Monoculture Worlds

There are a number of worlds around the galaxy where the evolution of life has been effectively halted by a single species (and sometimes a single organism) that came to dominate the biosphere.  These are known to planetary scientists as monoculture worlds.  In the whole galaxy, not a single monoculture world harbors intelligent life, except where explorers have come from another planet.

Usually, monocultures are dominated by early-evolving plant-like organisms that grow so aggressively as to block out the sunlight that its competitors would need, while producing potent poisons that kill off nascent herbivores.  These dominant organisms often do have other, simpler symbiotic partners that exist only to the extent that they serve the dominant species. 

On a few monoculture worlds, the dominant species has evolved past the need to eliminate competition, and the dominant species prove edible.  The aggressive growth cycle of these species can provide an abundant food source for nearby inhabited worlds and colonies.  This aggressive growth also presents a hazard, for when these dominant organisms are transplanted, they often become invasive pests.

Icebox Worlds

Rastaban 2 once had a thriving ecosystem before the gravity of a rogue planet drew it out beyond the habitable zone.

Icebox worlds are places that were once warm enough to support life, but froze over after life started there, leaving a whole biosphere of frozen life-forms.  A number of these are found throughout the galaxy. 

This can happen for a number of reasons.  Some of these worlds were nudged from their habitable orbits by outside influences, another star passing close to their solar system, or the gravity of a rogue planet.  Others were warm when their stars were younger, but time has depleted the energy of their suns (often a red giant shrinking into a dwarf star).  Icebox worlds can also be caused by volcanism filling the atmosphere with opaque debris and smoke that causes a cooling feedback loop, and in rare cases a civilization arises that creates a nuclear winter scenario.

The first Icebox world to be discovered was Uranus's moon Oberon, which was found to have simple life forms frozen into its icy outer layer.

In some cases, when a planet is still in the habitable zone, terraforming equipment such as heat-mills and atmospheric conditioners is brought in, and the planet is colonized.

The home planets of the Gorox and the Wuxria are considered to be borderline Icebox, but not fully included in this category due to the persistence of advanced lifeforms.