Artist Concept of "Planet X"

Researchers at Caltech discovered evidence suggesting there may be a "Planet X" residing deep in the solar system. Nicknamed "Planet Nine," this hypothetical planet would be about the size of Neptune and have a mass about 10 times that of Earth. They also believe it would have a highly elongated orbit and would take from 10,000 to 20,000 Earth years to make one full orbit around the Sun.

This is an artist concept showing what Planet X may look like.