A zoo planet is a planet with a Wildlife Sanctuary on it. Just place 15 Wildlife Sanctuaries on planets to transform them into zoo planets. You can use planets without Spice to place them, since you can't build colonies on them. The planet must also have a T-score of at least 1 and no sentient beings are present. (only on that planet) To get the Wildlife Sanctuary tool, get the Eco Hero 3 or the Zoologist 3 badge.