Which Planet is the World of Avatar?

[Winter]

Referring to the Nickelodeon show this is something that's tied into a theory my sister and I have had for a while now. See, in Avatar: The Last Airbender more time seemed to have past then was logical given how much the characters physically develop, they all seem to be closer to a year and a half or 2 years older at the end of the series despite only 3 seasons passing (Winter, Spring and Summer). So, my sister and I came up with a theory that a year in the world of Avatar is longer then a year on Earth.

We figured that World of Avatar has an orbit that last for around the same time it takes Mars to obit the sun, around 687 days but could also be at the same distance as the Ateroid Belt in our system. This was more or less backed up in the Legend of Korra when we finally saw the planets in the world of Avatar.



As you can see the third planet from the sun has Rings which means it can't be the world of Avatar which means there are only 3 options for which world where the one the series takes place in. Given that we don't know the size of the sun in Avatar it could mean that the Goldie Locks Zone where a planet needs to be in order to have life could be further away then what we have in our system. My guess it's either the 4th or 5th rock from the sun as any further then that might be outside of the GLZ.

So, now which world do you think it is?