Need little more information please:
Are you trying to change the name of the planets after you place them? The map editor does have some bugs, but there are ways to work around them.
This problem applys to all roman numericly related objects.
I hope they get things fixed real so I can get on naming systems correctly.
The problem arises because when deciding on what to call the next planet the Map editor searches all the other planets to see waht they are called. And if its named differently it cocks up.