Ghost Worlds and CTEs

The Dangers of Terraforming

The Buildings stood like sentinels when they approached. Green grass blanketed the land, growing tall and soft. They ventured into what should have been the town square, but not a single resident was around to greet them. The metal structures were silent. The generators wern't running, the lights were all off, and even the water purifier stood still and without purpose.

"Where is everyone?" Oracle said as he reached up to remove his helmet, the atmosphere was breathable after all.
"Stop," Mouse replied, reaching out on instinct, then recoiling when their hands nearly touched. "We can't expose the environment to human biology, and vice versa, just because no one's around."

Oracle nodded. "Who built this place?"

"Not us," Mouse replied. She activated her implant, a process that took little more than a thought. Deposits of memory appeared and disappeared from her mind. "I can't find any mention of it. This must be a brand-new colony." She stopped to sift through old data, analyzing ship paths, projected projects, black sites, and terraforming logs. Her eyes lit up, as if remembering a long forgotten dream. "Got it. It was terraformed by the…" she paused shuddering as she tried to pronounce the name, "Tick, Tcklihtya? They're an avian species...carbon based. There should be over 100 colonists here."

Argus chimed in, speaking over the radio from the ship, "Maybe they're migratory."

Roadrunner joined the chatter on the mic, "No."
"Find something?" Mouse asked turning to see him shining a light through a window.

"Ghost World.." He replied. "It has to be."

Like the stars in stellar nurseries across the cosmos, we are destined to die. Our gene pools deplete, or we destroy ourselves before it can happen. Maybe someone else comes along and wipes us out. Regardless, we are destined to nourish the soil for the next species to exploit. As eons pass, we supply the fossil fuels they burn. It can't be stopped. Sometimes, terraforming a planet can interrupt this process. It can wake something up.
Even the most desolate world can hide something in a deep pocket beneath its surface, a tiny sliver of what lived before. This biome thrives where it is, but give it a better environment to flock to, and things can spiral out of control.
Stranger things can lurk on a distant world. Who knows how many creatures will rise to breathe the air you provide, expand in the new wilderness you build, or eat what you never intended to be eaten.

Focused query: Ghost worlds

Ghost worlds are a type of planet. They are a paradise, all things considered. The atmosphere is perfect for the species that created it, and the entire landscape is teaming with gorgeous views and complex life.
Colonies may litter the surface of the planet, but be warned. A ghost world is nothing but a death trap. Touch the surface without protection, and you're likely to meet the fate of its missing colonists.
Ghost worlds are so named because despite their beauty and against all reason, no one has bothered to colonize it. Those who tried will, thankfully, leave their bodies behind as a warning to any who seek to follow in their example.

C.T.Es

A CTE, or Catastrophic Terraformation Event, occurs during the terraforming process of another planet. Life is not special. It isn't unique. Some species appear and go extinct at a constant rate. On a long enough timeline, the survival rate of any species will inevitably reach zero. Such is the way of life. Rinse and repeat.
Enormous effort goes into preventing CTEs. Drones scour the surface, dig as far down as they can go. They search every crevice, scour every ocean. The resources needed to terraform a planet are costly, and it would be a shame to waste.
It's easy to mark a planet as a bad candidate and move on. Smaller problems can be forcibly removed, but sometimes, something gets overlooked. Hundreds of colonists can end up dead for one reason or another. It happens enough to be considered an occupational hazard.

Triggers

Any living organism, no matter how small, is a trigger for a CTE. Even detecting carbon monoxide in the atmosphere is a dead giveaway for carbon based species. To verify a terraforming candidate, the planet is studied vigorously. Stations will be built in orbit to analyze data and samples from the surface.
Diagnosing it is trickier. There is no guarantee a CTE will occur even with the presence of life. When humanity settled on Safeharbor, it did so with few problems, a small miracle considering we knew nothing of the dangers at the time. This is why the stations are placed. Any old planet can be altered. Finding the ones you want to change is not as important as finding the ones you don’t. It's a much longer list, by far.
CTEs are not only biological, however. Sometimes terraforming goes wrong on a mechanical level. Restarting the planet's core and creating a magnetic field can cause chaotic weather, tectonic activity, and other disastrous phenomena. These are just easier to account for and easier to handle. They are also a waiting game. Eventually these things will subside. A ghost world will be that way indefinitely.

Ethics

Many in the galactic community hate the very idea of terraformation. They see an ethical problem in altering planets beyond their natural state. It’s not how nature intended it to be, so why change it?
Terraforming in general costs resources that usually come from somewhere else, further depriving that planet. Life on these worlds could die in the process, or thrive when they are best kept at bay.
Most species do not share this sentiment, humanity included. We see the universe as a canvas. We will paint what we so choose. It should be said that were awfully bad at it. We still have yet to figure out the minute details.

Mouse and Oracle rushed forward to look. The light passed through the glass, revealing numerous bodies at various stages of decomposition. Each body rested on a bed, save for some that looked like they died crawling to the door. The creatures had wings and long hooked beaks. Their bodies were coated in matted feathers, stained by blueish blood.

"Damn," Oracle said, resisting nausea.

"They came seeking a new life, and they ended up dying of an outbreak." Mouse said. "Argus start decontamination."