How many planets are there in this solar system?

  • 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 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?


    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.

    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".

    For an authoritative statement on the planetary status of Pluto see the IAU's statement on the matter.

    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.

  • Undo

    Undo Correct answer

    9 years ago

    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.

    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.

    Isn't it actually likely that there are other dwarf planets, we just didn't discover them yet?

    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?

    @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?

    You can add more dwarf planets: Sedna, Ixion, Orcus, Quaoar, and Varuna. There may be more as well.

    @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).

    This answer would benefit from adding LDC3's list as potential dwarf planets which are not awarded the designation at present purely because we do not know enough about them yet; this would make it clearer that the lower boundary of that category is hazy and subject to observational limitations.

    I've gotta say it. I'm still hoping for a ninth planet being discovered in the Kuiper Belt, even though articles I've read lately question its existence. I'd like to see the terms major and minor planets adopted to avoid confusion. I think there should be "major and "minor" moons, also with major moons being massive enough to be spherical. While we're at it I think a double planet system should mean that the center of gravity is not inside a planet (ie. yes for Pluto/ Charon and no for Earth/ Moon).

    amazing, I was not much aware of this fact. can you help me what duration does iua take to decide the given body is a planet or not? secondly if a small mter line asteroid fill the demands will it be treated as planet considering the fact that it's distance fro the sun is so much to overcome other gravitational forces

License under CC-BY-SA with attribution

Content dated before 7/24/2021 11:53 AM