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Planet Nine is a hypothetical ninth planet in the outer region of the Solar System.

These alignments suggest that an undiscovered planet may be shepherding the orbits of the most distant known Solar System objects. Astronomers Konstantin Batygin and Michael Brown have suggested that Planet Nine may be the core of a giant planet that was ejected from its original orbit by Jupiter during the genesis of the Solar System. Others suggest that the planet was captured from another star, was once a rogue planet, or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star. Although sky surveys such as Wide-field Infrared Survey Explorer (WISE) and Pan-STARRS did not detect Planet Nine, they have not ruled out the existence of a Neptune-diameter object in the outer Solar System. The ability of these past sky surveys to detect Planet Nine was dependent on its location and characteristics. Unless Planet Nine is observed, its existence remains purely conjectural. Subsequent discoveries of objects including 2017 OF201 and 2023 KQ14 have challenged the hypothesis, as their orbits are not aligned with the other TNOs and would be unstable if Planet Nine were present.

Forbes' work is considered similar to more recent Planet Nine theories in that the planets would be responsible for a clustering of the orbits of several objects, in this case the clustering of aphelion distances of periodic comets near about 100–300 AU. Like Planet Nine it would cause the perihelia of objects with semi-major axes greater than 300 AU to oscillate, delivering some into planet-crossing orbits and others into detached orbits like that of Sedna.

In early 2016, California Institute of Technology's Batygin and Brown described how the similar orbits of six ETNOs could be explained by Planet Nine and proposed a possible orbit for the planet.

If this disk had a distant inner edge, 100–200 AU, a planet encountering Neptune would have a 20% chance of being captured in an orbit similar to that proposed for Planet Nine, with the observed clustering more likely if the inner edge is at 200 AU.

Objects with semi-major axis greater than 250 AU are strongly anti-aligned with Planet Nine, with perihelia opposite Planet Nine's perihelion. Objects with semi-major axes between 150 and 250 AU are weakly aligned with Planet Nine, with perihelia in the same direction as Planet Nine's perihelion.

They also conducted a survey of Neptune-crossing objects with inclinations below 40 degrees and semi-major axes between 100 and 1000 AU and argued that the results aligned with the presence of Planet Nine, which would produce a ratio of Neptune-crossers to objects with a perihelion beyond Neptune's orbit of 3%, compared to 0.5% in the absence of Planet Nine.

The orbit parameters for Planet Nine favored by Batygin and Brown after an analysis using these objects were:

The Planet Nine hypothesis includes a set of predictions about the mass and orbit of the planet.

Observation and analysis of the orbital dynamics of Kuiper Belt objects constrain the possible orbital parameters of a Planet Nine, and at the current rate of new observations, University of Michigan professor Fred Adams believes enough data will have been gathered to pinpoint Planet Nine or rule out its existence by 2035.

The analysis, using Batygin and Brown's orbital parameters for Planet Nine, suggests that the lack of perturbations to Saturn's orbit is best explained if Planet Nine is located at a true anomaly of 117.8°+11°−10°.

If these orbits are hyperbolic due to close encounters with Planet Nine the analysis estimates that Planet Nine is currently near aphelion with a right ascension of 83–90° and a declination of 8–10°.

Alternatively, they could be in resonance with Planet Nine, but have orbital orientations that circulate instead of being confined by Planet Nine if it had a semi-major axis of 505 AU.

Quick Facts

• Astronomers Konstantin Batygin and Michael Brown have suggested that Planet Nine may be the core of a giant planet that was ejected from its original orbit by Jupiter during the genesis of the Solar System.
• Others suggest that the planet was captured from another star, was once a rogue planet, or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star.
• Although sky surveys such as Wide-field Infrared Survey Explorer (WISE) and Pan-STARRS did not detect Planet Nine, they have not ruled out the existence of a Neptune-diameter object in the outer Solar System.
• Objects with semi-major axes between 150 and 250 AU are weakly aligned with Planet Nine, with perihelia in the same direction as Planet Nine's perihelion.
• If these orbits are hyperbolic due to close encounters with Planet Nine the analysis estimates that Planet Nine is currently near aphelion with a right ascension of 83–90° and a declination of 8–10°.

Source material: Wikipedia - "Planet Nine". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.