What is the minimum distance for a planet to be able to form from a star?

I’ve been playing Starchitect for a while, and I’m trying to create a terrestrial planet orbiting a F-type star with minimal distance. I have tried 0.005 and 0.01 AU, both of which failed. The upper bound is 0.1 0.03 AU, with a terrestrial planet orbiting there as well. So what is the lower bound?

Update: After some experimenting, the new upper bound is 0.03 AU.