Planet Y

Planet Nine has taken on some brutal constraints up to last July. Then in August, Amir Siraj, Christopher F. Chyba, and Scott Tremaine instead proposed a planet with different parameters. The study has now trickled into NASA Space News.

The study is of the classical Kuiper Belt. Some KB-Objects have entered capture or at least resonance with that monster Neptune: Triton, the Pluto system, Orcus. These are excluded (unless/until someone can figure out whence they came). The paper looks instead at the reservoir as has avoided, thus far, this fate.

Our solar system has an "invariable plane", close to (but not quite) the ecliptic plane of the Earth's present revolution around Sol. The paper defines the invariable plane from the angular momentum vector of the whole system. The KBOs within 80 AU, where not Neptune-resonant, remain Invariable; but those past that, cluster at their own angle, which is not the Invariable.

Siraj-Chyba-Tremaine would resolve this by proposing a Mars-to-Venus-ish mass in the 100s AU inclined over 10°. At that distance I'd expect a lot of bright ice and lacking Pluto's dark tholins. They think the Vera Rubin should be able to see it, with its LSST survey.