Planet Conditions

[Akurgan]

in the manual(ingame and book) it says that the Planets range from Pleasant to Deathly.

now ive seen:

unpleasant
mild
good
optimal

is there more?
what is the exact order of conditions, ranging from worst to greatest?
theres a few planets of mine that i want the best condition, and some Messages have said
" the planet X has reached its best possible atmosphere condition, further development of the conditions are not needed( or something like that )
but... none of my planets are ranked "Pleasant"

are some planets unable to go all the way to 'best condition".. example after 4 years of climate control... planet Z is maxed at condtion "Mild"?

thanx guys