The hypothetical world known as Planet X or Planet Nine hasn’t yet been found, but thanks to the search, astronomers have discovered smaller worlds on the solar system’s edge.
Mapping such objects could lead to the big discovery: a planet that’s thought to be at least several times bigger than Earth, lurking at least 200 times farther away from the sun. Or it could lead to a different explanation for the puzzling, highly elongated orbits of some of the objects that lie far beyond Pluto.
Planet Nine’s existence was proposed last year as the most elegant way to account for the orbits of worlds such as Sedna and 2012 VP113 (which has been nicknamed “Biden” in honor of the veep). Ever since then, astronomers have been surveying the skies in hopes of tracking it down.