In January 2015, a team of astronomers from the California Institute for Technology (Caltech) stumbled upon a potentially groundbreaking discovery. The alignment of several of the most distant worlds in our Solar System seemed to all have the same perihelion, or closest point to the Sun. This suggested that something big had to have pulled them into these orbits, and flung them out hundreds of times the distance between the Sun and Earth.

All this suggested a roughly Neptune-sized world so distant that it hadn’t ever been spotted. Caltech astronomers Konstantin Batygin and Michael Brown dubbed it Planet Nine. This hypothetical planet may have a mass that's about 10 times that of Earth and orbit about 20 times farther from the Sun than Neptune, the eighth and farthest known planet from the Sun. Planet Nine may take between 10,000 and 20,000 Earth years to complete one orbit around the Sun, according to NASA.