Wiktionary
n. (context astronomy English) a planet that shares similar physical characteristics, such as size, density and mass, with another planet
Wikipedia
Sister Planet is a science fiction short story by Poul Anderson. It was first published in Satellite Science Fiction in 1959 and has been anthologised many times. It was included in The Golden Age of Science Fiction (anthology).
Usage examples of "sister planet".
The mighty Pioneer, climbing like an angel of the Lord, flaming sword at hand, is even now on her glorious way to our sister planet.
It seems they're doing salvage on the wrecked world, and Iskalon's sister planet of Gamandar is next on their list.
If there was a vessel used for mining any type of planet, it was here, at Dubrillion's sister planet, Destrillion, part of what Lando called his prototype fleet.
After all, only a week earlier Graves had survived an experience over on Opal's sister planet, Quake, that by the sound of it had been as harrowing as anything that Birdie had been through.
It shows an explosion or an impact near the south pole of Charon, the sister planet of Pluto.