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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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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". –  Keith Thompson Oct 10 '13 at 1:16

3 Answers 3

up vote 35 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

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

Pluto is not longer a planet since 2006 when it was 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
    
At least in English, the usual spelling is "Mercury"; I've never seen it spelled "Mercurius". –  Keith Thompson Oct 10 '13 at 0:00

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 periods up to a factor of 10 longer or shorter. 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:

enter image description here

Suddenly, a natural hard line emerges. There's a finite set of bodies that have "cleared their orbits", and some other bodies which are well, well behind in that respect. Note also that the vertical scale is logarithmic: Neptune's planetary discriminant is ~10,000 bigger than Ceres'.

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, from which I took the data for the image. If you don't mind skipping over some technical bits, go for the original paper where this was proposed,

What is a planet? Steven Soter. The Astronomical Journal 132 no.6 (2006), p. 2513. arXiv:astro-ph/0608359.

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Very good summary on the criterion. This was new to me. –  Arne Oct 9 '13 at 21:26
    
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 at 6:52

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