Planet X? Why not a tiny black hole instead?

Enlarge (credit: SXS Lensing)

Planet X has a long and storied history of non-existence. For about 130 years, astronomers have debated the existence of an additional planet or planets to explain discrepancies in the orbits of the known planets (mainly Neptune and Uranus). Later, the list of discrepancies was expanded to cover trans-Neptunian objects. But none of the Planet X candidates discovered, including Pluto, have the mass or location to explain observations.

Primordial black holes have now been proposed as the latest planet X (or planet 9, since Pluto was demoted).

Orbital weirdness

Planet X's origin starts with the discovery of Neptune. Neptune was not found by accident: observations of oddities in the orbit of Uranus were used to calculate the location of Neptune, and it was subsequently found.

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