Jantanis (ZG2)

A super-earth world which appears to be habitable but it absolutely isn't. Covered by thick clouds, it has a rather dense atmosphere made up of highly toxic materials. All vegetation has been contaminated and withered away long ago, leaving toxic substances in the ground to contaminate the surface and give the remaining rocks an unnatural purple color. The sand is actually copper sulphate, with the prolonged exposure to water keeping it blue. Said water, of course, is just as toxic as everything on the planet.

Jantanis was the home of the Xantalicans, a highly religious tripedal alien species which have made the artificial material Ditherium. It has similar properties to lava but is less dense and normally does not glow, only in certain refined forms. It was stored under high pressure in underground containers. The Xantalicans, however, have overused this substance which lead it to become even more dangerous. Both due to the excessive Ditgerium usage and poor reliability of the storage systems, the underground containers eventually bursted, spilling the substance on the surface, with the unshielded Ditherium releasing massive amounts of toxic elements into the atmosphere and ground. This eventually lead to their total extinction in their calendar year of 3093, equivalent to the human year of 1533 AD. Some ruins still exist today, including a well preserved observation tower on the south pole, alongside a ruined space center, a Ditherium refinery, a laboratory complex, and a few religious monuments.

Originally suggested by user SamTheFox (now RudimentaryCheezit) or chillymations (i have no clue anymore)