Volatiles are mentioned in the description of the planet/moon. An icy planet will mention that there are large amounts trapped as solids etc. They consist of the gases nitrogen, hydrogen and ammonia, amongst others.
Crucially they are what makes a crater misty and give the surface of Maia A7 what looks like and is, really, a thin atmosphere. Along with metal content and being tidally locked to a star (only one constant fixed light source), they may be part of the key to solving this. Misty craters tend to form on the edge of the light/dark zone.
This gives less area to search. Planets tidally locked to another planet will still receive light to it's entire surface, at some point, except the poles (which could still be searched as possible sites).
So, a micro-atmosphere in a misty crater, for respiration, a source of metal from the planet to grow, heavy elements from the nebula and the right temperature may be what is needed for these barnacles to thrive. I believe the planet where all this comes together is out there, in the Pleiades.
MB has stated that Merope is NOT where barnacles should be looked for. What is not present at Merope is obviously something (singular, or plural) that is needed by the barnacle to thrive.
I have started eliminating types of planets and narrowing the search. Where they wouldn't be, is just as important as where they might be. They aren't at Merope.
Thanks again for all your hard work, as you have saved me a lot of time. I was working and still am working, with a notebook which is rapidly filling up with scribbles!