So let's say we have a planet not too dissimilar in size to those in our Solar System. It doesn't have to be spherical. But let's say we want a river to flow round the object. We want the flow to last as long as possible. We can consider two cases seperately: in our Solar system (with the Sun), and in isolation (no Sun). So what shape should the planet be, how big should it be, how fast should it be rotating, and how/how fast/ what direction should the river be flowing in order to create the best system for the river to flow as long as possible (i.e. it will have to slow down with time, but what is the way it will slow the rapidly)? Question: How can the river flow round an object that has an irregular shape? Answer: You aren't allowed to make a tiny circular river, but have to make one that you can recognize as going (reasonably close to) the greatest outer geodesic of the (possibly) irregular shape. Question: What is the planet made of? Answer: Again, you get to decide. This isn't a competition but I'm interested in hearing your ideas and thoughts on this one, as I have thought about it a few times in the past.