I feel there's a bit much emphasis on tidally locked planets in this discussion. Therefore I would like to comment on 2 points which I think are the most "harmful" to our search.
- The trailer scene which we assume shows Barnacles is in the dark, therefore Large Barnacles are only found in the dark.
- The trailer scene which we assume shows Barnacles is in the dark, therefore it must be a tidally locked planet.
Those two points are completely baseless. The trailer could be shot at night, the trailer could be shot on a planet far away from a star or it could indeed be on a tidally locked planet. But we have absolutely no indicator which it could be. And the darkness could very well serve an exclusively dramatic purpose.
In my opinion it would be much more useful to try to understand what the Barnacles "want", what a suitable environment might be for Large Barnacles. I think it's safe to assume that the Large Barnacles have been dubbed that way because they look similar to the barnacles we all know and love, and not because those "interstellar explorers" have run a lengthy analysis to figure out that they actually are similar.
If we assume those things in the trailer are indeed Large Barnacles then I would tend to exclude planets with a really low gravity. Why? Because I think the gravity of that world would favor tall and slender entities over those with a broad base as was shown in the trailer. This kind of structure to me looks like it would be a much better fit on a world with gravity equal to or greater than Earth's.
Agreed, especially when you need to take into account what things are tidally locked to.
If it's permanent darkness people are looking for, only objects that are tidally locked to a star count - these are likely to be hot - maybe too hot for meta-alloys.
Given the way ED does light sources, maybe even only the primary star.
Moons can be tidally locked to their planet, binary pairs may be tidally locked to each other - neither of these results in areas of permanent darkness.