In the argot of the original, Expansion-era space explorers, certain worlds proved so dangerous to survey crews that they were dubbed bad planets. This was not a scientific term arising from rigid criteria, but a folk reputation accrued through successive death tolls. Later, in the Combine era, the designation ULC (Unremittingly Life-Compromising) arose to replace it in official stellar mapping databases. The formula for ULC designation is too complicated to explain here but looks for the following characteristics:

• requires protective gear to safely explore
• subjects a crew to conditions or life-forms likely to compromise that protective gear
• life-forms comprise a hyper-predatory ecosystem (HPE), producing aggressive fauna (or flora) whose defenses and attacks make them a grave and constant threat even to trained, well-armed personnel
• features conditions likely to foster rapid psychological debilitation

During its heyday, the Combine legally forbade travel to ULC worlds and set up patrols and armed buoys to enforce quarantines.

Such measures may still be operative in the core worlds but have fallen away in the Bleed. Since the war, lasers have revived the original terminology. As aggressive contractors, you might not avoid a bad planet altogether, as Combine exploratory teams stayed away from ULCs. Still, you’ll certainly take every possible precaution, knowing your crew faces highly life-threatening circumstances from the moment they land until their successful return to orbit.

from The Justice Trade, page 30