According to Dynkin's classification, we have as possibilities these only, where n is the number of nodes:
To me it seems not obvious why there should not be a $E_9$? Further clicking got me to the following:
"Roughly speaking, symmetries of the Dynkin diagram lead to automorphisms of the Bruhat-Tits building associated with the group." from here.
Is there easy answer without going through the details of root systems?