736  \times 10^{24}         \\
     Uranus      &amp;   4.2  \times 10^2        &amp;   2.9  \times 10^4        &amp;   3.84    \times 10^5        &amp;   8.6832  \times 10^{25}     \\
     Neptun       &amp;   3.0  \times 10^2        &amp;   2.4  \times 10^4    &amp;   2.73  
  \times 10^5        &amp;   1.0243  \times 10^{26}     \\
    Mercury      &amp;   1.3  \times 10^2        &amp;   9.1  \times 10^4        &amp;   1.95   \times 10^3        &amp;   3.3022  \times 10^{23}       \\
     Mars      &amp;   5.4  \times 10^1        &amp;   5.1  \times 10^3        &amp;   9.42   \times 10^2        &amp;   6.4185  \times 10^{23}         \\
     Ceres      &amp;   4.0  \times 10^{−2}        &amp;   0.33        &amp;   8.32  \times 10^{−4}        &amp;   9.43  \times 10^{20}         \\
     Pluto      &amp;   2.8  \times 10^{−2}        &amp;   0.08        &amp;   2.95  \times 10^{−3}        &amp;   1.29  \times 10^{22}             \\
     Eris      &amp;   2.0  \times 10^{−2}        &amp;   0.10        &amp;   2.15  \times 10^{−3}        &amp;   1.67  \times 10^{22}            \\  
     Haumea      &amp;   7.8  \times 10^{−3}        &amp;   0.02       &amp;   2.41  \times 10^{−4}        &amp;   4.0  \times 10^{21}             \\
     Makemake      &amp;   7.3  \times 10^{−3}        &amp;   0.02       &amp;   2.22  \times 10^{−4}        &amp;   4.0  \times 10^{21}        \\
\end{array}
$$</span></p>

<p>In <a href="https://arxiv.org/abs/astro-ph/0608359" rel="noreferrer">this paper</a> by Sober he also made some nice plots showing the categorical difference between the two types of objects depending on their M, m and their semi-major axis a: 
<a href="https://i.sstatic.net/AXOu0.png" rel="noreferrer"><img src="https://i.sstatic.net/AXOu0.png" alt=""></a>
<a href="https://i.sstatic.net/L7PTu.png" rel="noreferrer"><img src="https://i.sstatic.net/L7PTu.png" alt=""></a></p>

<p>So, you see, while the IAU definition of "clearing an orbit" really is ambiguous in its interpretation, following its intent, there are quantifiable ways to show the existing difference between planets and dwarf-planets. </p>

CC BY-SA 4.0
2020-02-25T13:36:27.907Z
But they have arbitrary definitions of "neighbourhood zone" that are obviously pretty strict. Most of its year, Eris is outside the Kuiper belt. The values you show however obviously take into account Eris' path only within the Kuiper belt. There needs to be a definition of "orbit zone". If one means the hill sphere then all three bodies (Ceres, Pluto and Eris) should be planets because their mass is greater than that of all other bodies together crossing their hillspheres.
user30007
CC BY-SA 4.0
2020-02-25T14:23:53.853Z
@user30007 if the clear plots and table cannot convince you that there is a distinct difference between current planets and current dwarf planets - then probably nothing can. It's a distinction in *naming* to make clear what type of object one talks about - which in turn implies different histories and effects being important. It's less one of a single property of the bodies involved.
planetmaker
CC BY-SA 4.0
2020-02-25T14:30:34.077Z
@planetmaker First the IAU should better find another term than "dwarf planet" because such term would mean it is actually a planet. As you say, one must make clear what type of object we're talking about, so it shouldn't depend on its orbital parameters anyway but solely on what the body itself is. I'm not opposed to a definition that would allow eight planets (and a hypothetical ninth one far beyond the Kuiper belt) but it must be a clear one. How do YOU consider the planets' neighbourhood zone? Is it 10 times its radius from it or perhaps 100 or 1000 times the radius of them?
user30007
CC BY-SA 4.0
2020-02-25T14:37:50.797Z
@user3007 -- The planetary discriminant described in the Wikipedia page is not arbitrarily defined but defined from first principles.
antlersoft
CC BY-SA 4.0
2020-02-25T14:52:41.147Z
@user30007 If you want to put it into a formula, I'd like to follow Soter's or Allan Stern's parameter - it clearly gives a distinction based on dynamical properties from first principles. As humorous as it may be, this might do his own position to keep Pluto as planet damage, but makes him a good scientist.
planetmaker
CC BY-SA 4.0
2020-02-25T15:18:29.263Z
@antlersoft So how are their orbital zones defined?
user30007
CC BY-SA 4.0
2020-02-25T15:20:51.910Z
@planetmaker Dr Stern considers both types of bodies planets and like me, he states that a planet shouldn't depend on its orbital parameters but solely on what it is. The definitions of orbital zones must be arbitrary but maybe antlersoft will tell us how they're defined.
user30007
CC BY-SA 4.0
2020-02-25T16:36:51.963Z
@user30007 You already start with a false assumption: Eris is not a dominant mass in its orbit. Its perihelion is inside Plutos oribit, thus there is at least one object in its orbiti of similar size. There are many more, if you consider its highly eccentric orbit between 38 and 97 AU. Inclination is of little importance when considering *whether* bodies interact on their dynamical timescale - it mostly defines how they scatter eachother, not the *if*.
planetmaker
CC BY-SA 4.0
2020-02-25T17:57:45.747Z
@planetmaker As I wrote, Eris intersects with Pluto from a vertical point of view, but this doesn't mean they come so close that they would attract themselves strongly or even collide with each other. You have to think 3-dimensionally. I dunno the minimum orbit intersection distance between them but it surely are millions of miles or more.
user30007
CC BY-SA 4.0
2020-02-25T23:09:58.360Z
If you don't know, how can you be sure?
planetmaker
CC BY-SA 4.0
2020-02-26T06:19:22.400Z
@planetmaker The Pluto-Eris MOID is surely as wide or wider than the Neptune-Pluto MOID. I ask antlersoft to explain the principles he is talking about.
user30007
CC BY-SA 4.0
2020-02-25T12:56:59.300Z
2020-02-25T14:46:36.827Z
2020-02-25T14:46:36.827Z
SpaceCore
<p>Because the "orbital neighbourhood" of Eris (and Ceres and Pluto) is defined as the entire Kuiper belt or in Ceres' case the entire Main belt in the planetary dominance discriminants user SpaceBread is showing. If the IAU defined the orbital dominance region as the body's hill sphere all three bodies would have to be classified as planets. Except if they took a strict approach that no body must even cross their hill sphere, in such case there wouldn't exist any planets at all. And they'd perhaps take the strict approach since, as you say, Eris actually isn't part of the Kuiper belt but just crosses it. Therefore, the IAU took the approach to call the entire belts their "orbital region" and from a 2-dimensional point of view rather than a 3-dimensional one (despite Pluto's and Eris' orbits being highly inclined to the ecliptic) in order to fit their definition of 2006.</p>

<p>According to the argument by some you provide, that another body dominates more than the three bodies, you're right that Neptune dominates Pluto's orbit but one might also conclude that Jupiter dominates Ceres' and farther on that Neptune dominates the entire Kuiper belt and bodies crossing it (like Eris). Still, Eris is the most massive known object beyond Neptune.      </p>

CC BY-SA 4.0
2020-05-19T07:31:37.360Z
2020-05-19T07:31:37.360Z
The Architect
<p>The 2006 definition of a planet states that a planet  is a celestial body that</p>

<p><em>(a) is in orbit around the Sun</em></p>

<p><em>(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape</em></p>

<p><em>(c) has cleared the neighbourhood around its orbit.</em></p>

<p>Point c is why Pluto allegedly is no longer a planet. However some "proponents" of the definition, namely those who use logic, figured out that according to this vague definition, if we take it literal and set a arbitrary border on "clearing the orbit", then either Pluto is still a planet or we have no planets at all. Therefore these guys actually set up an own definition, thereby replacing point c by the requirement that a planet must be the dominant mass in its orbit. This is not what the 2006 definition is telling, so these anti-Plutoers aren't actually proponents of the 2006 definition. They have their own definition according to which <em>really</em> only Pluto is no planet because Neptune dominates its orbit.</p>

<p>However their definition still doesn't exclude Eris from planethood. Its orbit is far from any of the eight recognized planets. From a vertical point of view, Eris' orbit intersects with that of Pluto. However Eris' orbit is highly tilted to the ecliptic and the two bodies don't really come close enough that their gravities would influence each other strongly. Eris is more massive than any Kuiper belt object (including Pluto). Eris itself is not really a KBO because it only intersects the Kuiper belt while most of its year being outside it. So there is no reason to ban Eris from planethood if point c states (according to some anti-Plutoers) that a planet must be the dominant mass throughout its orbit. So why don't these guys consider Eris a ninth planet?  </p>

CC BY-SA 4.0
2020-02-25T09:39:53.420Z
I don't know why two people have down voted without leaving a helpful comment or at least a clue. There have been some questions that seem to be pushing a viewpoint rather than asking for an objective answer, so it's possible that it's related to that.
uhoh
CC BY-SA 4.0
2020-02-25T11:31:13.517Z
[Pile-on down voting without a single comment explaining why or how to edit, is this how a site should work? How close was my guess?](https://astronomy.meta.stackexchange.com/q/575/7982)
uhoh
CC BY-SA 4.0
2020-02-25T15:02:54.507Z
-1 for beating a dead horse. There's a new category of solar system bodies. The official definition is weak. Arguing about it here is Not Useful.
Mike G
CC BY-SA 4.0
2020-02-25T15:17:58.233Z
@MikeG It is because it's weak we have to argue about it. This doesn't mean we should change it immediately. Perhaps we should wait until the hypothetical planet beyond the Kuiper belt is either discovered or disproved. That might happen soon.
user30007
CC BY-SA 4.0
2020-02-25T23:41:04.193Z
`-1` because this is an "argument" and not a proper Stack Exchange question.
uhoh
CC BY-SA 4.0
2020-02-26T06:16:56.547Z
@uhoh It is a proper question: why is Eris not considered a planet in their definition despite being the most dominant mass in its orbital path?
user30007
CC BY-SA 4.0
2024-03-25T05:15:10.240Z
Eris does look like a planet, but that's not the determining factor to consider it one according to the IAU. While it covers two of the three requirements, it does not check all of the boxes, giving it a status of a dwarf planet.
4NT4R3S
CC BY-SA 4.0
2024-03-22T15:20:35.377Z
I think that Eris should be considered a planet should be a planet because it fulfills all but one of the requirements. and I think that if objects in space fulfill at least two of the current requirements they should be considered a planet. Also does this not look like a planet? [![This looks like a planet, because of the fact that life forms could live on it and because it looks like It is about the size of Mars](https://i.sstatic.net/VsIBc.jpg)](https://i.sstatic.net/VsIBc.jpg)
Oliver Balgeman
CC BY-SA 4.0
2024-03-23T10:36:19.130Z
This does not provide an answer to the question. Once you have sufficient [reputation](https://astronomy.stackexchange.com/help/whats-reputation) you will be able to [comment on any post](https://astronomy.stackexchange.com/help/privileges/comment); instead, [provide answers that don't require clarification from the asker](https://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead). - [From Review](/review/late-answers/34898)
GrapefruitIsAwesome
<mass><orbital-elements><kuiper-belt><definition><trans-neptunian-object>
Why isn't Eris considered a planet despite being the body of dominant mass?
user30007
# Why isn't Eris considered a planet despite being the body of dominant mass?

(Asked by: username_0 on 2020-02-25)

The 2006 definition of a planet states that 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 shape*

*(c) has cleared the neighbourhood around its orbit.*

Point c is why Pluto allegedly is no longer a planet. However some "proponents" of the definition, namely those who use logic, figured out that according to this vague definition, if we take it literal and set a arbitrary border on "clearing the orbit", then either Pluto is still a planet or we have no planets at all. Therefore these guys actually set up an own definition, thereby replacing point c by the requirement that a planet must be the dominant mass in its orbit. This is not what the 2006 definition is telling, so these anti-Plutoers aren't actually proponents of the 2006 definition. They have their own definition according to which *really* only Pluto is no planet because Neptune dominates its orbit.

However their definition still doesn't exclude Eris from planethood. Its orbit is far from any of the eight recognized planets. From a vertical point of view, Eris' orbit intersects with that of Pluto. However Eris' orbit is highly tilted to the ecliptic and the two bodies don't really come close enough that their gravities would influence each other strongly. Eris is more massive than any Kuiper belt object (including Pluto). Eris itself is not really a KBO because it only intersects the Kuiper belt while most of its year being outside it. So there is no reason to ban Eris from planethood if point c states (according to some anti-Plutoers) that a planet must be the dominant mass throughout its orbit. So why don't these guys consider Eris a ninth planet?

**Tags:** mass, orbital-elements, kuiper-belt, definition, trans-neptunian-object

> **Comments:**
> **username_1**: I don't know why two people have down voted without leaving a helpful comment or at least a clue. There have been some questions that seem to be pushing a viewpoint rather than asking for an objective answer, so it's possible that it's related to that.  
> **username_1**: [Pile-on down voting without a single comment explaining why or how to edit, is this how a site should work? How close was my guess?](https://astronomy.meta.stackexchange.com/q/575/7982)  
> **username_2**: -1 for beating a dead horse. There's a new category of solar system bodies. The official definition is weak. Arguing about it here is Not Useful.  
> **username_0**: @MikeG It is because it's weak we have to argue about it. This doesn't mean we should change it immediately. Perhaps we should wait until the hypothetical planet beyond the Kuiper belt is either discovered or disproved. That might happen soon.  
> **username_1**: `-1` because this is an "argument" and not a proper Stack Exchange question.  
> **username_0**: @username_1 It is a proper question: why is Eris not considered a planet in their definition despite being the most dominant mass in its orbital path?  
> **username_4**: I think that Eris should be considered a planet should be a planet because it fulfills all but one of the requirements. and I think that if objects in space fulfill at least two of the current requirements they should be considered a planet. Also does this not look like a planet? [![This looks like a planet, because of the fact that life forms could live on it and because it looks like It is about the size of Mars](https://i.sstatic.net/VsIBc.jpg)](https://i.sstatic.net/VsIBc.jpg)  
> **username_5**: This does not provide an answer to the question. Once you have sufficient [reputation](https://astronomy.stackexchange.com/help/whats-reputation) you will be able to [comment on any post](https://astronomy.stackexchange.com/help/privileges/comment); instead, [provide answers that don't require clarification from the asker](https://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead). - [From Review](/review/late-answers/34898)  
> **username_3**: Eris does look like a planet, but that's not the determining factor to consider it one according to the IAU. While it covers two of the three requirements, it does not check all of the boxes, giving it a status of a dwarf planet.  

---

## 2 answers

### Answer by username_6 on 2020-02-25. Score: 10

You are correct that the IAU definition of "clearing the orbit" has the problem of being not explicitly quantified. And a complete clearing was obviously never the intention behind the definition. I like [this statement](https://ui.adsabs.harvard.edu/abs/2008DPS....40.0701S/abstract) by Steven Soter:

> The IAU definition of a planet as a heliocentric body that "has cleared the neighborhood around its orbit” is problematic. Clearance is never complete because the asteroid and comet reservoirs are leaky, and resonant planet-crossing orbits can be stable. A more accurate criterion for planetary status is "dynamical dominance". An object is dynamically dominant if it sweeps up or scatters other objects from its orbital zone in a time much less than the age of the system (theoretical criterion), and/or if its mass is much greater than the total mass of all other objects in its orbital zone (observational criterion), where "orbital zone” can also be simply quantified.

However, there really are substantial difference in between Eris/Pluto and the regular planets concerning their dominance in their respective orbits. The Wikipedia article on "[Clearing the neighbourhood](https://en.wikipedia.org/wiki/Clearing_the_neighbourhood)" has a neat overview over a few of the proposed ways to make this difference more quantifiable.

Probably the most straight forward definition is the fraction $\mu = M/m$ between the mass of the object in question $M$ and the combined mass of other objects in its orbits zone $m$. But there are other options as well:

$$
\begin{array} { r |r |r |r |r |r |r |r }
\mathrm{ Name} & \mathrm{Margot's \,\,} \Pi & \mathrm{Soter's \,\, } \mu & \mathrm{Stern–Levison\, parameter \,\, } \Lambda & \mathrm{Mass} (kg) \\
\hline
Jupiter & 4.0 \times 10^4 & 6.25 \times 10^5 & 1.30 \times 10^9 & 1.8986 \times 10^{27} \\
Saturn & 6.1 \times 10^3 & 1.9 \times 10^5 & 4.68 \times 10^7 & 5.6846 \times 10^{26} \\
Venus & 9.5 \times 10^2 & 1.3 \times 10^6 & 1.66 \times 10^5 & 4.8685 \times 10^{24} \\
Earth & 8.1 \times 10^2 & 1.7 \times 10^6 & 1.53 \times 10^5 & 5.9736 \times 10^{24} \\
Uranus & 4.2 \times 10^2 & 2.9 \times 10^4 & 3.84 \times 10^5 & 8.6832 \times 10^{25} \\
Neptun & 3.0 \times 10^2 & 2.4 \times 10^4 & 2.73
\times 10^5 & 1.0243 \times 10^{26} \\
Mercury & 1.3 \times 10^2 & 9.1 \times 10^4 & 1.95 \times 10^3 & 3.3022 \times 10^{23} \\
Mars & 5.4 \times 10^1 & 5.1 \times 10^3 & 9.42 \times 10^2 & 6.4185 \times 10^{23} \\
Ceres & 4.0 \times 10^{−2} & 0.33 & 8.32 \times 10^{−4} & 9.43 \times 10^{20} \\
Pluto & 2.8 \times 10^{−2} & 0.08 & 2.95 \times 10^{−3} & 1.29 \times 10^{22} \\
Eris & 2.0 \times 10^{−2} & 0.10 & 2.15 \times 10^{−3} & 1.67 \times 10^{22} \\
Haumea & 7.8 \times 10^{−3} & 0.02 & 2.41 \times 10^{−4} & 4.0 \times 10^{21} \\
Makemake & 7.3 \times 10^{−3} & 0.02 & 2.22 \times 10^{−4} & 4.0 \times 10^{21} \\
\end{array}
$$

In [this paper](https://arxiv.org/abs/astro-ph/0608359) by Sober he also made some nice plots showing the categorical difference between the two types of objects depending on their M, m and their semi-major axis a:
[![](https://i.sstatic.net/AXOu0.png)](https://i.sstatic.net/AXOu0.png)
[![](https://i.sstatic.net/L7PTu.png)](https://i.sstatic.net/L7PTu.png)

So, you see, while the IAU definition of "clearing an orbit" really is ambiguous in its interpretation, following its intent, there are quantifiable ways to show the existing difference between planets and dwarf-planets.

> **Comments:**
> **username_0**: But they have arbitrary definitions of "neighbourhood zone" that are obviously pretty strict. Most of its year, Eris is outside the Kuiper belt. The values you show however obviously take into account Eris' path only within the Kuiper belt. There needs to be a definition of "orbit zone". If one means the hill sphere then all three bodies (Ceres, Pluto and Eris) should be planets because their mass is greater than that of all other bodies together crossing their hillspheres.  
> **username_7**: @username_0 if the clear plots and table cannot convince you that there is a distinct difference between current planets and current dwarf planets - then probably nothing can. It's a distinction in *naming* to make clear what type of object one talks about - which in turn implies different histories and effects being important. It's less one of a single property of the bodies involved.  
> **username_0**: @username_7 First the IAU should better find another term than "dwarf planet" because such term would mean it is actually a planet. As you say, one must make clear what type of object we're talking about, so it shouldn't depend on its orbital parameters anyway but solely on what the body itself is. I'm not opposed to a definition that would allow eight planets (and a hypothetical ninth one far beyond the Kuiper belt) but it must be a clear one. How do YOU consider the planets' neighbourhood zone? Is it 10 times its radius from it or perhaps 100 or 1000 times the radius of them?  
> **username_8**: @user3007 -- The planetary discriminant described in the Wikipedia page is not arbitrarily defined but defined from first principles.  
> **username_7**: @username_0 If you want to put it into a formula, I'd like to follow Soter's or Allan Stern's parameter - it clearly gives a distinction based on dynamical properties from first principles. As humorous as it may be, this might do his own position to keep Pluto as planet damage, but makes him a good scientist.  
> **username_0**: @username_8 So how are their orbital zones defined?  
> **username_0**: @username_7 Dr Stern considers both types of bodies planets and like me, he states that a planet shouldn't depend on its orbital parameters but solely on what it is. The definitions of orbital zones must be arbitrary but maybe username_8 will tell us how they're defined.  
> **username_7**: @username_0 You already start with a false assumption: Eris is not a dominant mass in its orbit. Its perihelion is inside Plutos oribit, thus there is at least one object in its orbiti of similar size. There are many more, if you consider its highly eccentric orbit between 38 and 97 AU. Inclination is of little importance when considering *whether* bodies interact on their dynamical timescale - it mostly defines how they scatter eachother, not the *if*.  
> **username_0**: @username_7 As I wrote, Eris intersects with Pluto from a vertical point of view, but this doesn't mean they come so close that they would attract themselves strongly or even collide with each other. You have to think 3-dimensionally. I dunno the minimum orbit intersection distance between them but it surely are millions of miles or more.  
> **username_7**: If you don't know, how can you be sure?  
> **username_0**: @username_7 The Pluto-Eris MOID is surely as wide or wider than the Neptune-Pluto MOID. I ask username_8 to explain the principles he is talking about.  

---

### Answer by username_9 on 2020-05-19. Score: 0

Because the "orbital neighbourhood" of Eris (and Ceres and Pluto) is defined as the entire Kuiper belt or in Ceres' case the entire Main belt in the planetary dominance discriminants user SpaceBread is showing. If the IAU defined the orbital dominance region as the body's hill sphere all three bodies would have to be classified as planets. Except if they took a strict approach that no body must even cross their hill sphere, in such case there wouldn't exist any planets at all. And they'd perhaps take the strict approach since, as you say, Eris actually isn't part of the Kuiper belt but just crosses it. Therefore, the IAU took the approach to call the entire belts their "orbital region" and from a 2-dimensional point of view rather than a 3-dimensional one (despite Pluto's and Eris' orbits being highly inclined to the ecliptic) in order to fit their definition of 2006.

According to the argument by some you provide, that another body dominates more than the three bodies, you're right that Neptune dominates Pluto's orbit but one might also conclude that Jupiter dominates Ceres' and farther on that Neptune dominates the entire Kuiper belt and bodies crossing it (like Eris). Still, Eris is the most massive known object beyond Neptune.


---

