<p>According to the <a href="https://en.wikipedia.org/wiki/IAU_definition_of_planet" rel="noreferrer">2006 IAU definition of a planet</a>, a celestial body must meet all 3 criteria to be considered a planet.  The Moon fails the first criteria "1. Is in orbit around the Sun".  The Moon orbits Earth, not the Sun, and thus it doesn't have a legitimate "neighbourhood" (solar orbit) to clear in that context.</p>

<p>Also to note, according to the <a href="https://en.wikipedia.org/wiki/Clearing_the_neighbourhood#Disagreement" rel="noreferrer">Clearing the neighbourhood</a> article, this criteria is contentious as it is vaguely written and Earth, Mars, Jupiter and Neptune have not "cleared the neighbourhood" of their respective solar orbits.</p>

CC BY-SA 4.0
2018-11-22T03:04:08.877Z
You think this problem as too simple one. First of all, this question can be related to the idea that the "Clearing the neighbourhood" condition is inadequate and ambiguous for planet definition. More deeply, this question can be related to the idea that 2006 IAU definition of planet is logically ambiguous and clumsy. 2006 IAU definition of planet begins with Moon is already set as a satellite, not a planet. If you can set a celestial body whether planet or not a planet, why other conditions are needed? All you need is just a list of planetary bodies, not other conditions.
pdh0710
CC BY-SA 4.0
2018-11-22T03:42:40.697Z
Perhaps I misunderstood your question.  It just seems to me that your pretense to isolate the "clearing the neighbourhood" criteria from the others is to ignore the reality of the moon.  You seem to be asking if the moon has the mass to "clear the neighbourhood" as if it was orbiting the sun, perhaps in a hypothetical sense?  Is that correct?
Blaelph
CC BY-SA 4.0
2018-11-22T03:57:36.733Z
Yes, it's similar. And I'd also considered the idea that Earth-Moon system is a double planet system. Though I do not agree with the idea, I think the idea is worth considering. If Earth-Moon system is a double planet system, Earth is orbiting around the center of mass of Earth-Moon. Then the center of mass of Earth-Moon is orbiting around Sun.
pdh0710
CC BY-SA 4.0
2018-11-22T08:37:20.537Z
Downvoted due to the second paragraph. "Clearing the neighborhood" does not mean capturing or ejecting every last spec of matter. The question itself alludes to this, using two of the three contending mathematical formulations for what qualifies a body as having the potential to have cleared its neighborhood.
David Hammen
CC BY-SA 4.0
2018-11-23T17:31:50.510Z
So, if the earth-like body would orbit Sirius according to the first criteria, it would not be considered a planet ?
user3715778
CC BY-SA 4.0
2018-11-24T13:05:54.187Z
@pdh0710 Seem to remember coming across definition of twin planetary system involved centre of mass of system had to lie outside the diameter of either body - Earth/moon mass centre is about 4.6 Km from Earth's centre.
Jiminy Cricket.
CC BY-SA 4.0
2018-11-24T21:15:25.540Z
@Duckisaduckisaduck : Although the position of barycenter is often said to be a criterion for satellite, it can not be an appropriate criterion. Currently Earth-Moon barycenter lies inside Earth. However, as Moon is drifting away from Earth, eventually it will lie outside Earth. Another notable case is that Sun-Jupiter barycenter lies outside Sun. And though Pluto-Charon barycenter lies outside Pluto, Charon is a satellite of Pluto.
pdh0710
CC BY-SA 4.0
2018-12-06T01:51:44.037Z
@user3715778 - I was quoting the article verbatim.  I would assume their usage of "Sun" indicates the star of any solar system, not specifically ours.
Blaelph
CC BY-SA 4.0
2018-12-06T01:58:31.890Z
@David Hammen - I think the point of the 3rd criteria was to exclude objects like Ceres from the definition of a planet.  Ceres does meet the first 2 criteria: orbits the sun (1) and has sufficient mass to assume hydrostatic equilibrium (2).  And obviously it has a quite cluttered "neighborhood" being in the asteroid belt.
Blaelph
CC BY-SA 4.0
2018-11-22T02:14:32.827Z
2018-11-22T02:14:32.827Z
Blaelph
<p>What you have shown is that the Moon is large enough so that it would be a planet by the IAU definition if the Earth were not present. But, because the Moon does orbit the Earth, it is not a planet by the IAU definition. If the "clearing the neighborhood" criterion were sufficient by itself to define a planet, then the other criteria would not be part of the definition.</p>

CC BY-SA 4.0
2018-11-23T01:54:43.083Z
What I have shown is that Moon satisfies the qualification of the planet, according to the "Clearing the neighbourhood" conditions,
Of course, satisfying qualification can not mean that Moon is a planet.
However, the condition causes a contradictory situation, as I mentioned : Moon is classified as a satellite, not a planet, without rules and conditions. But Moon satisfies the qualification of the planet.......
2006 IAU definition of planet induces this contradictory situation, and instigates the old debate whether Earth-Moon system is a double planet system, instead settles the old debate.
pdh0710
CC BY-SA 4.0
2018-11-23T02:12:53.970Z
Additionally... Moon orbits around Earth? In  Pluto–Charon system, it is not clear whether Pluto and/or Charon orbit around Sun, or orbit around their barycenter, or Charon orbits around Pluto.
In Earth-Moon system, the situation is not much different.
pdh0710
CC BY-SA 4.0
2018-11-23T16:42:04.917Z
This is a good example showing what a disaster the 2006 definition was and the [process leading to it](http://news.bbc.co.uk/2/hi/science/nature/5283956.stm) was every bit as ugly as any gerrymandered decision. And is the Moon really in orbit about the Earth?  The gravitational force between the Sun-Moon is greater than the force between the Earth-Moon, so it is very easy to argue that the Moon is in orbit about the Sun with the Earth providing a periodic perturbation to the orbit.
Dave
CC BY-SA 4.0
2018-11-23T01:02:27.663Z
2018-11-23T01:02:27.663Z
Mark Foskey
<p>(Please excuse my English)</p>

<p>Above question arose during my investigation on the double planet/dwarf planet system. As we know, the definition of satellite and the definition of double planet/dwarf planet system are closely related. And currently there is no clear definition of the satellite, and so is the double planet/dwarf planet system.<br />
I thought a simple and obvious criterion is better. However, the simple size criterion is not sufficient for complex combinations of celestial bodies. In other hand, the position of barycenter is often said to be a criterion for satellite, but it can not be an appropriate criterion as described <a href="https://astronomy.stackexchange.com/questions/28484/moon-is-a-planet-by-clearing-the-neighbourhood-condition?noredirect=1#comment50692_28487">here</a>.<br />
What I thought as a criterion is : If two celestial bodies bound by gravity are classified as same class, we should classify the two bodies as a double body system. For example, if two stars are bound by gravity, we classify it as double star system. And if two black holes are bound, we classify it as a double black hole system.<br />
So if two planet/dwarf planet bodies are bound, it is natural to classify it as a double planet/dwarf planet system.</p>

<p>For test, I applied the criterion to Pluto-Charon system. Pluto is classified as a dwarf planet by 2006 IAU definition. If we consider Charon as a seperated body, Charon also satisfies the conditions for dwarf planet. So we can classify Pluto-Charon system as a double dwarf planet system.</p>

<p>Next, I applied the criterion to Earth-Moon system. As I mentioned in above question, I do not agree that Moon is a planet and Earth-Moon system is a double planet system. I thought Earth and Moon should be classified to different classes, because there are too many differences between two bodies.<br />
However, Moon as a seperated body satisfies IAU planet conditions(especially <em>Clearing the neighborhood</em> condition) as I described in above question. I was embarrassed. So I asked my calculation is correct.</p>

<p>I have thought for a long time that there are many problems with the <em>Clearing the neighborhood</em> condition for the planet definition. Above situation increased my distrust of the <em>Clearing the neighborhood</em> condition. Thus, as I replied the comments and answers, I became over-expressing my opinion on the condition.</p>

CC BY-SA 4.0
2018-11-25T19:14:42.020Z
I forgot the fact that the two definitions do not necessarily have to be mutually exclusive. 
Here, the two definitions are the definition of satellite and the definition of double planet/dwarf planet system.
So, in a double planet system, the smaller planet can be a satellite.
Whether the two definitions are mutually exclusive or not seems to be a matter of preference.
pdh0710
CC BY-SA 4.0
2018-11-25T19:52:42.360Z
As we can see in the comments and answers here, 2006 IAU definition of planet has two serious problems .......
1) The definition of satellite is missing.
2) The definition of double planet/dwarf planet system is impossible
.......
To prevent misunderstandings, I agree that the 2006 IAU definition re-classified Pluto as a non-planet body.
Although many people are sorry about it, the re-classification of Pluto is necessary and inevitable.
However, the way of re-classification does not look good.
pdh0710
CC BY-SA 4.0
2018-11-25T01:39:41.700Z
2018-11-25T09:47:40.450Z
2018-11-25T09:47:40.450Z
pdh0710
<blockquote>
<p>At present, the <a href="http://en.wikipedia.org/wiki/Moon" rel="nofollow noreferrer">Moon</a> is defined as a <a href="http://en.wikipedia.org/wiki/Natural_satellite" rel="nofollow noreferrer">satellite</a> of the <a href="http://en.wikipedia.org/wiki/Earth" rel="nofollow noreferrer">Earth</a>, but over time the Earth-Moon <a href="http://en.wikipedia.org/wiki/Barycenter" rel="nofollow noreferrer">barycenter</a> will drift outwards (see <a href="http://en.wikipedia.org/wiki/Tidal_acceleration" rel="nofollow noreferrer">tidal acceleration</a>) and could eventually become situated outside of both bodies.<sup><strong>25</strong></sup> This development would then upgrade the Moon to planetary status at that time, according to the definition. The time taken for this to occur, however, would be billions of years, long after many <a href="http://en.wikipedia.org/wiki/Astronomy" rel="nofollow noreferrer">astronomers</a> expect the <a href="http://en.wikipedia.org/wiki/Sun" rel="nofollow noreferrer">Sun</a> to expand into a <a href="http://en.wikipedia.org/wiki/Red_giant" rel="nofollow noreferrer">red giant</a> and destroy both Earth and Moon.<sup><strong>26</strong></sup></p>
<p><sup><strong>25</strong></sup> <sub>Robert Roy Britt (2006). <em><a href="http://web.archive.org/web/20100822072302/http://www.space.com/scienceastronomy/moon_mechanics_0303018.html" rel="nofollow noreferrer">Moon Mechanics: What Really Makes Our World Go 'Round'</a></em></sub></p>
<p><sup><strong>26</strong></sup> <sub>Robert Roy Britt (2006). <em><a href="http://edition.cnn.com/2006/TECH/space/08/18/moon.planet" rel="nofollow noreferrer">Earth's moon could become a planet</a></em></sub></p>
</blockquote>
<p><span class="math-container">$\qquad\qquad\qquad\qquad\qquad\qquad\qquad$</span> — <em><a href="http://en.wikipedia.org/wiki/IAU_definition_of_planet#Criticism" rel="nofollow noreferrer"><strong>Wikipedia :</strong> Criticism of the IAU definition of 'planet'</a></em></p>

CC BY-SA 4.0
2018-11-26T21:22:23.523Z
Yes, this is already mentioned in the comments and answers here.
pdh0710
CC BY-SA 4.0
2018-11-26T18:44:11.590Z
2018-11-26T18:44:11.590Z
2020-06-17T09:47:40.653Z
Lucian
<p>Whether the Moon (Earth's natural satellite) is a planet is an old debate in planetary science.<br />
Though I do not agree that Moon is a planet, I found something difficult to clearly understand.</p>

<p>According to the <a href="https://en.wikipedia.org/wiki/IAU_definition_of_planet" rel="nofollow noreferrer">2006 IAU definition of planet</a>, a planet must satisfy <em>Clearing the neighbourhood</em> condition.
And some criteria for the condition are introduced in the Wikipedia article about <a href="https://en.wikipedia.org/wiki/Clearing_the_neighbourhood" rel="nofollow noreferrer">Clearing the neighbourhood</a>.</p>

<ul>
<li>Stern–Levison's <span class="math-container">$\Lambda$</span></li>
</ul>

<p><span class="math-container">$$\Lambda = \frac {m^2}{a^{\frac 3 2}} k$$</span></p>

<ul>
<li>Margot's <span class="math-container">$\Pi$</span></li>
</ul>

<p><span class="math-container">$$\Pi=\frac {m}{m^{\frac 5 2}a^{\frac 9 8}} k $$</span></p>

<p>In the above equations, <span class="math-container">$m$</span> is the mass of the body, <span class="math-container">$a$</span> is the body's semi-major axis, and other factors are described in the Wikipedia article. Both criteria are said to be satisfied if the calculated value is greater than 1.</p>

<p>My calculations are simple. Let <span class="math-container">$m_{Moon} = 0.0123 \times m_{Earth}$</span> and Moon's other factors (such as <span class="math-container">$a$</span>, <span class="math-container">$k$</span> and <span class="math-container">$M$</span>) have the same values as Earth's. Then I can get...</p>

<p><span class="math-container">$$\Lambda_{Moon} = 0.0123^2 \times \Lambda_{Earth} = 23.147$$</span></p>

<p><span class="math-container">$$\Pi_{Moon} = 0.0123 \times \Pi_{Earth} = 9.963$$</span></p>

<p>Both <span class="math-container">$\Lambda_{Moon}$</span> and <span class="math-container">$\Pi_{Moon}$</span> thus satisfy the <em>Clearing the neighbourhood</em> condition. Though I do not calculate the Soter's <span class="math-container">$\mu$</span>, it will not change my results.</p>

<p>What is wrong with my calculations and results? Do I miss something about the condition and the criteria? Or is there something wrong with the <em>Clearing the neighbourhood</em> condition?</p>

<hr>

<p><strong>Edit</strong> in response to comments:</p>

<p>This question can be related to the idea that the <em>Clearing the neighborhood</em> condition is inadequate and ambiguous for the planet definition.
More deeply, this question can be related to the idea that 2006 IAU definition of planet is logically ambiguous and clumsy.<br />
For example, the 2006 definition of planet defines the Moon as a satellite and not a planet.
If one celestial body can be defined as a planet or not without rules and conditions, why other conditions are needed? All we need is just a planet list, not other planet conditions.</p>

<p>And...</p>

<p>Frankly speaking, the <em>Clearing the neighborhood</em> condition is like comedy. 
The condition says "Planet must sweep out other bodies" ...
Then says "Except satellites" ...
Then again says "Except bodies having no significant size" ...
Then again "Except bodies having resonant orbit" ...
Then again "It may take billions of years" ...
Then again "Sweep-out process and time are influenced by other planets and bodies, actually" .......</p>

<p>It is obvious that sometimes the <em>Clearing the neighborhood</em> theory is useful. But it is inappropriate for the rule and definition requiring strictness.</p>

CC BY-SA 4.0
2018-11-23T10:28:19.543Z
The 2006 IAU definition was designed to deal with non-satellites, such as Pluto and other large Kuiper Belt objects (and Ceres as well). Complaining that the "clearing the neighborhood" condition is superfluous for the Moon is missing the point.
Peter Erwin
CC BY-SA 4.0
2018-11-23T11:19:34.010Z
@PeterErwin : Your point is already mentioned and commented many times by others. And I also know the meaning of 2006 IAU definition. Do you think astronomers who claim that Moon is a planet are missing the point?
pdh0710
CC BY-SA 4.0
2018-11-23T13:21:32.137Z
@PeterErwin : I added sentences begin with "Frankly speaking, the Clearing the neighbourhood condition is like comedy" in the above question's `Edit` section.
pdh0710
CC BY-SA 4.0
2018-11-24T15:32:40.853Z
I'm voting to close this question as off-topic because this appears not to be a bona fide question but an invite to discuss the IAU definition of a planet.
James K
CC BY-SA 4.0
2018-11-24T21:26:44.527Z
@JamesK : I agree to close this question. At first this was just a question. However, as I replied the comments, I became over-expressing my opinion on the Clearing the neighbourhood condition. How can I close this question and join to the discussion on IAU definition of a planet?
pdh0710
CC BY-SA 4.0
2018-11-25T02:48:25.527Z
@pdh0710 welcome to Stack Exchange! This site is a bit different than typical Q&A sites. The structure is fairly rigid and designed to discourage extensive discussion of anything except how to clarify the question and improve the answer(s). Both [What types of questions should I avoid asking?](https://astronomy.stackexchange.com/help/dont-ask) and [How do I ask a good question?](https://astronomy.stackexchange.com/help/how-to-ask) explain this. If I were you, I'd delete the block-quoted EDIT section because that's just not how Stack Exchange questions should be used.
uhoh
CC BY-SA 4.0
2018-11-25T03:42:13.657Z
@uhoh : Thank you for your advice. I also think the `Edit` section has some problems. But the original purpose of the `Edit` section is to prevent repeated comments and queries. As you can see, this question has been involved with more complex and broader issues than I expected. So I hesitate to delete the section.
pdh0710
CC BY-SA 4.0
2018-11-25T04:15:16.877Z
@uhoh : I edited the paragraph. My intention for the paragraph is "Why it is like comedy? Because.......". Would you please let me know whether my intention is expressed well? (I'm not good at English)
pdh0710
CC BY-SA 4.0
2018-11-25T05:31:38.487Z
@pdh0710 I get a better feeling for what you would like to say now. I've adjusted the wording and formatting. You can undo it if you like by clicking `edited` then look for  `roll back` in the previous edit. Or you can edit further. The `>` block quote should only be used when quoting other people, it's not meant a highlight to your own comments unless you are quoting yourself from a previous publication. Still, Stack Exchange questions are not to be used to forward your own thoughts and opinions, nor to ask for thoughts and opinions of others.
uhoh
CC BY-SA 4.0
2018-11-25T05:49:30.523Z
@uhoh : Oh, thank you. It looks very good and what I wanted to express are contained well. The only problem is... It looks too nice. It does not look like a comedy at all.
pdh0710
<planet><the-moon>
Moon is a planet by "Clearing the neighborhood" condition?
pdh0710
# Moon is a planet by "Clearing the neighborhood" condition?

(Asked by: username_0 on 2018-11-22)

Whether the Moon (Earth's natural satellite) is a planet is an old debate in planetary science.  
Though I do not agree that Moon is a planet, I found something difficult to clearly understand.

According to the [2006 IAU definition of planet](https://en.wikipedia.org/wiki/IAU_definition_of_planet), a planet must satisfy *Clearing the neighbourhood* condition.
And some criteria for the condition are introduced in the Wikipedia article about [Clearing the neighbourhood](https://en.wikipedia.org/wiki/Clearing_the_neighbourhood).

* Stern–Levison's $\Lambda$

$$\Lambda = \frac {m^2}{a^{\frac 3 2}} k$$

* Margot's $\Pi$

$$\Pi=\frac {m}{m^{\frac 5 2}a^{\frac 9 8}} k $$

In the above equations, $m$ is the mass of the body, $a$ is the body's semi-major axis, and other factors are described in the Wikipedia article. Both criteria are said to be satisfied if the calculated value is greater than 1.

My calculations are simple. Let $m\_{Moon} = 0.0123 \times m\_{Earth}$ and Moon's other factors (such as $a$, $k$ and $M$) have the same values as Earth's. Then I can get...

$$\Lambda\_{Moon} = 0.0123^2 \times \Lambda\_{Earth} = 23.147$$

$$\Pi\_{Moon} = 0.0123 \times \Pi\_{Earth} = 9.963$$

Both $\Lambda\_{Moon}$ and $\Pi\_{Moon}$ thus satisfy the *Clearing the neighbourhood* condition. Though I do not calculate the Soter's $\mu$, it will not change my results.

What is wrong with my calculations and results? Do I miss something about the condition and the criteria? Or is there something wrong with the *Clearing the neighbourhood* condition?

---

**Edit** in response to comments:

This question can be related to the idea that the *Clearing the neighborhood* condition is inadequate and ambiguous for the planet definition.
More deeply, this question can be related to the idea that 2006 IAU definition of planet is logically ambiguous and clumsy.  
For example, the 2006 definition of planet defines the Moon as a satellite and not a planet.
If one celestial body can be defined as a planet or not without rules and conditions, why other conditions are needed? All we need is just a planet list, not other planet conditions.

And...

Frankly speaking, the *Clearing the neighborhood* condition is like comedy.
The condition says "Planet must sweep out other bodies" ...
Then says "Except satellites" ...
Then again says "Except bodies having no significant size" ...
Then again "Except bodies having resonant orbit" ...
Then again "It may take billions of years" ...
Then again "Sweep-out process and time are influenced by other planets and bodies, actually" .......

It is obvious that sometimes the *Clearing the neighborhood* theory is useful. But it is inappropriate for the rule and definition requiring strictness.

**Tags:** planet, the-moon

> **Comments:**
> **username_1**: The 2006 IAU definition was designed to deal with non-satellites, such as Pluto and other large Kuiper Belt objects (and Ceres as well). Complaining that the "clearing the neighborhood" condition is superfluous for the Moon is missing the point.  
> **username_0**: @PeterErwin : Your point is already mentioned and commented many times by others. And I also know the meaning of 2006 IAU definition. Do you think astronomers who claim that Moon is a planet are missing the point?  
> **username_0**: @PeterErwin : I added sentences begin with "Frankly speaking, the Clearing the neighbourhood condition is like comedy" in the above question's `Edit` section.  
> **username_2**: I'm voting to close this question as off-topic because this appears not to be a bona fide question but an invite to discuss the IAU definition of a planet.  
> **username_0**: @JamesK : I agree to close this question. At first this was just a question. However, as I replied the comments, I became over-expressing my opinion on the Clearing the neighbourhood condition. How can I close this question and join to the discussion on IAU definition of a planet?  
> **username_3**: @username_0 welcome to Stack Exchange! This site is a bit different than typical Q&A sites. The structure is fairly rigid and designed to discourage extensive discussion of anything except how to clarify the question and improve the answer(s). Both [What types of questions should I avoid asking?](https://astronomy.stackexchange.com/help/dont-ask) and [How do I ask a good question?](https://astronomy.stackexchange.com/help/how-to-ask) explain this. If I were you, I'd delete the block-quoted EDIT section because that's just not how Stack Exchange questions should be used.  
> **username_0**: @username_3 : Thank you for your advice. I also think the `Edit` section has some problems. But the original purpose of the `Edit` section is to prevent repeated comments and queries. As you can see, this question has been involved with more complex and broader issues than I expected. So I hesitate to delete the section.  
> **username_0**: @username_3 : I edited the paragraph. My intention for the paragraph is "Why it is like comedy? Because.......". Would you please let me know whether my intention is expressed well? (I'm not good at English)  
> **username_3**: @username_0 I get a better feeling for what you would like to say now. I've adjusted the wording and formatting. You can undo it if you like by clicking `edited` then look for  `roll back` in the previous edit. Or you can edit further. The `>` block quote should only be used when quoting other people, it's not meant a highlight to your own comments unless you are quoting yourself from a previous publication. Still, Stack Exchange questions are not to be used to forward your own thoughts and opinions, nor to ask for thoughts and opinions of others.  
> **username_0**: @username_3 : Oh, thank you. It looks very good and what I wanted to express are contained well. The only problem is... It looks too nice. It does not look like a comedy at all.  

---

## 4 answers

### Answer by username_4 on 2018-11-22. Score: 5

According to the [2006 IAU definition of a planet](https://en.wikipedia.org/wiki/IAU_definition_of_planet), a celestial body must meet all 3 criteria to be considered a planet. The Moon fails the first criteria "1. Is in orbit around the Sun". The Moon orbits Earth, not the Sun, and thus it doesn't have a legitimate "neighbourhood" (solar orbit) to clear in that context.

Also to note, according to the [Clearing the neighbourhood](https://en.wikipedia.org/wiki/Clearing_the_neighbourhood#Disagreement) article, this criteria is contentious as it is vaguely written and Earth, Mars, Jupiter and Neptune have not "cleared the neighbourhood" of their respective solar orbits.

> **Comments:**
> **username_0**: You think this problem as too simple one. First of all, this question can be related to the idea that the "Clearing the neighbourhood" condition is inadequate and ambiguous for planet definition. More deeply, this question can be related to the idea that 2006 IAU definition of planet is logically ambiguous and clumsy. 2006 IAU definition of planet begins with Moon is already set as a satellite, not a planet. If you can set a celestial body whether planet or not a planet, why other conditions are needed? All you need is just a list of planetary bodies, not other conditions.  
> **username_4**: Perhaps I misunderstood your question.  It just seems to me that your pretense to isolate the "clearing the neighbourhood" criteria from the others is to ignore the reality of the moon.  You seem to be asking if the moon has the mass to "clear the neighbourhood" as if it was orbiting the sun, perhaps in a hypothetical sense?  Is that correct?  
> **username_0**: Yes, it's similar. And I'd also considered the idea that Earth-Moon system is a double planet system. Though I do not agree with the idea, I think the idea is worth considering. If Earth-Moon system is a double planet system, Earth is orbiting around the center of mass of Earth-Moon. Then the center of mass of Earth-Moon is orbiting around Sun.  
> **username_5**: Downvoted due to the second paragraph. "Clearing the neighborhood" does not mean capturing or ejecting every last spec of matter. The question itself alludes to this, using two of the three contending mathematical formulations for what qualifies a body as having the potential to have cleared its neighborhood.  
> **username_6**: So, if the earth-like body would orbit Sirius according to the first criteria, it would not be considered a planet ?  
> **username_7**: @username_0 Seem to remember coming across definition of twin planetary system involved centre of mass of system had to lie outside the diameter of either body - Earth/moon mass centre is about 4.6 Km from Earth's centre.  
> **username_0**: @Duckisaduckisaduck : Although the position of barycenter is often said to be a criterion for satellite, it can not be an appropriate criterion. Currently Earth-Moon barycenter lies inside Earth. However, as Moon is drifting away from Earth, eventually it will lie outside Earth. Another notable case is that Sun-Jupiter barycenter lies outside Sun. And though Pluto-Charon barycenter lies outside Pluto, Charon is a satellite of Pluto.  
> **username_4**: @username_6 - I was quoting the article verbatim.  I would assume their usage of "Sun" indicates the star of any solar system, not specifically ours.  
> **username_4**: @username_5 - I think the point of the 3rd criteria was to exclude objects like Ceres from the definition of a planet.  Ceres does meet the first 2 criteria: orbits the sun (1) and has sufficient mass to assume hydrostatic equilibrium (2).  And obviously it has a quite cluttered "neighborhood" being in the asteroid belt.  

---

### Answer by username_8 on 2018-11-23. Score: 3

What you have shown is that the Moon is large enough so that it would be a planet by the IAU definition if the Earth were not present. But, because the Moon does orbit the Earth, it is not a planet by the IAU definition. If the "clearing the neighborhood" criterion were sufficient by itself to define a planet, then the other criteria would not be part of the definition.

> **Comments:**
> **username_0**: What I have shown is that Moon satisfies the qualification of the planet, according to the "Clearing the neighbourhood" conditions, Of course, satisfying qualification can not mean that Moon is a planet. However, the condition causes a contradictory situation, as I mentioned : Moon is classified as a satellite, not a planet, without rules and conditions. But Moon satisfies the qualification of the planet....... 2006 IAU definition of planet induces this contradictory situation, and instigates the old debate whether Earth-Moon system is a double planet system, instead settles the old debate.  
> **username_0**: Additionally... Moon orbits around Earth? In  Pluto–Charon system, it is not clear whether Pluto and/or Charon orbit around Sun, or orbit around their barycenter, or Charon orbits around Pluto. In Earth-Moon system, the situation is not much different.  
> **username_9**: This is a good example showing what a disaster the 2006 definition was and the [process leading to it](http://news.bbc.co.uk/2/hi/science/nature/5283956.stm) was every bit as ugly as any gerrymandered decision. And is the Moon really in orbit about the Earth?  The gravitational force between the Sun-Moon is greater than the force between the Earth-Moon, so it is very easy to argue that the Moon is in orbit about the Sun with the Earth providing a periodic perturbation to the orbit.  

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### Answer by username_0 on 2018-11-25. Score: 1

(Please excuse my English)

Above question arose during my investigation on the double planet/dwarf planet system. As we know, the definition of satellite and the definition of double planet/dwarf planet system are closely related. And currently there is no clear definition of the satellite, and so is the double planet/dwarf planet system.  
I thought a simple and obvious criterion is better. However, the simple size criterion is not sufficient for complex combinations of celestial bodies. In other hand, the position of barycenter is often said to be a criterion for satellite, but it can not be an appropriate criterion as described [here](https://astronomy.stackexchange.com/questions/28484/moon-is-a-planet-by-clearing-the-neighbourhood-condition?noredirect=1#comment50692_28487).  
What I thought as a criterion is : If two celestial bodies bound by gravity are classified as same class, we should classify the two bodies as a double body system. For example, if two stars are bound by gravity, we classify it as double star system. And if two black holes are bound, we classify it as a double black hole system.  
So if two planet/dwarf planet bodies are bound, it is natural to classify it as a double planet/dwarf planet system.

For test, I applied the criterion to Pluto-Charon system. Pluto is classified as a dwarf planet by 2006 IAU definition. If we consider Charon as a seperated body, Charon also satisfies the conditions for dwarf planet. So we can classify Pluto-Charon system as a double dwarf planet system.

Next, I applied the criterion to Earth-Moon system. As I mentioned in above question, I do not agree that Moon is a planet and Earth-Moon system is a double planet system. I thought Earth and Moon should be classified to different classes, because there are too many differences between two bodies.  
However, Moon as a seperated body satisfies IAU planet conditions(especially *Clearing the neighborhood* condition) as I described in above question. I was embarrassed. So I asked my calculation is correct.

I have thought for a long time that there are many problems with the *Clearing the neighborhood* condition for the planet definition. Above situation increased my distrust of the *Clearing the neighborhood* condition. Thus, as I replied the comments and answers, I became over-expressing my opinion on the condition.

> **Comments:**
> **username_0**: I forgot the fact that the two definitions do not necessarily have to be mutually exclusive.  Here, the two definitions are the definition of satellite and the definition of double planet/dwarf planet system. So, in a double planet system, the smaller planet can be a satellite. Whether the two definitions are mutually exclusive or not seems to be a matter of preference.  
> **username_0**: As we can see in the comments and answers here, 2006 IAU definition of planet has two serious problems ....... 1) The definition of satellite is missing. 2) The definition of double planet/dwarf planet system is impossible ....... To prevent misunderstandings, I agree that the 2006 IAU definition re-classified Pluto as a non-planet body. Although many people are sorry about it, the re-classification of Pluto is necessary and inevitable. However, the way of re-classification does not look good.  

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### Answer by username_10 on 2018-11-26. Score: 0

> At present, the [Moon](http://en.wikipedia.org/wiki/Moon) is defined as a [satellite](http://en.wikipedia.org/wiki/Natural_satellite) of the [Earth](http://en.wikipedia.org/wiki/Earth), but over time the Earth-Moon [barycenter](http://en.wikipedia.org/wiki/Barycenter) will drift outwards (see [tidal acceleration](http://en.wikipedia.org/wiki/Tidal_acceleration)) and could eventually become situated outside of both bodies.**25** This development would then upgrade the Moon to planetary status at that time, according to the definition. The time taken for this to occur, however, would be billions of years, long after many [astronomers](http://en.wikipedia.org/wiki/Astronomy) expect the [Sun](http://en.wikipedia.org/wiki/Sun) to expand into a [red giant](http://en.wikipedia.org/wiki/Red_giant) and destroy both Earth and Moon.**26**
>
> **25** Robert Roy Britt (2006). *[Moon Mechanics: What Really Makes Our World Go 'Round'](http://web.archive.org/web/20100822072302/http://www.space.com/scienceastronomy/moon_mechanics_0303018.html)*
>
> **26** Robert Roy Britt (2006). *[Earth's moon could become a planet](http://edition.cnn.com/2006/TECH/space/08/18/moon.planet)*

$\qquad\qquad\qquad\qquad\qquad\qquad\qquad$ — *[**Wikipedia :** Criticism of the IAU definition of 'planet'](http://en.wikipedia.org/wiki/IAU_definition_of_planet#Criticism)*

> **Comments:**
> **username_0**: Yes, this is already mentioned in the comments and answers here.  

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