I also got interested in this question a few years back and while browsing for candidates I learned about Neso, the outermost (known) moon of Neptune. https://e

I also got interested in this question a few years back and while browsing for candidates I learned about Neso, the outermost (known) moon of Neptune. https://en.wikipedia.org/wiki/Neso_(moon) https://en.wikipedia.org/wiki/Neso_(moon) It is on average further away from Neptune than Mercury is from the Sun! It takes more than 26 years to orbit Neptune, it still hasn't completed a full orbit since we have discovered it. Also, I think there could be "binary-moons", if a binary asteroid gets captured as an irregular moon. Asteroids with natural satellites exist and have been found. There could also be very big trojans moons/planets in other star systems.