**Don't worry about the distance**

Don't both trying to calculate the distance to the Sun. The exact number of astronomical units has no qualitative effect on the world or story.

What I can tell you is that a suitable distance certainly exists. And I'll tell you how I know too. Put your planet too far away and it will not evaporate at all. Put it too close and it evaporates in ten minutes. Somewhere in between is a distance where it takes 10 billion years to evaporate fully. 

Put your planet there.