The "paradise moon" likely has an archeotech climate control system- maybe even energy shield projectors to protect it from Crematoria's sun's radiation.
Maybe a combination of a death world, small graveworld, and a paradise moon.
You know what this means.
AdMech Archmagi: "We must be allowed to meditate before this archeotech artifact, and study it for Mankind's benefit! You want compensation? Fine, we'll get you a Banelblade for each century we're allowed to maintain a research station on this moon! Here's the first- formerly known as the Dread King and the Dakka Boss, it was recovered from Armageddon's battlefields and cleansed of the Ork taint- accept it as a down payment!"
I presume these nobles overthrew the planetary governor- living representative of the Emperor's authority- in the chaos and Chaos following Cadia's fall, justifying the Chapter's conquest as necessary to restore lawful order to the planet?
The deathworld has a lot of raw material and was ruled by Age of Darkness nobles that lay to rest on a graveworld in the system. Perfect for the chapter to liberate and establish roots.
Will the spirits haunting the graveworld, draw Chaos cultists to it?
Graveworld is haunted, obviously.