Hafjian is an inhabited planet, a Federation colony planet located somewhere in the galaxy's Alpha or Beta Quadrants. The world's high-gravity conditions forced the inhabitants to wear Lieber exoskeletons to function under these conditions.
In the early 2250s decade, Hikaru Sulu and his family temporarily lived on Hafjian. Upon being posted to the USS Enterprise Sulu was advised by Doctor Leonard McCoy that living on that world might cause complications with his heart after reaching the age of 70. (TOS novel: Enterprise: The First Adventure)