Some planets only appear once in a while when your are banging planets. Example the planet class Amfar usually takes about 100+ bangs to get one, where as the m class earth appears over and over. What setting makes the planet harder to bang?

You can't change how often it will appear, that is an internal configuration. Although, the more planets you add over the stock planet types or non-gold planet types, the more difficult it is to get them. There is a trick though, if you fill the sector up with planets the gold planets will start appearing pretty much everytime you pop a new planet. Although leaves you to deal with NavHaz once you have obtained the planets you were shooting for.

so the stock planets show up more than the gold planets. So if I want a planet to bang less frequently make it gold correct

I believe it has to do more with the number of available planets VS the number of planets allowed per sector, then it has to do with it being an edited planet. Additionally it seems as if the more planets there are available, the harder the higher numbered ones are to obtain. I dont know what the exact formula used is, although it has to be partly randomized.

I notice that even playing stock planets it is more difficult to get a Volcanic or Mountainous than a Desert, but even still you are pretty much guaranteed one within twenty or so attempts. Maybe it is something like the first few have higher obtainment odds, for example the first four are considered common (I am just using an example) and anything over that starts getting more and more difficult to get, with the numbers above that four having a smaller chance at obtainment. The marker could just as well be 5, 6, 7, etc., this to me explains why it takes hundreds of attempts to get those higher planet numbers, which are always created as gold planets. Also this is more then likely effected by the maximum allowable planets per sector. Then again maybe the planets class/subclass is also figured into the equation.

I would say put your more valued planets on the bottom of the list and those to be used as common up on top, if you want to make it a rare commodity. Players are going to shoot for the best planets obviously anyways, but the goal is to make them work for it.

http://www.tw-cabal.com/strategy/Planetmaking.html

A small addendum to that article. It focuses on getting planets without leaving haz behind. However, the amount of haz in a sector and the amount of planets over the limit in the sector also factor into what planets you will get. In general, you will have better results with added gold planets if you first overload the sector by one planet, but it will leav eyou 10% nav haz.