<p>In addition to Undo's fine answer, I would like to explain a bit about the motivation behind the definition.</p>

<p>When Eris was discovered, it turned out to be really, <em>really</em> similar to Pluto. This posed a bit of a quandary: should Eris be accepted as a new planet? Should it not? If not, then why keep Pluto? Most importantly, this pushed to the foreground the question</p>

<blockquote>
  <p>what, <em>exactly</em>, is a planet, anyway?</p>
</blockquote>

<p>This had been ignored until then because everyone "knew" which bodies were planets and which ones were not. However, with the discovery of Eris, and the newly-realized potential of more such bodies turning up, this was no longer really an option, and <em>some</em> sort of hard definition had to be agreed upon.</p>

<p>The problem with coming up with a hard definition that decides what does make it to planethood and what doesn't is that nature very rarely presents us with clear, definite lines. Size, for example, is not a good discriminant, because solar system bodies come in a continuum of sizes from Jupiter down to meter-long asteroids. Where does one draw the line there? Any such size would be completely arbitrary.</p>

<p>There <em>is</em>, however, one characteristic that has a sharp distinction between some "planets" and some "non-planets", and it is <em>the amount of other stuff in roughly the same orbit</em>. This is still slightly arbitrary, because it's hard to put in numbers exactly what "roughly" means in this context, but it's more or less unambiguous. </p>

<p>Consider, then a quantity called the "planetary discriminant" µ, equal to the ratio of the planet's mass to the total mass of other bodies that cross its orbital radius and have non-resonant periods (so e.g. Neptune doesn't count as sharing Pluto's orbit) up to a factor of 10 longer or shorter (to rule out comets, which has little effect in practice). This is still a bit arbitrary (why 10?) but it's otherwise quite an objective quantity.</p>

<p>Now take this quantity and calculate it for the different bodies you might call planets, comparing it to both the objects' mass,</p>

<p><img src="https://i.sstatic.net/iKpkE.png" alt="Planetary discriminant vs mass"></p>

<p>and their diameter,</p>

<p><img src="https://i.sstatic.net/MzTOZ.png" alt="Planetary discriminant vs diameter"></p>

<p>or with an arbitrary horizontal axis, in order of decreasing discriminant,</p>

<p><img src="https://i.sstatic.net/yqfo1.png" alt="Planetary discriminant on an arbitrary horizontal axis, in order of decreasing discriminant"></p>

<p>Suddenly, <strong>a natural hard line emerges</strong>. If you look only at the mass and the diameter of the objects (shown in the insets above the plots), then there is a pretty continuous spread of values, with bigger gaps between the gas giants and the terrestrial planets than between Mercury and Eris/Pluto. However, if you look at the planetary discriminant, on the vertical axis, you get a very clear grouping into two distinct populations, separated by over four orders of magnitude. There's a finite set of bodies that have "cleared their orbits", and some other bodies which are well, well behind in that respect.</p>

<p>This is the main reason that "clearing its orbital zone" was chosen as a criterion for planethood. It relies on a distinction that is actually there in the solar system, and very little on arbitrary human decisions. It's important to note that this criterion need not have worked: this parameter might also have come out as a continuum, with some bodies having emptier orbits and some others having slightly fuller ones, and no natural place to draw the line, in which case the definition would have been different. As it happens, this is indeed a good discriminant.</p>

<p>For further reading, I recommend <a href="http://en.wikipedia.org/wiki/Clearing_the_neighbourhood" rel="noreferrer">the Wikipedia article on 'Clearing the neighbourhood'</a>, as well as the original paper where this criterion was proposed,</p>

<blockquote>
  <p>What is a planet? S Soter,  <a href="http://dx.doi.org/10.1086/508861" rel="noreferrer"><em>The Astronomical Journal</em> <strong>132</strong> no.6 (2006), p. 2513</a>. <a href="http://arxiv.org/abs/astro-ph/0608359" rel="noreferrer">arXiv:astro-ph/0608359</a>.</p>
</blockquote>

<p>which is in general very readable (though there are some technical bits in the middle which are easy to spot and harmless to skip), and from which I took the discriminant data for the plots above.</p>

<hr>

<p><sup><strong>Edit:</strong> I must apologize for having included, in previous versions of this post, an incorrect plot, caused by taking <a href="http://web.archive.org/web/20131015071825/http://en.wikipedia.org/wiki/Clearing_the_neighbourhood" rel="noreferrer">data from Wikipedia</a> without verifying it. In particular, the planetary discriminant for Mars was wrong (1.8×10<sup>5</sup> instead of 5.1×10<sup>3</sup>), which now puts it below Neptune's instead of just below Saturn's, but the overall conclusions are not affected. The Mathematica code for the graphics is available at <code>Import["http://goo.gl/NaH6rM"]["https://i.sstatic.net/CQA4T.png"]</code>.</sup></p>

<hr>

<p>... and, as a final aside: Pluto is <em>awesome</em>. It was visited in July 2015 by the <a href="https://en.wikipedia.org/wiki/New_Horizons" rel="noreferrer">New Horizons</a> probe, which found a world that was much more rich, dynamic, and active than anyone expected, including what appear to be churning lakes of solid nitrogen ringed by mountains of water ice, among other marvels.</p>

<p><a href="https://en.wikipedia.org/wiki/File:Pluto-01_Stern_03_Pluto_Color_TXT.jpg" rel="noreferrer"><img src="https://i.sstatic.net/Gz9sM.jpg" alt="Pluto as seen from the New Horizons flyby, in enhanced colour"></a></p>

<p>(Note the image has been colour-enhanced to bring out the variety of surface materials; the true-colour version of this image is <a href="https://commons.wikimedia.org/wiki/File:Nh-pluto-in-true-color_2x_JPEG.jpg" rel="noreferrer">here</a>.) I, personally, don't feel it's at all necessary to 'grandfather' Pluto into the list of planets to really feel the awe at the amazing place it is - it's perfectly OK for it to be a cool place with cool science, that is also not a planet.</p>

CC BY-SA 3.0
2016-06-21T19:53:23.453Z
Thanks for coming back and updating this! Still a fantastic answer.
called2voyage
CC BY-SA 3.0
2016-06-21T19:57:02.587Z
@called2voyage To be honest I'm really embarrassed about this episode - the data on Wikipedia were dead wrong, and I simply didn't catch it through not doing my due diligence. (Even worse, the corresponding [edit history](https://en.wikipedia.org/w/index.php?title=Clearing_the_neighbourhood&oldid=575282009) in Wikipedia is garbled, which is worrying by itself.) Hopefully the new plots can make up for a mistake of that calibre on such a visible post.
Emilio Pisanty
CC BY-SA 3.0
2016-06-21T19:58:01.490Z
@EmilioPisanty I appreciate your diligence on the matter. I refer to this answer frequently, so it is good to know that you are doing your best to ensure it is accurate.
called2voyage
CC BY-SA 4.0
2024-05-26T20:40:48.563Z
Thanks for this great summary. It looks to me that the scientific community was looking for a good reason to leave Pluto out (to maintain the educational value of having a bunch of named bodies easy to remember, bring interest on the matter to young generations, etc.), and well, they found one tuning this parameter to their needs. I came looking for this detailed explanation of "cleaning up the neighbourhood", wondering how they managed to leave Jupiter out with all the moons and trojan asteroids etc., and where does the "neighbourhood" extend up to... but well it's something
Azurlake
CC BY-SA 3.0
2014-07-04T06:52:43.883Z
+1, partly just for publicizing that plot! I hadn't seen it before either, but it really lends weight to the inclusion of that "clear-the-neighbourhood" criterion.
Warrick
CC BY-SA 3.0
2014-10-09T20:43:34.707Z
Would expanding or shrinking the zone searched for similar stuff leave the graph basically unchanged?  If there is any easy way to compute what the discriminant would be if a larger or smaller zone were used, that might help show that the factor of 10 may have been arbitrary, but the difference between planets and other things is not dependent upon that arbitrary factor.
supercat
CC BY-SA 3.0
2014-10-09T22:22:48.837Z
@supercat That is a good question, and I honestly don't know. You should ask it separately!
Emilio Pisanty
CC BY-SA 3.0
2013-10-09T21:26:50.720Z
Very good summary on the criterion. This was new to me.
Arne
CC BY-SA 3.0
2013-10-10T01:11:58.117Z
I was going to post a comment saying that any definition of "planet" must be arbitrary, given that, for example, Ceres and Mercury are much more similar to each other than Mercury and Jupiter. But that graph is fascinating. I note that it says much more about the bodies' spacial relationships to their neighbors than about the bodies themselves.
Keith Thompson
CC BY-SA 3.0
2013-10-10T15:42:48.610Z
@KeithThompson Yeah, but as it turns out there's no definition that depends only on the bodies themselves that matches our intuition for "planethood". Put it this way: If Mercury's orbit was as full of other crap as Pluto's, we'd have noticed it ages ago. (And of course, for Ceres we did: it took fifteen months between the discoveries of Ceres and Pallas.)
Emilio Pisanty
CC BY-SA 3.0
2013-10-09T16:10:00.887Z
2016-06-21T20:12:05.920Z
2016-06-21T20:12:05.920Z
Emilio Pisanty
<p>The correct answer is 8 (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune).</p>

<p>Pluto is not longer a planet since 2006 when the IAU adopted a <a href="http://en.wikipedia.org/wiki/Definition_of_planet" rel="noreferrer">formal definition of planet</a></p>

CC BY-SA 3.0
2017-10-01T22:43:34.567Z
The answer isn't that simple. There could be 8 or 100
user35971
CC BY-SA 3.0
2013-10-09T00:17:12.543Z
[+1] but I still have the problem of not having any reliable proof to that. I added an edit to my question to explain why Wikipedia doesn't really help finding the answer to my question. If you could provide **an astronomical trusted source**, I would gladly accept the answer...
e-sushi
CC BY-SA 3.0
2013-10-09T00:04:46.763Z
2016-01-23T13:19:29.780Z
2016-01-23T13:19:29.780Z
Claudio Redi
<p>Well, it depends on how you define a planet.</p>

<p>According to <a href="https://astronomiac.com/how-many-planets-are-there-in-the-solar-system/" rel="nofollow noreferrer">IAU</a> (InternationalAstronomical Union), an object should satisfy three conditions for it to be called a planet</p>

<ol>
<li>It should revolve around the sun.</li>
<li>It should have enough gravity to form itself into a sphere.</li>
<li>It should have cleared space around itself (it should be a dominant object in that area)</li>
</ol>

<p>Pluto was demoted as a planet because it failed the <strong>third criteria</strong>. 
Currently, <a href="https://astronomiac.com/how-many-planets-are-there-in-the-solar-system/" rel="nofollow noreferrer">there are only Eight planets</a> that satisfy the three conditions</p>

<ol>
<li>Mercury</li>
<li>Venus</li>
<li>Earth</li>
<li>Mars</li>
<li>Jupiter</li>
<li>Saturn</li>
<li>Uranus</li>
<li>Neptune</li>
</ol>

<p>There are five dwarf planets officially recognised by IAU </p>

<ol>
<li>Pluto</li>
<li>Ceres</li>
<li>Eris</li>
<li>Haumea</li>
<li>Makemake</li>
</ol>

CC BY-SA 4.0
2019-05-18T14:54:37.560Z
Thank you for your contribution.  It's not clear to me how this differs from the existing answers. We have quite a few unanswered questions, and others with answers that could be improved on.  Have a look at the list of new and unanswered questions to find one that you can answer. This will be more useful to the community than repeating content already found in other answers.
James K
CC BY-SA 4.0
2019-05-18T05:00:20.870Z
2019-05-18T05:00:20.870Z
Prajyot
<p>According to a geophysical planet definition, as well as the IAU planets and dwarf planets, the largest moons are also planets: the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton.</p>
<p><a href="https://en.wikipedia.org/wiki/Geophysical_definition_of_planet" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Geophysical_definition_of_planet</a></p>
<p>Although other moons of Saturn and Uranus and also Charon look round to me also.</p>

CC BY-SA 4.0
2022-12-07T10:24:24.977Z
I'm not that fond of that wikipedia-page, as it states Pluto-Charon as double planets. If that were true, why are the Earth and our Moon not counted as double planets, I mean the mass ratio does not differ that much: ±1/3 versus ±1/5)?
Dominique
CC BY-SA 4.0
2021-07-23T18:28:16.743Z
2021-07-23T18:52:17.613Z
2021-07-23T18:52:17.613Z
sno
<p>This is not exactly what you asked, but I'm writing it anyway because it's relevant to the title question.</p>
<p>Strictly speaking, the answer is <strong>at least 8, possibly more</strong>. The &quot;8&quot; are Mercury / Venus / Earth / Mars / Jupiter / Saturn / Uranus / Neptune, as detailed in the other answers. These celestial bodies are unambiguously planets. Others (like Pluto, Ceres ...) are generally treated as dwarf planets, at least by professional astronomers.</p>
<p>The &quot;possibly more&quot; part is because the data isn't good enough to rule out the existence of other large planets around the Sun. See the <a href="https://en.wikipedia.org/wiki/Planet_Nine" rel="nofollow noreferrer">Planet Nine</a> article on Wikipedia. It's even possible that there is a &quot;<a href="https://en.wikipedia.org/wiki/Planets_beyond_Neptune#Kuiper_cliff_and_%22Planet_Ten%22" rel="nofollow noreferrer">Planet Ten</a>&quot;. These planets are hypothetical, however, and it will take a direct detection to convince everyone that they definitely exist.</p>

CC BY-SA 4.0
2023-02-07T02:48:30.010Z
2023-02-07T02:48:30.010Z
Allure
<p>The other answers told how many planets are recognized, how many dwarf planets that aren't IAU-recognized planets are recognized and one that indeed includes Eris. In a month it will be fifteen years since the infamous decision for an extremely vague and therefore stretched definition that was trying hard to keep the number of planets as low as possible for some reason. The majority among ~4% of the IAU who attended the meeting feared if they accepted Ceres, Pluto and Eris, there would eventually be &quot;hundreds&quot; of planets.</p>
<p>Instead of the current allegedly &quot;official&quot; position, I will portray a history of what <em>should</em> have been done if there wasn't any panic about &quot;hundreds of planets&quot; and if one were to go by logic and science according to how planets and asteroids have been defined pre-2006. Let's see if the fear was even justified.</p>
<p>Since 1930 there were <strong>9 planets</strong>: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. This number was <em>subjectively correct</em> from 1930 to 2004 (not 06). In 2004 the Hubble telescope shot the first image of Ceres sharp enough to recognize features and its spherical shape:</p>
<p><a href="https://i.sstatic.net/8cEF1.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/8cEF1.jpg" alt="Ceres Planethood Proof" /></a></p>
<p>At that time Ceres was considered the largest asteroid, and even today some still call it an asteroid for some reason (though they probably mean &quot;asteroid belt object&quot;). An asteroid is an irregular body, and Hubble proved that Ceres is near-perfectly spherical and therefore indeed a planet as it was recognized in the first half of the 19th century. This should have sufficed pre-2006 to reclassify it a planet again and even if you don't stretch the current definition too much it may be a planet as it's the largest and most massive object of the main belt. Whatever, Hubble proved Ceres was a planet, not an asteroid, and following the publication of the shot, Ceres should have gained planethood back. <em>This wasn't done which was the first falsification in the number and recognition of planets.</em></p>
<p>If they had done so they would consequently have had to give planethood to the larger body Sedna in expectance that it must be a planet too. So if they did what would have been logical according to the then-understanding of celestial bodies, they would have accepted Ceres and Sedna as planets and there would be <strong>11 planets</strong> now.</p>
<p>Later the year, 2005 FY9, later named Makemake, has been discovered. 2005 FY9 is larger than Ceres and Sedna and therefore should have got planethood as well, and it would get it probably along with Quaoar which is larger than Ceres as well, and Orcus the &quot;anti-Pluto&quot; which has about the same size as Ceres and an orbit similar to Pluto. Hence there would be <strong>14 planets</strong> now.</p>
<p>In 2004/05 Haumea was discovered which was named a dwarf planet by the IAU in 2008. However, Haumea is a Jacobi ellipsoid (rugby ball shape), and back then a planet needed to be &quot;spherical&quot;, not to &quot;overcome rigid body forces to pull itself into hydrostatic equilibrium&quot;. Therefore, rather than gaining planet status it would have been considered the largest asteroid, thereby replacing Ceres which got planethood back in 2004. If Haumea was recognized a planet, Vesta would have to be recognized too which isn't perfectly spherical either but more so than Haumea, and is internally differentiated, and Pallas and Varuna perhaps too.</p>
<p>In 2007, 2007OR10, later named Gonggong, has been discovered. Of course it would have been recognized a planet too as it is larger than Ceres, Orcus and Sedna. There would now be <strong>15 planets</strong>.</p>
<p>In 2017 and 18, the VLT shot sharp images of 10 Hygiea proving a near-spherical shape:</p>
<p><a href="https://i.sstatic.net/qIMGB.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/qIMGB.jpg" alt="Hygiea sphericity VLT" /></a></p>
<p>Published in 2019, many consider Hygiea the smallest dwarf planet now. Hygiea would have beeen recognized a planet under the expectation that subsequent study would prove it to be one, maybe a probe would have been sent to study it from close up. As you see, planets are obviously considered more &quot;important&quot; than bodies unrecognized as planets. There would be <strong>16 planets</strong> now. There would be 20 planets if you add Haumea, Vesta, Pallas and Varuna. If Hygiea turned out to actually be spherical enough and differentiated, one may someday add 2002MS4 and Salacia as well so there would be 22 planets. In 2014 and 16 a hypothetical super-Earth or ice giant has been proposed to explain the very eccentric orbits of 6 TNOs; if you add it there are 23 planets (or <strong>17 planets</strong> if you don't count the six controversial ones).</p>
<p>And no more planets would come unless we discover yet another Earth-sized or larger planet beyond the hypothetical one, or discover a dwarf planet that has an orbit similar to Sedna and comes to perihelion, or send a space probe to a planet candidate (however, the other &quot;dwarf planet candidates&quot; are unlikely to be near-perfectly spherical or differentiated). WISE/neoWISE ruled out the possibility of Saturn- and Jupiter-sized planets to a certain distance.</p>
<p>The fear of &quot;hundreds of planets&quot; was unnecessary and is disproved. And even if there would have been, I don't see anything wrong with it. Non-professionals would only have to learn the eight largest ones and Ceres and Pluto for historical reasons perhaps.</p>

CC BY-SA 4.0
2023-10-16T10:27:06.027Z
There are two problems with only considering "near sphericity". (1) The "nearly" part was only supposed to make allowance for oblateness due to rotation, plus gravitational distortion by nearby bodies; Hygiea is clearly "lumpy" and so would not qualify. (2) at least 32 known bodies in the solar system meet this criterion: the 8 acknowledged planets, the 9 largest known Kuiper belt objects, and the 15 largest moons (include Earth's moon). (Ceres 1 is the 32nd largest object, and confirmed to be spherical, so I'm assuming all the heavier objects are likewise spherical.)
Martin Kealey
CC BY-SA 4.0
2023-10-18T02:51:11.420Z
"Planet" was greek for "wanderer", that is, a "star" that moved about, and visible to the naked eye. Pluto is quite invisible to the naked eye, so on that basis alone it should be excluded from the planets. https://astronomy.stackexchange.com/questions/51131/is-uranus-the-farthest-object-in-solar-system-that-you-can-see-with-naked-eyes
Martin Kealey
CC BY-SA 4.0
2021-07-23T13:33:08.227Z
2021-07-23T13:33:08.227Z
John
<p>So, in school (that's a long time ago) they have been teaching us there are 9 planets in our solar system.</p>
<ol>
<li>Mercury</li>
<li>Venus</li>
<li>Earth</li>
<li>Mars</li>
<li>Jupiter</li>
<li>Saturn</li>
<li>Uranus</li>
<li>Neptune</li>
<li>Pluto</li>
</ol>
<p>But every now and then I keep reading stories about another &quot;dwarf planet&quot; (Eris, discovered in 2005) that - depending on what source tells the story - is another planet according to the astronomical definition, while other sources say that it isn't a planet. Some even say Pluto isn't a planet anymore either.</p>
<p>The result: I'm confused due to the contradicting stories. Even <a href="http://en.wikipedia.org/wiki/Eris_%28dwarf_planet%29" rel="noreferrer">Wikipedia isn't clear about Eris</a> and only writes (emphasis mine):</p>
<blockquote>
<p>NASA <em><strong>initially</strong></em> described it as the Solar System's tenth planet.</p>
</blockquote>
<p>Initially? So, is it a 10th planet or not? Fact is, there is another &quot;something&quot; out there and it surely seems to look like a planet. Yet, some people keep stating there are 9 planets in our solar system, while others say there are <em>more</em> than 9 planets, and then again there are people stating that the latest definition of &quot;planet&quot; has kicked out Pluto too so there are actually <em>fewer</em> than 9 planets in our solar system.</p>
<p>Trying to get a definite, official, and astronomically correct answer I can actually rely on, I'm therefore asking: <em><strong>How many planets are there in this solar system?</strong></em></p>
<p><strong>EDIT</strong></p>
<p>The <a href="http://en.wikipedia.org/wiki/Definition_of_planet" rel="noreferrer">&quot;Definition of planet&quot; at Wikipedia</a> doesn't really help either, as it states:</p>
<blockquote>
<p>Many astronomers, claiming that the definition of planet was of little scientific importance, preferred to recognize Pluto's historical identity as a planet by &quot;grandfathering&quot; it into the planet list.*</p>
<p><sub>* Dr. Bonnie Buratti (2005), &quot;Topic — First Mission to Pluto and the Kuiper Belt; &quot;From Darkness to Light: The Exploration of the Planet Pluto&quot;&quot;, Jet Propulsion Laboratory. Retrieved 2007-02-22.</sub></p>
</blockquote>
<p>So, if you link somewhere to provide proof, it would be great if you could point me to a more trusted source than Wikipedia. Ideally, an astronomical trusted source and/or paper.</p>

CC BY-SA 3.0
2015-07-13T04:24:41.190Z
For an authoritative statement on the planetary status of Pluto see the  [IAU's statement](http://www.iau.org/public/themes/pluto/) on the matter.
Conrad Turner
CC BY-SA 3.0
2016-04-25T12:35:54.713Z
I would just like to add that Eris was named after the greek Goddess of strife and discord, because it was her discovery that forced the IAU to write down the definition of a planet, thus demoting Pluto. It also has a moon, Dysnomia, which is the daughter of Eris, and the goddess of anarchy.
Nico
CC BY-SA 3.0
2013-10-10T01:16:14.850Z
Ceres was classified as a planet for a number of years after its discovery in 1801. It was reclassified after it was found to be just one (the largest) of a number of similar bodies. Pretty much the same thing happened with Pluto; astronomers reconsidered its status after they started discovering other similar bodies. Perhaps a consistent definition of "planet" is "substantial bodies in the Solar System of which there are only a few".
Keith Thompson
<solar-system><planet><dwarf-planets>
How many planets are there in this solar system?
e-sushi
# How many planets are there in this solar system?

(Asked by: username_0 on 2013-10-08)

So, in school (that's a long time ago) they have been teaching us there are 9 planets in our solar system.

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune
9. Pluto

But every now and then I keep reading stories about another "dwarf planet" (Eris, discovered in 2005) that - depending on what source tells the story - is another planet according to the astronomical definition, while other sources say that it isn't a planet. Some even say Pluto isn't a planet anymore either.

The result: I'm confused due to the contradicting stories. Even [Wikipedia isn't clear about Eris](http://en.wikipedia.org/wiki/Eris_%28dwarf_planet%29) and only writes (emphasis mine):

> NASA ***initially*** described it as the Solar System's tenth planet.

Initially? So, is it a 10th planet or not? Fact is, there is another "something" out there and it surely seems to look like a planet. Yet, some people keep stating there are 9 planets in our solar system, while others say there are *more* than 9 planets, and then again there are people stating that the latest definition of "planet" has kicked out Pluto too so there are actually *fewer* than 9 planets in our solar system.

Trying to get a definite, official, and astronomically correct answer I can actually rely on, I'm therefore asking: ***How many planets are there in this solar system?***

**EDIT**

The ["Definition of planet" at Wikipedia](http://en.wikipedia.org/wiki/Definition_of_planet) doesn't really help either, as it states:

> Many astronomers, claiming that the definition of planet was of little scientific importance, preferred to recognize Pluto's historical identity as a planet by "grandfathering" it into the planet list.\*
>
> \* Dr. Bonnie Buratti (2005), "Topic — First Mission to Pluto and the Kuiper Belt; "From Darkness to Light: The Exploration of the Planet Pluto"", Jet Propulsion Laboratory. Retrieved 2007-02-22.

So, if you link somewhere to provide proof, it would be great if you could point me to a more trusted source than Wikipedia. Ideally, an astronomical trusted source and/or paper.

**Tags:** solar-system, planet, dwarf-planets
> **Comments:**
> **username_3**: Ceres was classified as a planet for a number of years after its discovery in 1801. It was reclassified after it was found to be just one (the largest) of a number of similar bodies. Pretty much the same thing happened with Pluto; astronomers reconsidered its status after they started discovering other similar bodies. Perhaps a consistent definition of "planet" is "substantial bodies in the Solar System of which there are only a few".  
> **username_1**: For an authoritative statement on the planetary status of Pluto see the  [IAU's statement](http://www.iau.org/public/themes/pluto/) on the matter.  
> **username_2**: I would just like to add that Eris was named after the greek Goddess of strife and discord, because it was her discovery that forced the IAU to write down the definition of a planet, thus demoting Pluto. It also has a moon, Dysnomia, which is the daughter of Eris, and the goddess of anarchy.  

---

## 6 answers

### Answer by username_4 on 2013-10-09. Score: 102

In addition to Undo's fine answer, I would like to explain a bit about the motivation behind the definition.

When Eris was discovered, it turned out to be really, *really* similar to Pluto. This posed a bit of a quandary: should Eris be accepted as a new planet? Should it not? If not, then why keep Pluto? Most importantly, this pushed to the foreground the question

> what, *exactly*, is a planet, anyway?

This had been ignored until then because everyone "knew" which bodies were planets and which ones were not. However, with the discovery of Eris, and the newly-realized potential of more such bodies turning up, this was no longer really an option, and *some* sort of hard definition had to be agreed upon.

The problem with coming up with a hard definition that decides what does make it to planethood and what doesn't is that nature very rarely presents us with clear, definite lines. Size, for example, is not a good discriminant, because solar system bodies come in a continuum of sizes from Jupiter down to meter-long asteroids. Where does one draw the line there? Any such size would be completely arbitrary.

There *is*, however, one characteristic that has a sharp distinction between some "planets" and some "non-planets", and it is *the amount of other stuff in roughly the same orbit*. This is still slightly arbitrary, because it's hard to put in numbers exactly what "roughly" means in this context, but it's more or less unambiguous.

Consider, then a quantity called the "planetary discriminant" µ, equal to the ratio of the planet's mass to the total mass of other bodies that cross its orbital radius and have non-resonant periods (so e.g. Neptune doesn't count as sharing Pluto's orbit) up to a factor of 10 longer or shorter (to rule out comets, which has little effect in practice). This is still a bit arbitrary (why 10?) but it's otherwise quite an objective quantity.

Now take this quantity and calculate it for the different bodies you might call planets, comparing it to both the objects' mass,

![Planetary discriminant vs mass](https://i.sstatic.net/iKpkE.png)

and their diameter,

![Planetary discriminant vs diameter](https://i.sstatic.net/MzTOZ.png)

or with an arbitrary horizontal axis, in order of decreasing discriminant,

![Planetary discriminant on an arbitrary horizontal axis, in order of decreasing discriminant](https://i.sstatic.net/yqfo1.png)

Suddenly, **a natural hard line emerges**. If you look only at the mass and the diameter of the objects (shown in the insets above the plots), then there is a pretty continuous spread of values, with bigger gaps between the gas giants and the terrestrial planets than between Mercury and Eris/Pluto. However, if you look at the planetary discriminant, on the vertical axis, you get a very clear grouping into two distinct populations, separated by over four orders of magnitude. There's a finite set of bodies that have "cleared their orbits", and some other bodies which are well, well behind in that respect.

This is the main reason that "clearing its orbital zone" was chosen as a criterion for planethood. It relies on a distinction that is actually there in the solar system, and very little on arbitrary human decisions. It's important to note that this criterion need not have worked: this parameter might also have come out as a continuum, with some bodies having emptier orbits and some others having slightly fuller ones, and no natural place to draw the line, in which case the definition would have been different. As it happens, this is indeed a good discriminant.

For further reading, I recommend [the Wikipedia article on 'Clearing the neighbourhood'](http://en.wikipedia.org/wiki/Clearing_the_neighbourhood), as well as the original paper where this criterion was proposed,

> What is a planet? S Soter, [*The Astronomical Journal* **132** no.6 (2006), p. 2513](http://dx.doi.org/10.1086/508861). [arXiv:astro-ph/0608359](http://arxiv.org/abs/astro-ph/0608359).

which is in general very readable (though there are some technical bits in the middle which are easy to spot and harmless to skip), and from which I took the discriminant data for the plots above.

---

**Edit:** I must apologize for having included, in previous versions of this post, an incorrect plot, caused by taking [data from Wikipedia](http://web.archive.org/web/20131015071825/http://en.wikipedia.org/wiki/Clearing_the_neighbourhood) without verifying it. In particular, the planetary discriminant for Mars was wrong (1.8×105 instead of 5.1×103), which now puts it below Neptune's instead of just below Saturn's, but the overall conclusions are not affected. The Mathematica code for the graphics is available at `Import["http://goo.gl/NaH6rM"]["https://i.sstatic.net/CQA4T.png"]`.

---

... and, as a final aside: Pluto is *awesome*. It was visited in July 2015 by the [New Horizons](https://en.wikipedia.org/wiki/New_Horizons) probe, which found a world that was much more rich, dynamic, and active than anyone expected, including what appear to be churning lakes of solid nitrogen ringed by mountains of water ice, among other marvels.

[![Pluto as seen from the New Horizons flyby, in enhanced colour](https://i.sstatic.net/Gz9sM.jpg)](https://en.wikipedia.org/wiki/File:Pluto-01_Stern_03_Pluto_Color_TXT.jpg)

(Note the image has been colour-enhanced to bring out the variety of surface materials; the true-colour version of this image is [here](https://commons.wikimedia.org/wiki/File:Nh-pluto-in-true-color_2x_JPEG.jpg).) I, personally, don't feel it's at all necessary to 'grandfather' Pluto into the list of planets to really feel the awe at the amazing place it is - it's perfectly OK for it to be a cool place with cool science, that is also not a planet.

> **Comments:**
> **username_9**: Very good summary on the criterion. This was new to me.  
> **username_3**: I was going to post a comment saying that any definition of "planet" must be arbitrary, given that, for example, Ceres and Mercury are much more similar to each other than Mercury and Jupiter. But that graph is fascinating. I note that it says much more about the bodies' spacial relationships to their neighbors than about the bodies themselves.  
> **username_4**: @KeithThompson Yeah, but as it turns out there's no definition that depends only on the bodies themselves that matches our intuition for "planethood". Put it this way: If Mercury's orbit was as full of other crap as Pluto's, we'd have noticed it ages ago. (And of course, for Ceres we did: it took fifteen months between the discoveries of Ceres and Pallas.)  
> **username_7**: +1, partly just for publicizing that plot! I hadn't seen it before either, but it really lends weight to the inclusion of that "clear-the-neighbourhood" criterion.  
> **username_8**: Would expanding or shrinking the zone searched for similar stuff leave the graph basically unchanged?  If there is any easy way to compute what the discriminant would be if a larger or smaller zone were used, that might help show that the factor of 10 may have been arbitrary, but the difference between planets and other things is not dependent upon that arbitrary factor.  
> **username_4**: @supercat That is a good question, and I honestly don't know. You should ask it separately!  
> **username_5**: Thanks for coming back and updating this! Still a fantastic answer.  
> **username_4**: @called2voyage To be honest I'm really embarrassed about this episode - the data on Wikipedia were dead wrong, and I simply didn't catch it through not doing my due diligence. (Even worse, the corresponding [edit history](https://en.wikipedia.org/w/index.php?title=Clearing_the_neighbourhood&oldid=575282009) in Wikipedia is garbled, which is worrying by itself.) Hopefully the new plots can make up for a mistake of that calibre on such a visible post.  
> **username_5**: @EmilioPisanty I appreciate your diligence on the matter. I refer to this answer frequently, so it is good to know that you are doing your best to ensure it is accurate.  
> **username_6**: Thanks for this great summary. It looks to me that the scientific community was looking for a good reason to leave Pluto out (to maintain the educational value of having a bunch of named bodies easy to remember, bring interest on the matter to young generations, etc.), and well, they found one tuning this parameter to their needs. I came looking for this detailed explanation of "cleaning up the neighbourhood", wondering how they managed to leave Jupiter out with all the moons and trojan asteroids etc., and where does the "neighbourhood" extend up to... but well it's something  

---

### Answer by username_10 on 2013-10-09. Score: 12

The correct answer is 8 (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune).

Pluto is not longer a planet since 2006 when the IAU adopted a [formal definition of planet](http://en.wikipedia.org/wiki/Definition_of_planet)

> **Comments:**
> **username_0**: [+1] but I still have the problem of not having any reliable proof to that. I added an edit to my question to explain why Wikipedia doesn't really help finding the answer to my question. If you could provide **an astronomical trusted source**, I would gladly accept the answer...  
> **username_11**: The answer isn't that simple. There could be 8 or 100  

---

### Answer by username_12 on 2019-05-18. Score: 1

Well, it depends on how you define a planet.

According to [IAU](https://astronomiac.com/how-many-planets-are-there-in-the-solar-system/) (InternationalAstronomical Union), an object should satisfy three conditions for it to be called a planet

1. It should revolve around the sun.
2. It should have enough gravity to form itself into a sphere.
3. It should have cleared space around itself (it should be a dominant object in that area)

Pluto was demoted as a planet because it failed the **third criteria**.
Currently, [there are only Eight planets](https://astronomiac.com/how-many-planets-are-there-in-the-solar-system/) that satisfy the three conditions

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

There are five dwarf planets officially recognised by IAU

1. Pluto
2. Ceres
3. Eris
4. Haumea
5. Makemake

> **Comments:**
> **username_13**: Thank you for your contribution.  It's not clear to me how this differs from the existing answers. We have quite a few unanswered questions, and others with answers that could be improved on.  Have a look at the list of new and unanswered questions to find one that you can answer. This will be more useful to the community than repeating content already found in other answers.  

---

### Answer by username_14 on 2021-07-23. Score: 0

According to a geophysical planet definition, as well as the IAU planets and dwarf planets, the largest moons are also planets: the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton.

<https://en.wikipedia.org/wiki/Geophysical_definition_of_planet>

Although other moons of Saturn and Uranus and also Charon look round to me also.

> **Comments:**
> **username_15**: I'm not that fond of that wikipedia-page, as it states Pluto-Charon as double planets. If that were true, why are the Earth and our Moon not counted as double planets, I mean the mass ratio does not differ that much: ±1/3 versus ±1/5)?  

---

### Answer by username_16 on 2023-02-07. Score: -1

This is not exactly what you asked, but I'm writing it anyway because it's relevant to the title question.

Strictly speaking, the answer is **at least 8, possibly more**. The "8" are Mercury / Venus / Earth / Mars / Jupiter / Saturn / Uranus / Neptune, as detailed in the other answers. These celestial bodies are unambiguously planets. Others (like Pluto, Ceres ...) are generally treated as dwarf planets, at least by professional astronomers.

The "possibly more" part is because the data isn't good enough to rule out the existence of other large planets around the Sun. See the [Planet Nine](https://en.wikipedia.org/wiki/Planet_Nine) article on Wikipedia. It's even possible that there is a "[Planet Ten](https://en.wikipedia.org/wiki/Planets_beyond_Neptune#Kuiper_cliff_and_%22Planet_Ten%22)". These planets are hypothetical, however, and it will take a direct detection to convince everyone that they definitely exist.


---

### Answer by username_17 on 2021-07-23. Score: -2

The other answers told how many planets are recognized, how many dwarf planets that aren't IAU-recognized planets are recognized and one that indeed includes Eris. In a month it will be fifteen years since the infamous decision for an extremely vague and therefore stretched definition that was trying hard to keep the number of planets as low as possible for some reason. The majority among ~4% of the IAU who attended the meeting feared if they accepted Ceres, Pluto and Eris, there would eventually be "hundreds" of planets.

Instead of the current allegedly "official" position, I will portray a history of what *should* have been done if there wasn't any panic about "hundreds of planets" and if one were to go by logic and science according to how planets and asteroids have been defined pre-2006. Let's see if the fear was even justified.

Since 1930 there were **9 planets**: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. This number was *subjectively correct* from 1930 to 2004 (not 06). In 2004 the Hubble telescope shot the first image of Ceres sharp enough to recognize features and its spherical shape:

[![Ceres Planethood Proof](https://i.sstatic.net/8cEF1.jpg)](https://i.sstatic.net/8cEF1.jpg)

At that time Ceres was considered the largest asteroid, and even today some still call it an asteroid for some reason (though they probably mean "asteroid belt object"). An asteroid is an irregular body, and Hubble proved that Ceres is near-perfectly spherical and therefore indeed a planet as it was recognized in the first half of the 19th century. This should have sufficed pre-2006 to reclassify it a planet again and even if you don't stretch the current definition too much it may be a planet as it's the largest and most massive object of the main belt. Whatever, Hubble proved Ceres was a planet, not an asteroid, and following the publication of the shot, Ceres should have gained planethood back. *This wasn't done which was the first falsification in the number and recognition of planets.*

If they had done so they would consequently have had to give planethood to the larger body Sedna in expectance that it must be a planet too. So if they did what would have been logical according to the then-understanding of celestial bodies, they would have accepted Ceres and Sedna as planets and there would be **11 planets** now.

Later the year, 2005 FY9, later named Makemake, has been discovered. 2005 FY9 is larger than Ceres and Sedna and therefore should have got planethood as well, and it would get it probably along with Quaoar which is larger than Ceres as well, and Orcus the "anti-Pluto" which has about the same size as Ceres and an orbit similar to Pluto. Hence there would be **14 planets** now.

In 2004/05 Haumea was discovered which was named a dwarf planet by the IAU in 2008. However, Haumea is a Jacobi ellipsoid (rugby ball shape), and back then a planet needed to be "spherical", not to "overcome rigid body forces to pull itself into hydrostatic equilibrium". Therefore, rather than gaining planet status it would have been considered the largest asteroid, thereby replacing Ceres which got planethood back in 2004. If Haumea was recognized a planet, Vesta would have to be recognized too which isn't perfectly spherical either but more so than Haumea, and is internally differentiated, and Pallas and Varuna perhaps too.

In 2007, 2007OR10, later named Gonggong, has been discovered. Of course it would have been recognized a planet too as it is larger than Ceres, Orcus and Sedna. There would now be **15 planets**.

In 2017 and 18, the VLT shot sharp images of 10 Hygiea proving a near-spherical shape:

[![Hygiea sphericity VLT](https://i.sstatic.net/qIMGB.jpg)](https://i.sstatic.net/qIMGB.jpg)

Published in 2019, many consider Hygiea the smallest dwarf planet now. Hygiea would have beeen recognized a planet under the expectation that subsequent study would prove it to be one, maybe a probe would have been sent to study it from close up. As you see, planets are obviously considered more "important" than bodies unrecognized as planets. There would be **16 planets** now. There would be 20 planets if you add Haumea, Vesta, Pallas and Varuna. If Hygiea turned out to actually be spherical enough and differentiated, one may someday add 2002MS4 and Salacia as well so there would be 22 planets. In 2014 and 16 a hypothetical super-Earth or ice giant has been proposed to explain the very eccentric orbits of 6 TNOs; if you add it there are 23 planets (or **17 planets** if you don't count the six controversial ones).

And no more planets would come unless we discover yet another Earth-sized or larger planet beyond the hypothetical one, or discover a dwarf planet that has an orbit similar to Sedna and comes to perihelion, or send a space probe to a planet candidate (however, the other "dwarf planet candidates" are unlikely to be near-perfectly spherical or differentiated). WISE/neoWISE ruled out the possibility of Saturn- and Jupiter-sized planets to a certain distance.

The fear of "hundreds of planets" was unnecessary and is disproved. And even if there would have been, I don't see anything wrong with it. Non-professionals would only have to learn the eight largest ones and Ceres and Pluto for historical reasons perhaps.

> **Comments:**
> **username_18**: There are two problems with only considering "near sphericity". (1) The "nearly" part was only supposed to make allowance for oblateness due to rotation, plus gravitational distortion by nearby bodies; Hygiea is clearly "lumpy" and so would not qualify. (2) at least 32 known bodies in the solar system meet this criterion: the 8 acknowledged planets, the 9 largest known Kuiper belt objects, and the 15 largest moons (include Earth's moon). (Ceres 1 is the 32nd largest object, and confirmed to be spherical, so I'm assuming all the heavier objects are likewise spherical.)  
> **username_18**: "Planet" was greek for "wanderer", that is, a "star" that moved about, and visible to the naked eye. Pluto is quite invisible to the naked eye, so on that basis alone it should be excluded from the planets. https://astronomy.stackexchange.com/questions/51131/is-uranus-the-farthest-object-in-solar-system-that-you-can-see-with-naked-eyes  

---

