you are 100,000 feet above planet B and the ground range to your horizon point is 400 miles. What is the radius of planet B?
The satellite, Earth center and horizon tangent point form a right triangle. The Pythagorean theorem says
R^2 + x^2 = (R + h)^2
h is the altitude and x is the range. R is the earth radius
Solve for R.
x^2 = 2hR + h^2
R = (x^2-h^2)/(2h)