Notanban 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 Notanban'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.
The system cannot be older than 10 million years. Notanban is likely a bloated Neptune in mass. With a radius of 73,314 km and a 16.9 Earth hour day the ammonium-based life forms in the atmosphere would need to be travelling at more than 27,000 km/h in order to stay ahead of the terminator.