Astronomers have proposed Planet Y, a hypothetical world that could be orbiting unseen in the far reaches of our solar system. For a century at least, when astronomers have spoken of seeking an unknown planet, they’ve referred to it as Planet X. So, where do Planet X and Planet Y fit in? I also asked Siraj which hypothetical planet would conceivably be discovered first, Planet X/9 or Planet Y. Siraj answered:It’s hard to say whether Planet X/9 or Planet Y would be discovered first, if they both exist. So, the Vera Rubin Observatory is on the case to discover if Planet X or Planet Y might indeed exist.