Planet X: evidence mounts for undiscovered 9th planet in our solar system

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"... But for the past decade, astronomers have been finding it increasingly likely that something big – often called Planet X – might actually be out there. And a new study by a team from Princeton University has substantially raised the likelihood that it really exists. ...

The evidence comes from an apparently non-random distribution in the orientation of the orbits of smaller outer Solar System bodies known as trans-Neptunian objects or Kuiper Belt objects (KBOs). ...

At the moment, he says, the quest to actually find Planet X is on hiatus, awaiting the opening of the Vera C. Rubin Observatory, a half-billion-dollar (or more) project being built on a mountaintop in northern Chile. ..."

From the abstract:

"The plausibility of an unseen planet in the outer solar system, and the expected orbit and mass of such a planet, have long been a topic of inquiry and debate. We calculate the long-term orbital stability of distant TNOs, which allows us to expand the sample of objects that would carry dynamical information about a hypothetical unseen planet in the solar system.

Using this expanded sample, we find statistically significant clustering at the ∼3σ level for TNOs with semimajor axes >170 AU, in longitude of perihelion (ϖ), but not in inclination (i), argument of perihelion (ω) or longitude of node (Ω). Since a natural explanation for clustering in ϖ is an unseen planet, we run 300 n-body simulations with the giant planets, a disk of test particles representing Kuiper belt objects, and an additional planet with varied initial conditions for its mass, semimajor axis, eccentricity, and inclination. Based on the distribution of test particles after 1−2 Gyr, we compute relative likelihoods given the actual distribution of ϖ as a function of semimajor axis for distant TNOs on stable orbits using a significantly larger sample than previous work.

We find the best-fit unseen planet parameters to be: mass mp=4.4±1.1M⊕, semimajor axis ap=290±30AU, eccentricity ep=0.29±0.13, and inclination ip=6.8±5.0∘. Only 0.06% of the Brown & Batygin (2021) Planet Nine reference population produce probabilities within 1σ of the maximum within our quadrivariate model, indicating that our work identifies a distinct preferred region of parameter space for an unseen planet in the solar system."

Planet X: evidence mounts for undiscovered 9th planet

Orbit of a Possible Planet X (open access)

Figure 6:Left: the relative probability density distribution for the unseen planet as a function of right ascension and declination. Right: the relative probability density distribution from Brown & Batygin. Blue dashed lines indicate the ecliptic plane and $\pm {20}^{\circ }$ from the Galactic plane.