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So, in school (that's a long time age) 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 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 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.

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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 Oct 10 '13 at 1:16
    
For an authoritative statement on the planetary status of Pluto see the IAU's statement on the matter. – Conrad Turner Jul 13 '15 at 4:24
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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 Apr 25 at 12:35
up vote 50 down vote accepted

Since we're talking about terminology, we need to remember that none of this really matters, outside of clarity when communicating. Still, some people tend to have rather strong opinions on it, thus confusion about how many planets are really in the solar system arises.

The people

The most trusted source in Astronomy would have to be the people that set the generally accepted rules. The IAU (International Astronomical Union) has been in existence since 1919 and is comprised of 10814 Individual Members in 93 different countries worldwide. Of those countries, 73 are National Members.

The key activity of the IAU is the organization of scientific meetings. Every year the IAU sponsors nine international IAU Symposia. The IAU Symposium Proceedings series is the flagship of the IAU publications. Every three years the IAU holds a General Assembly, which offers six IAU Symposia, some 25 Joint Discussions and Special Sessions, and individual business and scientific meetings of Divisions, Commissions, and Working Groups. The proceedings of Joint Discussions and Special Sessions are published in the Highlights of Astronomy series. The reports of the GA business meetings are published in the Transactions of the IAU - B series.

The definition

At the 2006 IAU General Assembly in Prague, the accepted definition of a planet was debated vigorously. The outcome of the meeting was the currently accepted definition of a planet:

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.

What about Pluto?

With this in mind, the group decided on Pluto's fate. From page 2 of this official resolution document:

The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects.

The reason given in the FAQ on this page:

Q: Why is Pluto now called a dwarf planet?

A: Pluto now falls into the dwarf planet category on account of its size and the fact that it resides within a zone of other similarly-sized objects known as the transneptunian region.

Basically, they decided that it isn't officially a planet anymore because it didn't match criteria (c): has cleared the neighbourhood around its orbit. It hasn't done this, because it 'resides within a zone of other similarly-sized objects'. Therefore, it hasn't cleared its neighborhood.

Soo... what about the number of planets?

Q: Based on this new definition, how many planets are there in our Solar System?

A: There are eight planets in our Solar System; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Mnemonic: My Very Educated Mother Just Served Us Nachos.

But that's if you don't count Dwarf planets - if you do count them, you end up with five more:

  • Ceres
  • Pluto
  • Eris
  • Makemake
  • Haumea

So there are 8 planets in the solar system if you don't count Dwarfs, 13 if you do.

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Thanks for your perfectly clear answer, which lifts my confusion about all the contradicting writings I have been reading. I especially appreciate the time and effort you've put into your answer. Superb! [+1] and accepted. – e-sushi Oct 9 '13 at 0:43
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Isn't it actually likely that there are other dwarf planets, we just didn't discover them yet? – svick Oct 11 '13 at 13:31
    
I'd argue it doesn't fit (A) either; since the barycenter with Charon lies outside of the planet, it isn't orbitting the sun directly in the same manner as, say, Earth (the planet off the top of my head with the most shift due to a moon's mass). Was any thought by the IAU given to this? or was (C) the only reason for dwarf planet definition? – Sarah Bailey May 22 '15 at 13:01
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@SarahBourt A good question. A definition will always be tricky. It is within the realm of scientific possibility that an exoplanetary system could have two large planet-like masses in a bound state. It is theoretically possible to have two Jupiter sized planets bound to each other, with the barycenter outside either of them. Are these Jupiters planets, or not? What if two Earths were bound to each other? Maybe we'd call such a thing a "double planet". But then what of an Earth orbiting a Jupiter? The barycenter would be inside Jupiter still. Is it a moon, or a planet, then? – zibadawa timmy May 29 '15 at 20:34
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@LDC3 Only the five bodies mentioned in the answer are officially considered dwarf planets by the IAU. The reason is that we do not yet know enough about the bodies you mentioned to tell for sure if they are massive enough to reach hydrostatic equilibrium (although many suspect it). – Philipp Jun 22 at 0:02

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

and their diameter,

Planetary discriminant vs diameter

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

Planetary discriminant on an arbitrary horizontal axis, in order of decreasing discriminant

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', 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. arXiv: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 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"]["http://i.stack.imgur.com/CQA4T.png"].


... and, as a final aside: Pluto is awesome. It was visited in July 2015 by the 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

(Note the image has been colour-enhanced to bring out the variety of surface materials; the true-colour version of this image is here.) 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.

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Very good summary on the criterion. This was new to me. – Arne Oct 9 '13 at 21:26
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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 Oct 10 '13 at 1:11
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@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 Oct 10 '13 at 15:42
    
+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 Jul 4 '14 at 6:52
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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 Oct 9 '14 at 20:43

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

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[+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 Oct 9 '13 at 0:17

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