Scientists used to believe there was another planet beyond Neptune, even after Pluto was found, because the evidence at hand at the time didn’t make sense without another planet. Then a satellite visited Neptune and found out scientists has its mass calculated wrong, and suddenly that certainty of another planet was gone.
I’m not saying there isn’t a larger-than-rocky-smaller-than gas-giant sized planet way out beyond the belts. I’m saying that asserting there is a planet there when the evidence is circumstantial is chancy. I’m surprised Caltech is making such a statement.
Has anyone considered the reason the asteroids / comets / whatnot way out there all orbit to one side is because a planet broke up out there long time ago? Or perhaps their orbits are because they’re being pulled along by the Sun as it moves but they’re so far out that the Sun has moved from the “center” of the orbit and causes elongated catch ups – making their orbits trail behind the direction the solar system is moving through the Milky Way?