"There is no route" to neighboring uninhabited planet

I have a science vessel at the edge of my territory, and I wish for it to explore a neighboring system which is seemingly uninhabited. However, when I try to move the ship I get a "There is no route for the selected fleet to reach this system."

I’ve googled around and found many similar cases, but all have to do with being trapped by an FTL inhibitor or trying to pass through another civilization’s territory without being at war with them. As far as I can tell, however, this system is uninhabited, as was the system I’m departing from before I built my starbase.

Thinking this might be a border with a civilization I haven’t discovered yet and my sensors haven’t picked up (if that’s even possible at this range), I also set the ship’s stance to Passive, but still no dice.

So, what’s going on here?