When the fleet is stationed at a planet, the user wants the fleet associated with the planet and planet associated with the fleet:
Fleet <=> Planet
This is trivial by displaying them together. If you have a global list of fleets, make sure you can easily discover the planet its on and same for list of planets (again trivial).
When a transfer is planned, the user wants the following:
Source <=> Fleet <=> Destination
The source needs to be known given the fleet because the user can cancel the transfer still and this changes the state of the source as well as the destination.
When a transfer is in progress, the user wants to know:
Fleet <=> Destination
The source is irrelevant because whether the transfer exists or not is irrelevant to its state in the present and in the future because you cant cancel transfers in progress (the destination is relevant because its state will be changed when the transfer is complete)
So I would say that during planning, you could display something like
[RealPlanet (source)] -> RealFleet -> [VirtualPlanet (destination)]
near the source planet (so when you look at the source planet, you see that the real fleet that exists there is about to transfer to the destination planet (the virtualplanet could be a stylized representation of that planet, a pair of coordinates, a planet name, a button you can hover over to show the actual destination planet wherever it is...)
[VirtualPlanet (source)] -> VirtualFleet -> [RealPlanet (destination)]
near the destination planet to show that you have planned a transfer there, with virtual 'links' to the fleet and source planet in case you need to change something
During transfer you would remove the stuff near the source planet and remove the [VirtualPlanet (source)] from near the destination planet, only leaving the VirtualFleet symbol thingy.
Visually it could be like
(earth) Fleet1> -> [(Moon)]
[(earth) Fleet1>] -> (Moon)
looking at source (earth) and destination (moon) respectively. Basically everything that is located somewhere else is made 'virtual' (represented by [brackets] here)
Visually it would make sense to direct these incoming and outcoming fleet indicators such that they point toward the dest/src planet if possible, because the user probably maintains some kind of spatial map of the world in their head and uses this spatial map to remember what is going on where, so the spatial information is important in both quickly remembering where the fleet was going (without reading the details) as well as why ('oh yeah I have this massive celestial clash going on with that pirate faction in the top right corner of the universe, so this fleet must be going there') (this would allow you to even hide the source/dest details if the screen is too cluttered since the directional information should help the user remember that information to some extent)