Since we first heard about the Planet 9 hypothesis to explain the clustering of the periapses of numerous long-period detached Kuiper Belt Object orbits, many new objects out there have been found. The sample of objects has approximately doubled, and consequently the constraints on the nature of Planet 9 have been refined. Basically, the proposed planet is somewhat smaller, closer to the sun, and has a less eccentric orbit than first believed.
"Planet Nine is a factor of ~2 smaller in all quantities compared to what we reported in the original paper. The new estimate of the semi-major axis is a~400-500AU (could potentially be even smaller, but only marginally so). P9’s orbital eccentricity is about e~0.15-0.3. The inclination is around i~20 deg. Last but not least, the mass is about m~5 Earth masses. Planet Nine is probably not a relative of Neptune — it’s a Super-Earth."
If there were no Planet 9, the various orbital precessions of those detached KBOs would cause them to diverge and the periapsis clustering would vanish on timescales much shorter than the age of the solar system. Some object must be shepherding the objects to keep their orbital periapses clustered.
Running a bunch of N-body simulations, injecting various test masses in hypothetical orbits, and letting it scatter a bunch of smaller objects, we can see which values for the orbit/mass of the test object in our simulation will produce the outcome we see in the detached KBO orbits. Naturally a range of parameters will produce plausible fits to what we see, but the more objects we can compare to, the better an idea what must be perturbing them is, because our picture of what that outcome must be gets clearer.
Ignoring issues with stability, 2 or more different massive objects objects would have different orbital precession rates, completely erasing the periapsis clustering we see, as the "average" periapsis of the detached KBOs we see is anti-aligned to the periapsis of the perturbing body.
If that admittedly non-Bayesian description of a rather Bayesian problem makes sense.The Search for Planet 9
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