I have recently scanned a massive 5x mining, 5x artifact, rich resarch planet. How much would it be worth?

Depends on the buyers, if you do an auction you probably can go from 2-5k depending on who is competing. I would place a bid, but I am kind of broke atm

I agree with the price as stated i will offer you 2500CTP. Defense and attack depends as well

The following information would affect the price(if it's enemy occupied, if it's unoccupied then~ just the above posts):
Defense, Attack, Population, Invasion defence
High values will ofc negatively influenced the value

Passive stats
Defense and attack is kinda neutral, the true big buyers would want them but it will put off smaller ones. Cloak passive is defintely a +

Artifacts on the planet
unlimited(NPC, CTL, base raid etc) artifacts don't really worth too much, thou it does add a bit to the value. GP purchased ones defintely add to the value. But the most valueable are mission(limited) artifacts, especially ones with powerful effects, such as havoroids etc. Extremely rare ones such as the omni base also carry large values. Thou considering the rarity of blockade NPCs, it's possible drop by those can be considered valuable too...

Effect:
Specifically, if the planet have an analyised effect, it would worth extra. thou some worth more than others.

Ownership:
If the planet belongs to a big player or legion, it would put off some buyers. Especially in case it is owned by top 10 legions: if the defence value is sufficently high, only a small number of legions can take it, and a large portion of the possible buyers would not buy it because it belongs to their own legion and/or allied legion.

When making an auction, it would be a good idea to give the ball park value of atk, def and population (ball park means, for example a range of 1k*2^n atk/def and 100*2^n population. For example, 12135 atk 13564 def 1950 population would be 8-16k atk, 8-16k def, 1600-3200 population)
Further more, it might be a good idea to lable its rough legion rank(top 10, top 30, top 50 or unranked, for example). In case of top 10 legion.... I would suggest you do blind bidding(via PM), and request the buyers to list the legions that they will NOT invade a planet from... otherwise... well~
Unfortuntely, there is really no good way to list good artifacts(effect) while maintaining secrecy, so only make a special mention of "valueable artifact or effect on planet" on it. And when a deal is close to well... closing, chat about its value over PM(possibly narrow down buyers first then do a bit of private talk)

DO NOT give exact values or legion, because it might allow the actual owner learn of the sale and wall up.

And for future reference, might be better to not say it's an exotic, I would call it "generic" planet--a planet that does not have a type specific artifact(including dyson, gaia, demon, exotic etc). on the other hand, as terra, gas, toxic, oceanic etc have type-specific artifacts, it would carry extra value to specify the planet type. The more information you give out, the risker it becomes and the owner might make it go poof one way or another~

I think that is all that relates to the topic~