Helium, neon, argon, krypton, xenon, and radon are inert gases. Typically neon, argon, krypton or xenon are used. The group 8 elements include helium, neon, argon, krypton, xenon and radon. The bulbs are under vacuum or filled with argon, krypton or xenon. Besides argon, already mentioned, other noble gases, neon, helium, krypton, and xenon are also present. As of 2008, endohedral complexes with helium, neon, argon, krypton, and xenon have been created. The halogen is normally mixed with a noble gas, often krypton or xenon. The chemistry of the heavier noble gases, krypton and xenon, are well established. Also, this usually precludes the use of very expensive gases like krypton or xenon. Superman is born Kal-El on the alien planet Krypton. There are also unverified reports of a barium salt of a krypton oxoacid. Dru-Zod is a megalomaniacal Kryptonian, in charge of the military forces on Krypton. SpaceX Starlink use krypton as propellant for their electric propulsion system. The script ends with Superman flying off to Krypton in a spaceship. Thoron, which is in the same star system as Krypton.