A method to group things, using an astronomical metaphor

Grouping rules

General rules:
1. There is no limit to the number of objects contained in another
2. There are no smaller/larger versions of objects (i.e. no dwarf planet)
3. An object is not required to contain other objects, nor is it required to be contained by another object (i.e. a planet can exist on its own, without naming a system)
4. This metaphor does not acknowledge the existence of the multiverse. 

Rules for grouping:
1. The default unit is a planet
2. Planets can belong to a system
3. Systems can belong to a star cluster
4. Clusters make up a galaxy
5. Galaxies can belong to a galaxy cluster
6. Galaxy clusters belong to the universe

By starting at Planet, we have room to assign sub-elements as well:
1. A planet can include one or more moons
2. Moons can have multiple bases
3. Moon bases can have communities
4. Moon base communities can have individual people

This should be sufficient to categorize groups of things in a heirarchical manner.