A book-length story in Superman #119 (Dec 1957) tells the story of how Superman discovers Krypton’s “twin” planet, named Xenon. It turns out Xenon was the lost moon of Krypton that spun out of its orbit before Krypton was destroyed. Superman befriends an older man that is the double of Jor-El, his son, Kell Orr who is the spitting image of Superman, and an annoying female tele-reporter. It gives an opportunity for Wayne Boring to practice drawing people awkwardly sitting I guess?
After telling his story, Superman is given a tour of Xenon and discovers they are having planetary quakes, just like Krypton did. Superman suggests that Kell Orr and he swap places so that he can investigate the quakes. This gives an opportunity for some “inexperienced Superman” hijinx.
Meanwhile, Superman’s investigation have uncovered that Xenon has a uranium core, just as Krypton did and will suffer the same fate of a planet-wide explosion. Superman fetches Kell Orr and they go off to fetch Kryptonite. Superman needs to rescue Kell Orr, whose powers are not as great as Superman’s.
Eventually they are able to assemble enough kryptonite, at a distance, and drop it into the core of the planet, which neutralizes the radioactive core and prevents planetary destruction. Superman give a wistful turn as he returns to Earth to the new home he almost made.
