Persephone is the name given to a hypothetical planet in the solar system located beyond Neptune. It is also informally called Sol B1. This is a hypothetical planets corresponding to the generic term of Planet X.
Its existence was postulated in the late nineteenth century on the basis of anomalies in the orbit of Neptune, but following the steps of the probe Voyager 2, which showed that the mass of Neptune was poorly estimated, it is now admitted there is no need to use an additional planet to explain the orbit of Neptune.
It would be a gaseous planet more massive than Jupiter, and therefore it would be the world's largest and second largest body in the solar system. It would be in the external cloud about 30,000 AU from the Sun.