The Toxic Planet is a world very toxic to most life forms, which is one of many planet types and, one of 5 extreme planets in Galactic Civilizations III.
How to Identify?
Toxic worlds can be identified by the toxic icon next to the planet and when hovering your mouse over the planet, it will indicate it's a toxic world. If the planet is not able to be colonized it will give a message in the description that indicates "Requires Atmospheric Cleansing".
Toxic worlds are extremely toxic making it very hazardous to most life forms in the galaxy. Unfortunately, for most races this makes it a deadly planet where all but the most hearty lifeforms can survive. Because of this it is one of five extreme planets which require additional technologies to be able to survive in this environment.
One may try to research the technology Atmospheric Cleansing as soon as possible, especially in a galaxy crowded with races and limited expansion opportunities. Suggestion: Colonize normal planets first and extreme planets second. When choosing a Radioactive planets to colonize, choose something on the border of your neighbors ZOC (zone of control [influence]) to attempt to persuade inhabitants in the area to join your culture.
Which races able to immediately colonize?
Currently there are no races that are able to colonize this planet type at the start of the game, however, it is possible that a race would be added in the future in a DLC (Downloadable Content) or expansions that this will become possible.