Why Domed colonies on green dots

[Ruatha]

This is my first game and I'mmaking all the classic mistakes, have been helped by this forum, thanx.

I play with a methane gas giant as home world.
Now I've found my first planet that has a green dot, a large gas giant with methane and mild conditions. When I build my colony there it's domed. Am I doomed to have domed colonies forever? My homeplanet that is a methane gas giant isn't domed.