Why Pluto is not a Planet?

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