The Prometion Mining Planet is a world devoid of life, which is one of many planet types and, one of 2 mining planets in Galactic Civilizations III.
How to Identify?
Prometion mining worlds can be identified by the unique look to the planet and the promethion icon next to the planet description when hovering your mouse over the planet, it will indicate it's a prometion mining world. If the planet is able to be harvested.
Promethion mining planets is composed mostly of the rich resource promethion, which can be a very profitable resource. Typically these planets are found further away from its source star and can be mined by building a star base nearby.
Typically one would build a star base nearby to this planet type to mine the promethion resource. While building your star base keep in mind there may be other mineable resources around in Thulium Planets, asteroids, or black holes. I would suggest when building your mining star base to encompass as many of these resources as you can to minimize the number of star bases needed.