The Mysterious Planet by "Kenneth Wright" (Lester del Rey) (1953)
Lester del Rey contributed eight novels to Winston Publishing's Adventures in Science Fiction series. The Mysterious Planet is the weakest of them, which may explain why it was published under a unknown at the time pseudonym. (Tuck's 1978 The Encyclopedia of Science Fiction and Fantasy, for example, showed no knowledge that this was a del Rey* pen name.)
A tenth planet has been discovered beyond Pluto (which was still a planet back then). What at first appeared to be a highly eccentric orbit turns out to be something completely different. Planet X does not have an orbit; instead, it is traveling through the solar system heading towards earth. The solar Federation Navy sends its Ninth Wing to investigate. Aboard the Icarus, one of the ships sent on this mission, is newly minted Cadet Bob Griffith, who's father happens to be the Commander of the Ninth Wing. On their way they are met by a fleet of large black ships of unknown origin. Reluctantly, the Federation ships are drawn into a battle which they lose handily due to the enemy's highly advanced technology and weapons. The three surviving Federation ships limp back to their home port while the enemy ships make their way to Planet X.
Bob and two of his friends disobey orders and travel back to space in an attempt to get vital information about the enemy's technology but they fall into a trap and are captured. The three are brought to Planet X, which is actually a world from a distant sun that went nova. The humanoid people of Planet X want their planet to orbit our sun but they fear Earth and its warlike tendencies. Neither side, it turns out, wants a war but both feel they are being forced into one by the other side.
There's a lot of good stuff in this novel, but you have to sift through a number of time-word tropes, some plodding plotting, philosophical conundrums, and a simplistic resolution to find it. Del Rey, who could and did do much better, seems to have phoned this one in.
I am afraid you'd be better off reading any of his other contributions to this series, either under the del Rey name of his Philip St. John pseudonym.
* Del Rey, of course, was also a pen name. He claimed (and it was widely believed) that his true name was Ramon Felipe San Juan Mario Silvio Enrico Smith Heathcote-Brace Sierra del Rey y de los Verdes. It was only more than a decade after his death that his real name was Leonard Knapp.