This is so later on you don’t go back to a planet you’ve already been to thinking it still has resources. As you use up resources on a planet, fill this column in with X’s for the resources you exhausted. Rich is defined as having at least 9,000 for Iridium, Platinum, and Palladium and at least 1,000 for Element Zero. Given the rarity of human biotics, it seems likely. The key for the letters is at the bottom of the table. HAT-P-18b AND HAT-P-19b: TWO LOW-DENSITY SATURN-MASS PLANETS TRANSITING METAL-RICH K STARS. Only one of the requirements must be met to unlock that part of the galaxy.
I took the information from the Mass Effect wiki about the best planets for Iridium, Platinum, Palladium, and Element Zero and compiled it into a nice table you can edit as you play the game.
