The Secret of the Ninth Planet is a science-fiction novel written by Donald A. Wollheim and first published in the United States in 1959 by the Wollheim takes his heroes on a grand tour of the Solar System as that team struggles to prevent an alien force from blowing up the Sun. This is the last of three juvenile novels that Wollheim wrote for Winston, the other two being The Secret of Saturn's Rings and The Secret of the Martian Moons.