Planet Nine, meet … also Planet Nine. Maybe.
An Earth or Mars-sized world — or even two — may exist on the outskirts of the Kuiper Belt at an eight degree inclined orbit, shifting a number of Kuiper Belt orbits up to a similar inclination.
The planet-mass object would be about 60 AU from the Sun. One AU is the distance between the Sun and Earth, with Pluto at about 30 AU at closest approach.
The proposed planet, hypothesized by Kat Volk and Renu Malhotra of the University of Arizona, is different than the one proposed by Caltech’s Mike Brown and Konstantin Batygin. Brown and Batygin propose a much more distant ice giant a bit smaller than Neptune, whereas Volk and Malhorta’s planet is smaller and much closer in.
In fact, one does not preclude the existence of the other as the dwarf planets affected by Planet Nine’s orbit far, far away.
If it clears its orbit to meet the definition of a planet (and actually exists) we could probably consider it Planet Nine and rename Planet Nine to Planet Ten. Or if you’re a Pluto Truther, consider it Planet Ten, Eris as Planet Eleven (same size as Pluto so you have to count it), and Planet Nine as Planet Twelve.
Regardless, we could have a long lost sister to Earth and Mars lurking surprisingly close, in overall solar system terms.