When we play Star of Death on a planet where there are some ennemy leaders (Diplomat, Amiral etc), theses one are put elsewhere on another planet isn't ?

They are not dead ?

From the rules

If a Leader is present on a planet whose ownership
changes for any other reason than invasion, the
Leader automatically escapes and may be placed on
any friendly planet.
Leaders can never exist on neutral or enemy planets,
nor in space by themselves.

Since Star of Death does not mention Leaders and has not been changed in the FAQ or Errata then the Leaders will escape and go to another planet of that players choice.