<p>That actual <a href="https://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf" rel="noreferrer">IAU Resolution B5</a> adopted at the IAU General Assembly in 2006 states:</p>
<blockquote>
<p>(1) A planet is a celestial body that:</p>
<p>(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<br />
(c) has cleared the neighbourhood around its orbit.</p>
</blockquote>
<p>The last part being the <strong>neighbourhood around its (whole) orbit</strong> is the important part; it's not enough for Pluto to dominate its family of moons (and this last part is debatable as Pluto and Charon orbit a common barycenter (center of mass) which is between Pluto and Charon since Charon is so massive, compared to Pluto).</p>
<p>Since Pluto orbits in the Trans-Neptunian region with a similar orbit to other bodies, some of which are similar or larger in size and mass (Eris), and cross the orbit of Neptune, it can't really be said have to &quot;cleared the neighbourhood around its orbit&quot;. Indeed Pluto is the prototype of the <a href="https://en.wikipedia.org/wiki/Plutino" rel="noreferrer">Plutinos</a>; object which have a 3:2 mean motion resonance with Neptune (orbiting twice for every three orbits Neptune makes).</p>

CC BY-SA 4.0
2020-02-23T17:08:55.407Z
How are Mars's, Jupiter's, Saturn's, and Neptune's [trojans](https://en.wikipedia.org/wiki/Trojan_(astronomy)) reconciled with item (c) of the definition?
Eric Towers
CC BY-SA 4.0
2020-02-23T18:37:13.250Z
@EricTowers They are clearly dominated by the fact they exist only at Jupiter's Lagrange points.
corsiKa
CC BY-SA 4.0
2020-02-23T18:40:42.757Z
@corsiKa : So Jupiter has *not* cleared the neighborhood of its orbit?
Eric Towers
CC BY-SA 4.0
2020-02-23T18:44:10.070Z
@EricTowers No, it has - the trojans are not freely orbiting - they are cleared by being put in a stable orbit around the sun by the constant manipulation of Jupiter's gravity over time. When you clear your kitchen counter, you don't put your coffee pot in the cupboard - it belongs there - but you wouldn't leave it out with the filters everywhere, it's put away nice and tidy. So too are the trojans.
corsiKa
CC BY-SA 4.0
2020-02-23T18:46:06.460Z
@EricTowers Here's an answer from someone who works at NASA who expanded on it better than I could and not constrained by a comment: https://astronomy.stackexchange.com/a/19004/2239
corsiKa
CC BY-SA 4.0
2020-02-23T19:00:07.397Z
@corsiKa : You and I hear "clear the kitchen counter" very differently.  In my ear, that is the step immediately prior to "wipe it down with low concentration bleach", so the coffee pot and everything has been removed.  "Tidy the counter" seems to have the connotation you describe.  To your references: "cleared" is in a tense meaning that the clearing has been completed in the past.  Stern-Levinson's Lambda describes clearing if given the life of the universe and Margot's Pi if given ~5 billion more years, so neither describes a completed action.
Eric Towers
CC BY-SA 4.0
2020-02-23T19:01:20.650Z
@corsiKa : Also I agree with Stern (same Stern):  "If Neptune had cleared its zone, Pluto wouldn't be there."  ( Rincon, Paul (25 August 2006). "[Pluto vote 'hijacked' in revolt](http://news.bbc.co.uk/2/hi/science/nature/5283956.stm)". BBC News. Retrieved 2006-09-03. )
Eric Towers
CC BY-SA 4.0
2020-02-23T20:45:01.600Z
Stern sounds like he is more influenced by nostalgia and stubbornness than reason.  He also invokes the line drawing fallacy.  It's very obvious there's a big difference between Pluto and Jupiter.  In the Jupiter system, Jupiter is unquestionably the thing that is influencing everything else.  The center of gravity in the system is virtually identical the CoG of Jupiter itself.  In the Pluto system, Pluto is quite obviously not the central object, but rather the biggest object in a cluster of things, with the center being an arbitrary point in space which they all orbit.
eps
CC BY-SA 4.0
2020-02-23T20:46:09.440Z
Basically, to stamp your foot down on the "cleared the orbit" idea is a bit like saying species don't exist because there's fuzziness around the edges.
eps
CC BY-SA 4.0
2020-02-24T01:50:48.240Z
This is indeed the IAU definition of "planet", but this still doesnt solve the issue that many minor planet moons are likely to be captured.              
           
The problem here is that "cleared the orbit" is a very vague term: do you need to clear just *1* object, or all the objects in the neighborhood? How far is your neighborhood?
Nirvana
CC BY-SA 4.0
2020-05-21T11:45:34.643Z
@Nirvana All proposed standards for the terms do some sort of comparison on the mass of the would-be-planet versus the mass of everything else near its orbit.  There's a lot of possibilities there, but when applied to our own solar system they all create a very clear divide between the 8 planets versus objects like Pluto, with an order of magnitude (if not at least two) difference or more between their values.  Applications to other systems are not intended, as the definition specifically requires it to orbit the/our Sun.
zibadawa timmy
CC BY-SA 4.0
2020-11-27T10:52:09.210Z
Minor point but Pluto is larger than Eris.  New Horizons gave us a better understanding of Pluto's size.  Eris is more massive.  Pluto is slightly larger.   Eris is more reflective and more dense.    https://www.smithsonianmag.com/smart-news/dwarf-planet-pluto-bigger-expected-180955909/#:~:text=The%20measurement%20also%20means%20that%20Eris%2C%20another%20Kuiper,it%20was%201.25%20times%20more%20massive%20than%20Pluto.
userLTK
CC BY-SA 4.0
2020-02-23T05:56:36.843Z
2020-02-23T05:56:36.843Z
2020-06-17T09:47:40.653Z
astrosnapper
<p>No, Pluto is not a planet by that definition. However, that definition has a serious issue: Earth is also not a planet. There are objects that have not been cleared from the Earth's orbit. (see <a href="https://www.nationalgeographic.com/news/2011/7/1107128-trojan-asteroid-earth-planet-orbit-nasa-space-science/" rel="nofollow noreferrer">https://www.nationalgeographic.com/news/2011/7/1107128-trojan-asteroid-earth-planet-orbit-nasa-space-science/</a>)</p>

CC BY-SA 4.0
2020-02-23T18:37:37.873Z
That's a bold claim with no citation or supporting evidence. Care to elaborate?
corsiKa
CC BY-SA 4.0
2020-02-23T15:10:28.317Z
2020-02-23T20:09:25.213Z
2020-02-23T20:09:25.213Z
ScoopGracie
<p>In 2006, the International Astronomical Union redefined the definition of planet in order to exclude Pluto, Eris, and several other objects whose category was disputed. This new definition of a planet featured 3 criteria:</p>

<ol>
<li>A planet must be massive enough to achieve hydrostatic equilibrum and
become ellipsoidal. </li>
<li>A planet must orbit a star or brown dwarf (with the exception of planemo).</li>
<li>A planet must be massive enough to clear any nearby objects.</li>
</ol>

<p>Pluto and the other objects apparently failed to meet the 3rd criteria, as they had not cleared the nearby Kuiper belt objects, so they were reclassified under the term "dwarf planets".</p>

<p>But what if Pluto actually <em>has</em> met the third criteria? It's probably well-known by now that Pluto has 5 moons. The largest moon Charon is over half the size of  Pluto, and is frequently classed as a dwarf planet in its own right. The other 4 minor moons Nix, Styx, Hydra, and Kerberos, are much smaller then Charon, and are similar to the many asteroid-like moons found around gas planets. </p>

<p>The leading theory for how these smaller moons formed is that they are leftover debris from a collision between Pluto and another object, but it's just as likely that they were Kuiper belt objects that were captured by Pluto (just like the moons of Jupiter and Saturn). If they were captured, this means that Pluto has technically cleared 4 objects from its neighborhood, and so it meets the 3rd criteria. </p>

<p>The same logic can be applied to Eris, 2007 OR10, and the 400 other dwarf planets known to host moons. So should we have 392 planets, keep the current definition, or scrap or redefine the term "planet" entirely?</p>

<planet><natural-satellites><dwarf-planets><classification>
Up to 384 minor planets (including Pluto) qualify as planets?
Nirvana
# Up to 384 minor planets (including Pluto) qualify as planets?

(Asked by: username_0 on 2020-02-23)

In 2006, the International Astronomical Union redefined the definition of planet in order to exclude Pluto, Eris, and several other objects whose category was disputed. This new definition of a planet featured 3 criteria:

1. A planet must be massive enough to achieve hydrostatic equilibrum and
   become ellipsoidal.
2. A planet must orbit a star or brown dwarf (with the exception of planemo).
3. A planet must be massive enough to clear any nearby objects.

Pluto and the other objects apparently failed to meet the 3rd criteria, as they had not cleared the nearby Kuiper belt objects, so they were reclassified under the term "dwarf planets".

But what if Pluto actually *has* met the third criteria? It's probably well-known by now that Pluto has 5 moons. The largest moon Charon is over half the size of Pluto, and is frequently classed as a dwarf planet in its own right. The other 4 minor moons Nix, Styx, Hydra, and Kerberos, are much smaller then Charon, and are similar to the many asteroid-like moons found around gas planets.

The leading theory for how these smaller moons formed is that they are leftover debris from a collision between Pluto and another object, but it's just as likely that they were Kuiper belt objects that were captured by Pluto (just like the moons of Jupiter and Saturn). If they were captured, this means that Pluto has technically cleared 4 objects from its neighborhood, and so it meets the 3rd criteria.

The same logic can be applied to Eris, 2007 OR10, and the 400 other dwarf planets known to host moons. So should we have 392 planets, keep the current definition, or scrap or redefine the term "planet" entirely?

**Tags:** planet, natural-satellites, dwarf-planets, classification


---

## 2 answers

### Answer by username_1 on 2020-02-23. Score: 9

That actual [IAU Resolution B5](https://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf) adopted at the IAU General Assembly in 2006 states:

> (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.

The last part being the **neighbourhood around its (whole) orbit** is the important part; it's not enough for Pluto to dominate its family of moons (and this last part is debatable as Pluto and Charon orbit a common barycenter (center of mass) which is between Pluto and Charon since Charon is so massive, compared to Pluto).

Since Pluto orbits in the Trans-Neptunian region with a similar orbit to other bodies, some of which are similar or larger in size and mass (Eris), and cross the orbit of Neptune, it can't really be said have to "cleared the neighbourhood around its orbit". Indeed Pluto is the prototype of the [Plutinos](https://en.wikipedia.org/wiki/Plutino); object which have a 3:2 mean motion resonance with Neptune (orbiting twice for every three orbits Neptune makes).

> **Comments:**
> **username_2**: How are Mars's, Jupiter's, Saturn's, and Neptune's [trojans](https://en.wikipedia.org/wiki/Trojan_(astronomy)) reconciled with item (c) of the definition?  
> **username_3**: @EricTowers They are clearly dominated by the fact they exist only at Jupiter's Lagrange points.  
> **username_2**: @username_3 : So Jupiter has *not* cleared the neighborhood of its orbit?  
> **username_3**: @EricTowers No, it has - the trojans are not freely orbiting - they are cleared by being put in a stable orbit around the sun by the constant manipulation of Jupiter's gravity over time. When you clear your kitchen counter, you don't put your coffee pot in the cupboard - it belongs there - but you wouldn't leave it out with the filters everywhere, it's put away nice and tidy. So too are the trojans.  
> **username_3**: @EricTowers Here's an answer from someone who works at NASA who expanded on it better than I could and not constrained by a comment: https://astronomy.stackexchange.com/a/19004/2239  
> **username_2**: @username_3 : You and I hear "clear the kitchen counter" very differently.  In my ear, that is the step immediately prior to "wipe it down with low concentration bleach", so the coffee pot and everything has been removed.  "Tidy the counter" seems to have the connotation you describe.  To your references: "cleared" is in a tense meaning that the clearing has been completed in the past.  Stern-Levinson's Lambda describes clearing if given the life of the universe and Margot's Pi if given ~5 billion more years, so neither describes a completed action.  
> **username_2**: @username_3 : Also I agree with Stern (same Stern):  "If Neptune had cleared its zone, Pluto wouldn't be there."  ( Rincon, Paul (25 August 2006). "[Pluto vote 'hijacked' in revolt](http://news.bbc.co.uk/2/hi/science/nature/5283956.stm)". BBC News. Retrieved 2006-09-03. )  
> **username_4**: Stern sounds like he is more influenced by nostalgia and stubbornness than reason.  He also invokes the line drawing fallacy.  It's very obvious there's a big difference between Pluto and Jupiter.  In the Jupiter system, Jupiter is unquestionably the thing that is influencing everything else.  The center of gravity in the system is virtually identical the CoG of Jupiter itself.  In the Pluto system, Pluto is quite obviously not the central object, but rather the biggest object in a cluster of things, with the center being an arbitrary point in space which they all orbit.  
> **username_4**: Basically, to stamp your foot down on the "cleared the orbit" idea is a bit like saying species don't exist because there's fuzziness around the edges.  
> **username_0**: This is indeed the IAU definition of "planet", but this still doesnt solve the issue that many minor planet moons are likely to be captured.                           The problem here is that "cleared the orbit" is a very vague term: do you need to clear just *1* object, or all the objects in the neighborhood? How far is your neighborhood?  
> **username_5**: @username_0 All proposed standards for the terms do some sort of comparison on the mass of the would-be-planet versus the mass of everything else near its orbit.  There's a lot of possibilities there, but when applied to our own solar system they all create a very clear divide between the 8 planets versus objects like Pluto, with an order of magnitude (if not at least two) difference or more between their values.  Applications to other systems are not intended, as the definition specifically requires it to orbit the/our Sun.  
> **username_6**: Minor point but Pluto is larger than Eris.  New Horizons gave us a better understanding of Pluto's size.  Eris is more massive.  Pluto is slightly larger.   Eris is more reflective and more dense.    https://www.smithsonianmag.com/smart-news/dwarf-planet-pluto-bigger-expected-180955909/#:~:text=The%20measurement%20also%20means%20that%20Eris%2C%20another%20Kuiper,it%20was%201.25%20times%20more%20massive%20than%20Pluto.  

---

### Answer by username_7 on 2020-02-23. Score: -7

No, Pluto is not a planet by that definition. However, that definition has a serious issue: Earth is also not a planet. There are objects that have not been cleared from the Earth's orbit. (see <https://www.nationalgeographic.com/news/2011/7/1107128-trojan-asteroid-earth-planet-orbit-nasa-space-science/>)

> **Comments:**
> **username_3**: That's a bold claim with no citation or supporting evidence. Care to elaborate?  

---

