What triggers population moving to a new town on a colony planet?

[arquebus_x]

If I build a town hex, it always starts "abandoned" and I've not really been able to figure out what conditions lead to it getting populated. Is there a minimum population size (either on the planet overall, or on a city hex) that leads to the population moving to new town hexes? Does it vary by planet type (breathable vs landable)?

[Balthagor]

I realize this has gone unanswered for a while, my apologies. I've opened ticket 34942 and requested details from the coding team.

[Balthagor]

Some of the recent updates should have affected this. If you have a savegame where you think population are not moving around in logical ways, please send it over to us.