These periods are used to indicate the base and surface temperature according to if an orbital object is at its closest, farthest or intermediary orbit from its star.
Base temperature is a temperature based only on the distance of the object from it's star and the star's luminosity. The formula is pretty simple:
252 / sqrt( DistanceAU / sqrt( StarLuminosity ) ) [in Kelvin]
The surface temperature modifies the basic temperature with the object's geophysical and atmospheric properties; like greenhouse gases, optical depth and convection.
These data constitute the basic data required to process the regions.
So, since this part is completed I begin now the implementation of the regions themselves.
Just to give you an idea of the result since the start of the FUG's development, here's a XML file of a generated planetary system:
Google Docs XML file
Its generation is totally random, excepted for the star where I taken the Sun's data. The configuration of this system is fairly varied and even contain a planet with liquid water. There's no asteroid belt in this one, but anyway it's not the most interesting object to show :)