<p>I answered this same question at <a href="https://physics.stackexchange.com/a/141532/52112">physics.SE</a>. I specifically joined this part of the SE network to address this duplicate question at this site.</p>

<p><br></p>

<p>The astronomy community faced two crises with regard to what constitutes a "planet", first in the mid 19th century, and more recently at the start of the 21st century. The first crisis involved the asteroids. The second involved trans-Neptunian objects. Both crises challenged astronomers to question what a "planet" was.</p>

<p>1 Ceres, 2 Pallas, 3 Juno, and 4 Vesta were discovered in quick succession during the first decade of the 19th century. There was no international astronomical organization at the time of these discoveries; the International Astronomical Union wouldn't be formed for another century. Instead, the designation of what constituted a "planet" fell on the major astronomical almanacs such as the Berliner Astronomisches Jahrbuch (BAJ). Those discoveries at the start of the 19th century were treated as newly discovered "planets". This situation remained static for about 40 years.</p>

<p>That changed in 1845 with the discovery of 5 Astraea. During the 1850s, the list of objects orbiting the Sun grew to 50, and during the 1860s, the list grew to over 100. The response of the BAJ and others was to demote Ceres, Pallas, Juno, and Vesta from planethood status to some lesser status, either minor planet or asteroid. Astronomers didn't have a clear-cut concept of what constituted a planet other than that they should somehow be large. Ceres, the largest of the bunch, is not very large. The end result of all of these discoveries starting in 1845 was that the first four discovered asteroids were demoted from planethood status.</p>

<p>The second crisis started in 1992 with the discovery of (15760) 1992 QB<sub>1</sub>. By 2006, the number of trans-Neptunian objects had grown significantly. Were these things "planets", or something else? Some astronomers, notably Alan Stern, wanted the term "planet" to be extremely inclusive. Most astronomers balked at this idea.</p>

<p>Paradoxically, it was Alan Stern himself, along with Harold Levison, who provided the key criterion of "clearing the neighborhood" that lies at the heart of what the IAU deems to constitute a "planet." Their paper, <a href="http://adsabs.harvard.edu/full/2002HiA....12..205S" rel="nofollow noreferrer">Stern and Levison, "Regarding the criteria for planethood and proposed planetary classification schemes," <em>Highlights of Astronomy</em> 12 (2002): 205-213</a> suggested splitting "planet" into two categories, "überplanet" (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and "unterplanet" (Pluto+Charon, Eris, Ceres, Sedna, and a host of others).</p>

<p>Stern is being quite hypocritical when he rants that there is no clear-cut boundary between "planets" and "dwarf planets." The boundary is huge, and Stern knows this. The ratio of the square of an object's mass to its orbital radius about the Sun is key in determining whether an object can clear most of the junk from the vicinity of the object's orbit. There is a five order of magnitude difference between the smallest of the planets and the largest of the dwarf planets in terms of this ratio. This five order of magnitude difference figures predominantly in that paper by Stern and Levison.</p>

<p>The only difference between the proposal by Stern and Levison versus the voted-upon IAU resolution is that while Stern and Levison wanted to designate hundreds (and perhaps thousands) of objects into subcategories of "planet" ("überplanet" and "interplant"). On the other hand, the IAU chose to designate those objects as the mutually exclusive terms "planets" and "dwarf planets". This is consistent with how astronomers dealt with that first crisis. Planets should be "large." Stern and Levison provided the necessary ammunition to distinguish large from not so large.</p>

CC BY-SA 4.0
2021-06-16T05:59:29.083Z
A planet should be spherical and have a differentiation into a core, a mantle and a crust (or in case of gas giants into a core, a supercritical gas mantle and an atmosphere) in the first place. That's what always distinguished planets from asteroids. Stern and Levison admit btw that whether a planet has "cleared" its orbit depends on the distance from the Sun. They admit that Pluto would be a planet if it was within 1.7 AU from the Sun, and Eris within 2.1 AU, while Mercury and Mars wouldn't be planets at their distances. But those planetary discriminants are vague themselves. I gonna ask
John
CC BY-SA 4.0
2021-06-16T05:59:39.060Z
a question on this.
John
CC BY-SA 3.0
2014-10-16T13:27:21.087Z
Thank you, this (together with your answer on physics.SE) makes it quite clear!
Dieudonné
CC BY-SA 3.0
2014-10-16T10:57:29.030Z
2014-10-16T10:57:29.030Z
2017-04-13T12:40:15.377Z
David Hammen
<p>There is actually disagreement on this matter (within the IAU?). Dr. Alan Stern (lead of the New Horizons mission) for instance points out that "this rule is inconsistent" (e.g. see <a href="http://news.bbc.co.uk/2/hi/science/nature/5283956.stm" rel="nofollow">Pluto vote 'hijacked' in revolt</a>). Not only has Neptune not cleared its path, but the same holds for Earth, Mars and even Jupiter. Jupiter has a set of asteroids (the Trojans) which follow the same orbit (in the Lagrangian point L4 and L5).</p>

<p>Maybe the definition should be adjusted to include only those bodies as planets in the solar systems that have cleared all objects from their path except those objects that are in resonant orbits with the planet.</p>

<p><a href="http://en.wikipedia.org/wiki/Orbital_resonance" rel="nofollow">Resonant objects</a> are those objects whose Periods are related to the Planet's period by a ration of "small integer numbers". Pluto has a resonance with Neptune of 2:3. This means that for every two orbits completed by Pluto, Neptune will complete three orbits with the result that Neptune and Pluto never come close enough for Neptune to clear Pluto from its orbit.</p>

<p>In the case of the <a href="http://en.wikipedia.org/wiki/Jupiter_Trojan" rel="nofollow">Trojan asteroids</a>, the resonance is 1:1. the Trojans are alway either 60° in front of or behind of Jupiter and therefore never come close to Jupiter.</p>

CC BY-SA 3.0
2016-01-22T00:18:19.460Z
Would the definition of planet allow for the existence of two or more roughly-equal-sized planets in stable orbital resonance with each other, even though neither could represent the majority of the mass in its orbital zone?
supercat
CC BY-SA 3.0
2014-10-15T21:22:03.707Z
Alan is leading the Pluto probe. It is his job to do marketing for this event. Of course he takes advantage of the ridiculous Pluto/planet debacle to bring public attention to his mission and his team. Don't search for logic in PR.
LocalFluff
CC BY-SA 3.0
2014-10-16T00:08:45.160Z
Sooooo you disagree with the [assessment over at Physics.SE](http://physics.stackexchange.com/questions/141504/planets-and-pluto-neptune/141508#141508) that says Neptune *has* cleared its neighborhood?
Kyle Kanos
CC BY-SA 3.0
2014-10-16T06:29:23.773Z
@LocalFluff I hadn't considered the PR side. You may be right.
Dieudonné
CC BY-SA 3.0
2014-10-16T06:34:39.503Z
@KyleKanos No I don't disagree, though I would prefer if the IAU was more clear in its definition.
Dieudonné
CC BY-SA 3.0
2014-10-16T11:14:55.813Z
@Dieudonné - Think of it in terms of a constitutional amendment. Constitutional amendments are almost always short and more than a bit vague. Making sense of those somewhat cryptic amendments is a task left to to the legislative and judicial branches of government. The same applies to that IAU resolution, but here the details are left to the published academic media. When you look at those publications, the distinction is extremely clear.
David Hammen
CC BY-SA 3.0
2014-10-15T20:14:34.687Z
2014-10-15T20:14:34.687Z
Dieudonné
<p>IAU failed to understand that the concept "planet" is a cultural concept, not a scientific concept, not any more anyway. Of course there's no distinct objective scientific definition between what is a planet and what is not. Mercury and Jupiter in the same category? Yeah, right, as if that clarifies what we are talking about when we use the word "planet"!</p>

<p>Pluto was discovered in 1930. Back then a planet was a dot in the sky. Since then our knowledge has advanced. IAU should've decided that there were 9 planets discovered in our historical culture. And now we discover many objects which are much the same as some planets. Another plutoid here, a jupiteoid there. But there are only 9 planets in the entire universe, because "planet" is a cultural concept.</p>

<p>Now, what shall we call Ceres? It is not a planet, it is not an asteroid, it is not a plutoid. Are we back in 1801?</p>

<p>And so called "exoplanets", are they really planets? How do we know that they have cleared their orbits? IAU should consequently decide to classify them as "exodwarfplanets". No exoplanets have been confirmed.</p>

<p>How long is a string according to the confused IAU?</p>

CC BY-SA 3.0
2014-11-26T18:44:58.660Z
Hey, I know now! Ceres is a COMET! It has been observed to have plumes, or a coma as it is called! Just like its discoverer Giuseppe Piazzi first thought when he saw it. The revision of the revisions of the revisions have been revised!
LocalFluff
CC BY-SA 3.0
2014-11-26T18:45:07.023Z
+1 for bringing up exoplanets.  We would have thought a hot Jupiter impossible a few short years ago, who's to say we won't find a Jupiter with 3 other planets sharing its orbit?
IchabodE
CC BY-SA 3.0
2014-11-26T22:01:31.547Z
The cultural definition of "planet" has changed before. If you want to keep the number of planets at 9, how do you justify saying Pluto is a planet and Eris isn't? If a dozen Kuiper Belt objects had been discovered in the 1930s, we probably wouldn't be having this debate (though we might well be having a different one).
Keith Thompson
CC BY-SA 3.0
2014-11-26T23:41:57.647Z
This undefinition of planets was not cultural, it was an ill thought through decision of the IAU. "Planet" should've been left alone. Other terms are needed to describe how Venus and Titan are in the same class because they have atmospheres, and how Ceres and Europa are in the same class because they are icy bodies. How they orbit what was only of key importance in the ancient days when they were just wandering dots in the sky. How is today's definition of "planet" scientifically useful? (It is as if IAU had changed the embarrassing name of Uranus to "Urectum" to solve the problem ;-)
LocalFluff
CC BY-SA 3.0
2014-11-26T17:42:40.673Z
2014-11-26T17:57:11.647Z
2014-11-26T17:57:11.647Z
LocalFluff
<p>If one of the rules to be a planet is that it needs to clear ALL objects from their orbit, does this also make Neptune a non-planet? Since it has thus far failed to clear Pluto (and based on an answer from the physics board, the kuiper belt) from it's orbit. Or does this rule not really apply to planets and we should welcome Pluto and a few other dwarf-planets into our family of planets? </p>

CC BY-SA 3.0
2014-10-15T20:57:25.063Z
3753 Cruithne and Earth? http://en.wikipedia.org/wiki/3753_Cruithne
Wayfaring Stranger
CC BY-SA 3.0
2014-10-15T22:23:07.257Z
Also at [Physics](http://physics.stackexchange.com/questions/141504/planets-and-pluto-neptune).
HDE 226868
<solar-system><planet><dwarf-planets>
Planets and Pluto? Neptune?
Lois Leis
# Planets and Pluto? Neptune?

(Asked by: username_0 on 2014-10-15)

If one of the rules to be a planet is that it needs to clear ALL objects from their orbit, does this also make Neptune a non-planet? Since it has thus far failed to clear Pluto (and based on an answer from the physics board, the kuiper belt) from it's orbit. Or does this rule not really apply to planets and we should welcome Pluto and a few other dwarf-planets into our family of planets?

**Tags:** solar-system, planet, dwarf-planets
> **Comments:**
> **username_1**: 3753 Cruithne and Earth? http://en.wikipedia.org/wiki/3753_Cruithne  
> **username_2**: Also at [Physics](http://physics.stackexchange.com/questions/141504/planets-and-pluto-neptune).  

---

## 3 answers

### Answer by username_3 on 2014-10-16. Score: 6

I answered this same question at [physics.SE](https://physics.stackexchange.com/a/141532/52112). I specifically joined this part of the SE network to address this duplicate question at this site.

The astronomy community faced two crises with regard to what constitutes a "planet", first in the mid 19th century, and more recently at the start of the 21st century. The first crisis involved the asteroids. The second involved trans-Neptunian objects. Both crises challenged astronomers to question what a "planet" was.

1 Ceres, 2 Pallas, 3 Juno, and 4 Vesta were discovered in quick succession during the first decade of the 19th century. There was no international astronomical organization at the time of these discoveries; the International Astronomical Union wouldn't be formed for another century. Instead, the designation of what constituted a "planet" fell on the major astronomical almanacs such as the Berliner Astronomisches Jahrbuch (BAJ). Those discoveries at the start of the 19th century were treated as newly discovered "planets". This situation remained static for about 40 years.

That changed in 1845 with the discovery of 5 Astraea. During the 1850s, the list of objects orbiting the Sun grew to 50, and during the 1860s, the list grew to over 100. The response of the BAJ and others was to demote Ceres, Pallas, Juno, and Vesta from planethood status to some lesser status, either minor planet or asteroid. Astronomers didn't have a clear-cut concept of what constituted a planet other than that they should somehow be large. Ceres, the largest of the bunch, is not very large. The end result of all of these discoveries starting in 1845 was that the first four discovered asteroids were demoted from planethood status.

The second crisis started in 1992 with the discovery of (15760) 1992 QB1. By 2006, the number of trans-Neptunian objects had grown significantly. Were these things "planets", or something else? Some astronomers, notably Alan Stern, wanted the term "planet" to be extremely inclusive. Most astronomers balked at this idea.

Paradoxically, it was Alan Stern himself, along with Harold Levison, who provided the key criterion of "clearing the neighborhood" that lies at the heart of what the IAU deems to constitute a "planet." Their paper, [Stern and Levison, "Regarding the criteria for planethood and proposed planetary classification schemes," *Highlights of Astronomy* 12 (2002): 205-213](http://adsabs.harvard.edu/full/2002HiA....12..205S) suggested splitting "planet" into two categories, "überplanet" (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and "unterplanet" (Pluto+Charon, Eris, Ceres, Sedna, and a host of others).

Stern is being quite hypocritical when he rants that there is no clear-cut boundary between "planets" and "dwarf planets." The boundary is huge, and Stern knows this. The ratio of the square of an object's mass to its orbital radius about the Sun is key in determining whether an object can clear most of the junk from the vicinity of the object's orbit. There is a five order of magnitude difference between the smallest of the planets and the largest of the dwarf planets in terms of this ratio. This five order of magnitude difference figures predominantly in that paper by Stern and Levison.

The only difference between the proposal by Stern and Levison versus the voted-upon IAU resolution is that while Stern and Levison wanted to designate hundreds (and perhaps thousands) of objects into subcategories of "planet" ("überplanet" and "interplant"). On the other hand, the IAU chose to designate those objects as the mutually exclusive terms "planets" and "dwarf planets". This is consistent with how astronomers dealt with that first crisis. Planets should be "large." Stern and Levison provided the necessary ammunition to distinguish large from not so large.

> **Comments:**
> **username_5**: Thank you, this (together with your answer on physics.SE) makes it quite clear!  
> **username_4**: A planet should be spherical and have a differentiation into a core, a mantle and a crust (or in case of gas giants into a core, a supercritical gas mantle and an atmosphere) in the first place. That's what always distinguished planets from asteroids. Stern and Levison admit btw that whether a planet has "cleared" its orbit depends on the distance from the Sun. They admit that Pluto would be a planet if it was within 1.7 AU from the Sun, and Eris within 2.1 AU, while Mercury and Mars wouldn't be planets at their distances. But those planetary discriminants are vague themselves. I gonna ask  
> **username_4**: a question on this.  

---

### Answer by username_5 on 2014-10-15. Score: 3

There is actually disagreement on this matter (within the IAU?). Dr. Alan Stern (lead of the New Horizons mission) for instance points out that "this rule is inconsistent" (e.g. see [Pluto vote 'hijacked' in revolt](http://news.bbc.co.uk/2/hi/science/nature/5283956.stm)). Not only has Neptune not cleared its path, but the same holds for Earth, Mars and even Jupiter. Jupiter has a set of asteroids (the Trojans) which follow the same orbit (in the Lagrangian point L4 and L5).

Maybe the definition should be adjusted to include only those bodies as planets in the solar systems that have cleared all objects from their path except those objects that are in resonant orbits with the planet.

[Resonant objects](http://en.wikipedia.org/wiki/Orbital_resonance) are those objects whose Periods are related to the Planet's period by a ration of "small integer numbers". Pluto has a resonance with Neptune of 2:3. This means that for every two orbits completed by Pluto, Neptune will complete three orbits with the result that Neptune and Pluto never come close enough for Neptune to clear Pluto from its orbit.

In the case of the [Trojan asteroids](http://en.wikipedia.org/wiki/Jupiter_Trojan), the resonance is 1:1. the Trojans are alway either 60° in front of or behind of Jupiter and therefore never come close to Jupiter.

> **Comments:**
> **username_7**: Alan is leading the Pluto probe. It is his job to do marketing for this event. Of course he takes advantage of the ridiculous Pluto/planet debacle to bring public attention to his mission and his team. Don't search for logic in PR.  
> **username_8**: Sooooo you disagree with the [assessment over at Physics.SE](http://physics.stackexchange.com/questions/141504/planets-and-pluto-neptune/141508#141508) that says Neptune *has* cleared its neighborhood?  
> **username_5**: @LocalFluff I hadn't considered the PR side. You may be right.  
> **username_5**: @KyleKanos No I don't disagree, though I would prefer if the IAU was more clear in its definition.  
> **username_3**: @Dieudonné - Think of it in terms of a constitutional amendment. Constitutional amendments are almost always short and more than a bit vague. Making sense of those somewhat cryptic amendments is a task left to to the legislative and judicial branches of government. The same applies to that IAU resolution, but here the details are left to the published academic media. When you look at those publications, the distinction is extremely clear.  
> **username_6**: Would the definition of planet allow for the existence of two or more roughly-equal-sized planets in stable orbital resonance with each other, even though neither could represent the majority of the mass in its orbital zone?  

---

### Answer by username_7 on 2014-11-26. Score: 1

IAU failed to understand that the concept "planet" is a cultural concept, not a scientific concept, not any more anyway. Of course there's no distinct objective scientific definition between what is a planet and what is not. Mercury and Jupiter in the same category? Yeah, right, as if that clarifies what we are talking about when we use the word "planet"!

Pluto was discovered in 1930. Back then a planet was a dot in the sky. Since then our knowledge has advanced. IAU should've decided that there were 9 planets discovered in our historical culture. And now we discover many objects which are much the same as some planets. Another plutoid here, a jupiteoid there. But there are only 9 planets in the entire universe, because "planet" is a cultural concept.

Now, what shall we call Ceres? It is not a planet, it is not an asteroid, it is not a plutoid. Are we back in 1801?

And so called "exoplanets", are they really planets? How do we know that they have cleared their orbits? IAU should consequently decide to classify them as "exodwarfplanets". No exoplanets have been confirmed.

How long is a string according to the confused IAU?

> **Comments:**
> **username_7**: Hey, I know now! Ceres is a COMET! It has been observed to have plumes, or a coma as it is called! Just like its discoverer Giuseppe Piazzi first thought when he saw it. The revision of the revisions of the revisions have been revised!  
> **username_9**: +1 for bringing up exoplanets.  We would have thought a hot Jupiter impossible a few short years ago, who's to say we won't find a Jupiter with 3 other planets sharing its orbit?  
> **username_10**: The cultural definition of "planet" has changed before. If you want to keep the number of planets at 9, how do you justify saying Pluto is a planet and Eris isn't? If a dozen Kuiper Belt objects had been discovered in the 1930s, we probably wouldn't be having this debate (though we might well be having a different one).  
> **username_7**: This undefinition of planets was not cultural, it was an ill thought through decision of the IAU. "Planet" should've been left alone. Other terms are needed to describe how Venus and Titan are in the same class because they have atmospheres, and how Ceres and Europa are in the same class because they are icy bodies. How they orbit what was only of key importance in the ancient days when they were just wandering dots in the sky. How is today's definition of "planet" scientifically useful? (It is as if IAU had changed the embarrassing name of Uranus to "Urectum" to solve the problem ;-)  

---

