We have seen in the news recently the assertion that the trajectories of objects in the Kuiper Belt imply the existence of a planet nearly the size of Neptune lurking at the far reaches of our Solar System.
Respected planetary scientists Konstantin Batygin and Mike Brown of the California Institute of Technology (Caltech) in Pasadena assert that the clustering of distant objects is only explained by the existence of this "Planet X."
The ancient peoples knew of only the Moon, Mercury, Venus, Mars, Jupiter, and Saturn.
Uranus was first spotted in 1690. Neptune was found in 1846 in exactly the spot it was predicted to be based on other research.
Astronomer Percival Lowell devoted his life to the search for "Planet X," a hypothetical planet beyond Uranus and Neptune were displaced from their predicted positions by some gravitational force.
I attended Lowell Elementary School in Madison, WI, named after this astronomical great, and a sign of how progressive the city of my nurture was.
When Pluto was eventually discovered, some thought that "Planet X" had been discovered. But it turns out that Pluto has nowhere near the gravitational force to account for the anomalies in the orbits of Uranus and Neptune.
Planet X remains the subject of speculation and fascination.
I included it in my novel In Saecula Saeculorum as a force hurtling toward Earth threatening life on our Planet. A group of young people were trained unwittingly to travel back in time to gain information that could then be used to change the trajectory of Planet X in the current day and thus save our world. I invite you to check it out.