Companion is related to Pet. Pet is related to Family. Grandmother is part of Family. Grandmother is the opposite of Grandchild. 
you are likely to find Planet in/at/on Galaxy. Pan is a type of Planet. 
Companion <--RelatedTo--> Pet <--RelatedTo--> Family <--PartOf-- Grandmother <--Antonym--> Grandchild 
Galaxy <--AtLocation-- Planet <--IsA-- Pan 