Look at it this way.
- No one "finds them". They are like exotica and have a related quest or story line(varying level requirements). The idea as presented by the OP is that they are SCANNED so you have to FIND them. Having them mission-based just gives the high power legions more toys while the rest of the galaxy just stares at a planet they'll never keep long enough to benefit from. I know its hard to read that only one may be owned at a time. That level would also effect it. I also find it funny that in the same paragrpah you say "gives the high power legions more toys" while arguing the planets should be scan depedant...Who i wonder has higher scanning power? Oh wait high levels and their legions.
- From a coding standpoint its not simple, but its not difficult either. (you already lose planetary bonus' every time you join and leave a legion. The mechanism exists, just need to change the outcome from lost stats to a lost planet. The mechanism for losing planets already exists as well in the form the 5gp abandon) Those two mechanisms are completely separate. Tying them together would create more problems than is worth Dan's time. Not entirely true. Will it create an interesting bug or two? Of course, when does that not happen? However your definite yet scant statement leads me to believe you dont understand how to code games of this type.
- Planet benefits should be benefiting the legion as well as the owner. (See Above post) Never said it shouldn't. The OP was player benefit heavy.
- You must be in a legion to own one. Why?? See above, why create a planet with legion wide benifits then allow it to be taken by a solo player....talk about pointless. plus at the level required to take them chances are the player will be in a legion anyways. This being a matter of fact enables the above.
- Like Exotica their defense will diminish to encourage thefts and player combat as well as prevent a legion from dominating a certain perk. (well it will attempt to prevent that as best you can) Exotica is an example of a legion (select group of elite legions actually) dominating a perk. One top legion takes it and passes it around for the perk until another top legion takes it. The number of legions to control Exotica (even after the daily nerf effect) long enough to get the perks can be counted on your hands. With fingers left over. So you would quash innovation and new content because of the questionable activities of some? Instead of addressing those activities or those players? Also instead of just "tossing in the towel" why can't exotica be tweaked to allow greater access?

As for the Weasly twins you mentioned:
This is also simple.

When a player willing leaves a legion while owning an "exotica" they forfeit control of the planet. The planet abandons in the same way a planet does when 5gp is spent.

"well that screws the legion" Thats the point. You would want to have your strong active and loyal players be the invaders. Lest they leave, the planet go "rogue" and another legion scoop it up.
It makes the legion decide as a whole which planet benefits the legion. It makes the legion decide whom they trust to colonize. It also punishes the legion if they choose poorly.Sorry, but aside from NAPs why should my legion have the right to say which planet I can colonize?(Don't trip in your rush to help your legion. Your unwillingness to donate some credits or a planet slot to benefit the whole legion is very telling.) If a legion wants to control a KotH, then the legion should make the effort to take it as a group. Look at Exotica. If your legion wants it, they have to work together to get it to an invadeable state, then everyone mashes the Invade button till someone gets it, everyone fails, or it gets protected again.(this would not be different in that regard, just more choices than the singular Exotica) And if you have to be in the legion to enjoy the benefits of the planet that YOU spent YOUR credits colonizing/invading and YOUR credits and minerals building structures on, what's to stop them from booting you and taking it straight away?(sounds like you have trust issues) Having an auto-abandon feature just makes it easier to "abuse" the system that is already causing so much consternation with Exotica.