Notaban is a hydrogen-helium gas giant with traces of ammonia. Molecular nitrogen is present in the highest levels of the atmosphere, giving the planet its odd violet tint.
The upper levels of Notaban's atmosphere are inhabited by "shoals" of tiny ammonia-based lifeforms, no larger or more complex than the plankton of Earth's oceans. Held aloft by air pressure and wind, these bioluminescent creatures constantly flee from the approach of the terminator. Should they be exposed to the light and heat of Grissom, they die.