Find the maximum of Planck's radiation function

P = (15/π^4 (x^5 (Exp[(1/x)] - 1)));
{max, val} = Maximize[{P, x ≥ 0}, x]

{Infinity, {x -> Infinity}}

Plot[P, {x, 0, 1}, Epilog -> {Red, PointSize[0.025], Point[{x /. val, max}]}]

-Graphics-