Oxygen

Sol 3 is a Class-14 Deathworld on what used to be a thirteen-point scale until they found it.

Not only is the planet very geothermally volatile with active volcanic systems AND feature violent and chaotic weather systems...

"Earth" is the deepest gravity well they've ever witnessed chemical rocketry successfully achieve orbit from.

The biosphere is teeming with pathogens, so much so that the sapient population's own bodies rely on symbiotic microbial colonies in order to digest nutrients among other tasks.

And the macroscopic fauna are ALMOST as scary as the microscopic stuff: every biome packed with highly adapted predators.

At the top of this complex carnal carnival of carnivory, the "humans" who live there are unstoppable pursuit and persistence predators highly naturally gifted in ranged combat that historically used to just WALK their prey to death. The animals which ancient humans consumed could sprint to temporary safety, but humans will catch up, ALWAYS catch up, and the prey will still be tired when they have to sprint again. Eventually the fatigue outpaces them, and humans catch up for the last time. Just walk right up and bash them with a rock, they might not even have to throw it: dinner is ready!

Furthermore, it's not just the highly volatile oxygen that all the animals there breathe... Sol 3's atmosphere also even contains a constant background presence of radon. The biosphere is passively resistant to some levels of radiation. One of the cities was consumed in the fallout cloud of an exploding nuclear fission reactor(they STILL use water to cool their municipal fission reactors even now!), and although the humans fled, the animals that stayed there are FLOURISHING. Deformed and mutated, but thriving.

NOBODY SANE CHOOSES TO GO TO SOL 3.