The Radioactive Planet is a world devastated by radiation, which is one of many planet types and, one of 5 extreme planets in Galactic Civilizations III.
How to Identify?
Radioactive worlds can be identified by the radioactive icon next to the planet and when hovering your mouse over the planet, it will indicate it's a radioactive world. If the planet is not able to be colonized it will give a message in the description that indicates "Requires Atmospheric Cleansing".
Radioactive worlds are devastated by radiation either natural or by destruction with radioactive weapons or technology. 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 or racial abilities 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 to colonize normal planets first and extreme planets second. When choosing which 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 your influence.
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.