It's not true that there is no "minimum" orbital distance for a planet around
a dim star. There's something called tidal stress which is caused by the
difference in gravitational pull from the star on the close side of the planet
vs. the farther side. At the distance of the sun from the earth, the tidal
stress doesn't do more than puff up the ocean (which causes the ocean tides
you've probably heard of). :-) As an aside, the sun-earth tide is larger
than the moon-earth tide, or so I'm told.
Anyway, if your planet is too close to its M5 V star, tidal stress will
overcome the gravitational binding and the planet will break up. That's
why there's a minimum distance for stable orbits for planets.
For items which are not bound by gravity, e.g. a space ship, the minimum
orbit is MUCH smaller.
Martin Snow
[email removed]
[email removed]