When a planet, moving faster than Earth, entered the Solar System out beyond Pluto, the reporters, for want of a better name, termed it "Planet X." "Planet X", that astronomical curiosity, became, in a few short weeks, an object of stark terror to the billions of men scattered on the planets around our Sun, from blazing Mercury to frigid Pluto.

To Bob Griffith's father, Commander of the Ninth Wing of the Solar Federation's Space Navy, fell the task of investigating hte strange visitor...
Note: This was published under the pen name Kenneth Wright, although a 1978 paperback edition -- published by Del Rey books -- attributes it to Lester Del Rey.