19 - Wandl the Invader (version 2) by Ray Cummings

In a solar system with nine major planets, humanity first ventured into interplanetary commerce, trading with the ancient Martian civilization. But then, a shocking discovery emerged a tenth planet, an enigmatic maverick, was on a collision course with Earth and Mars. This rogue world disrupted astronautical calculations and stirred chaos among the two inhabited planets. Little did they know, this was WANDL THE INVADER—no barren rock, but a vibrant world inhabited by minds and monsters, arriving with a purpose far beyond mere astronomical anomaly! - Summary from the Original book jacket blurb. Sequel to Brigands of the Moon