Is everything known?
The 19th century was the era in which the globe was being definitively mapped, but still in the time when large stretches could still be marked terra incognita even if you didn't write "here there be dragons." For these planets to still be unknown stretches --
Well, there are going to have to serious limits in their ability to survey the planets from space. And still more serious ones in their ability to land. Large flat open spaces without severe winds, perhaps. That way everything has to radiate outwards.