Sorry for taking too long to answer, got a little sick;
The problem is, that if i blur too much the planet's image (not the edges), all those little lights will get erased or would look very bad.
Check this (brightness increased):
http://fc02.deviantart.net/fs70/f/2014/185/5/b/5_by_hundsdorfex-d7p7ldy.png
http://fc05.deviantart.net/fs71/f/2014/185/6/0/6_by_hundsdorfex-d7p7lfd.png
http://fc04.deviantart.net/fs71/f/2014/185/3/1/6_1_by_hundsdorfex-d7p7lkc.png