As shown in the first screen snapshot below, the Modify Planet item in the vault says, "Adds +1 to its usable tiles.".  I am going to use it on my core world named "Sagarmatha I" whose properties are shown in the second screen snapshot below.  Note that it is a Class 29 planet.

After I apply the Modify Planet item to Sagarmatha I it becomes a Class 32 planet as shown in the third screen snapshot below.  This means that the Modify Planet item adds +3 to the target planets usable tiles not +1.  I recommend that the text for the Modify Planet item be changed to say "+3" instead of the current "+1".