“The existence of another planet has been spoken about 100 times before,” Brown says. “But this is the first time in 150 years that we can say we have convincing evidence that the census of the solar system is incomplete.”

The discovery of the world that Brown and Batygin refer to in The Astronomical Journal simply as “Planet 9” began in 2003, with the discovery of a far more modest object named Sedna. A dwarf planet even smaller than Pluto, Sedna is a Kuiper Belt object (KBO), like Pluto one of a vast band of icy, rocky objects that surround the solar system beyond the orbit of Neptune. Brown was part of the team that found Sedna too, and if anything made the new world remarkable, it was its extreme distance from the sun—one that has it completing a single orbit in 11,400 years, compared to Pluto’s 248.