Why Pluto is not a Planet?

May 8, 2011

Pluto, a planet discovered in 1930 and named after Roman God of the Underworld; was omitted from the status of ‘Planets’ and was relegated to that of a ‘dwarf’. People still wonder what is the reason behind it and those who know about astronomy they can give you the answer. Why? Well it all has to do  with the definition of word ‘Planet’.
I didnt know a lot about the word ‘Planet’ until the PLUTO scenario was raised. And when IAU (International Astronomical Union) demoted the then planet PLUTO, I searched the definition of Planet and it gave me some very new information which I want to share with you people.

Let us begin with the discovery of planet Pluto.  Pluto was first observed when astronomers noticed that the orbit of Uranus is disturbed by another unknown planet which was then called as PLANET X. This event took place around 1840s and PLUTO was discovered in 1930.

The Solar System now had 9 planets.

That was the discovery of planet PLUTO. Now we take our focus back to definition of PLANET. The definition of planet was never strict and it evolved as our knowledge of universe grew.  In 19th century the term Planet was used to refer to the objects in Solar System. Thanks to the advancements in the field of optics, the observatories on Earth and in space discovered many objects similar to those in the solar systems and possibility of other planets increased. Then came the objects discovered in Kuiper Belt which is the region around Neptune planet.

Astronomers estimate that there are at least 70,000 icy objects, with the same composition  as Pluto.  So Pluto is just another Kuiper Belt object. All these objects were smaller then Pluto or in some cases closer to it. Astronomers realized that it was only a matter of time before an object larger than Pluto was discovered in the Kuiper Belt. Starting in 2000, with the discovery of Quaoar and Sedna all comparable to Pluto in terms of size and orbit, it became clear that either they all had to be called planets or Pluto would have to be reclassified.  And in 2005, Eris was discovered which was further out than the orbit of Pluto that was probably the same size, or even larger. With Eris being same as Pluto, made of the same ice/rock mixture, and more massive than Pluto, the concept that we have nine planets in the Solar System began to fall apart.

Astronomers decided they would make a final decision about the definition of a planet at the XXVIth General Assembly of the International Astronomical Union, which was held from August 14 to August 25, 2006 in Prague, Czech Republic. Vote was called out. According to then concept of planet not only Eris but Ceres (which had been thought of as the largest asteroid) and other objects discovered with mass/size larger or smaller then Pluto  be called planet increasing the number of planets to 12. In the end, astronomers voted for the controversial decision of demoting Pluto (Ceres and Eris also) down to the newly created classification of “dwarf planet”.

So what is the new definition of Planet. IAU says an object is called planet if:

1) is in orbit around the Sun
2) it needs to have enough gravity to pull itself into a spherical shape
3) has cleared the neighborhood around its orbit.

What does “cleared its neighborhood” mean? As planets form, they become the dominant gravitational body in their orbit in the Solar System. As they interact with other, smaller objects, they either consume them, or sling them away with their gravity. Pluto is only 0.07 times the mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.

Any object that doesn’t meet this 3rd criteria is considered a dwarf planet.

And so, Pluto is a dwarf planet.

So how many planets are there in our Solar System?

Hope you now have clear idea of word ‘Planet’ and why Pluto is not a Planet anymore. Waiting for your comments.

Image courtesy of BlendArtGallery

  • Guest

    I still believe that pluto is a planet, it has four moons and it orbits the sun

    • kenzie

      that doesn’t mean it’s a planet

  • Hi

    pluto is a planet have u ben two space it’s might be allmost the same sive as earth

  • FreDro

    Call it a planet and by the time we’re done finding objects in the Kuiper Belt there will be dozens of planets. A few of the kuiper belt objects we’ve observed so far are over half the mass of Pluto and some even bigger than Pluto. If Pluto is counted as a planet then the large objects in the asteroid belt have to be planets also. Moons can’t be considered as qualification Planet status because even asteroids can have moons. 

    All the mass of everything in the Kuiper belt combined is only between 10-20 percent of Earths mass. If it were one large rock it would be half the size of our moon. There isn’t enough mass for accretion and objects constantly collide and break into smaller pieces. One day Pluto or other kuiper belt objects out there may collide and the dwarf planets like Pluto may break into to smaller pieces. 
    Don’t be surprised if they change the classification for Jupiter and Saturn.
    Jupiter and Saturn could be called a Brown dwarf It’s a failed star unable so sustain nuclear fusion but puts out more heat than it absorbs from the sun. So Jupiter could be it’s own solar system and it’s moons are planets.

    Personally I think they should all be classified as Planets but with sub categories for Dwarf Planets,  Brown Dwarfs, and Regular Planets.

    • Anth2000ony


  • Kenzie

    I feel sad for pluto

    • Pluto RULZ

      me too its also a planet:C

  • Someone

    It’s just a name, people… As children we have the idea a star is `n shiny thing when the sky is dark. Yet, eventually, we learn, stars are burning balls of fire, planets, satelites, moons, dwarf-planets, asteroids, rocks, ice, whatever. If it is in the sky and you can see it, it’s a star. Like you grow out of that way of thinking, you should this.

  • Danfws

    LOOK! mercury,venus,earth,mars,jupiter,saturn,uranus, and neptune WOW!

  • Danfws

    earth: your going down! pluto: THIS IS SPARTA!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

  • Danfws

    now pluto is angry

  • kalan

    A GAS GIANT! AHHHHHHHHHHHHHHH!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

  • kalan

    never again a gas giant

  • kalan

    who,s hungry?

  • kalan

    who,s thirsty?

  • kalan


  • kalan


  • kalan


  • kalan

    earth:ME! venus:ME! mercury:ME! mars:ME! jupiter:ME! saturn:ME! uranus:ME! neptune:ME! pluto:ME?

  • kalan

    earth:OH NO YOU DON,T!! pluto:grrrrrrrrrrrrrrr!!!!!!!!!!!!!THIS IS SPARTA!!!!!!!!!!!!!

  • kalan

    saturn is fun!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

  • Chelsey Darren

    ummmmm? I think pluto is not planet because that planet is far to sun? the planet pluto is the last planet but i’m just calling that planet pluto because that unfair :( i feel sad for pluto too :(

  • Pingback: Our Most Famous Dwarf Planet | Holly's Astronomy Blog()

  • antiPlutoDemotion


  • spamless

    This article seems to be the parent of various other ones found online that copy some of your text. I am astonished to see a blatant grammar error that apparently originated in this article replicated manifold across the Internet.

    You write above, “Any object that doesn’t meet this 3rd criteria is considered a dwarf planet.” Bzzzt! “criteria” is the plural of “criterion.” It’s not the 3rd “criteria,” it’s the 3rd “criterion.” Sheesh!