The Thulium 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?
Thulium mining worlds can be identified by the unique look to the planet and the thulium icon next to the planet description when hovering your mouse over the planet, it will indicate it's a thulium mining world. If the planet is able to be harvested.
Thulium mining planets are composed mostly of the rich resource thulium, which can be a very profitable resource. Typically these planets are found relatively close to their 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 thulium resource. While building your star base keep in mind there may be other mineable resources around in Promethion 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.