Earlier this week, two astronomers at Caltech announced that they had observational evidence of a ninth planet orbiting in the far reaches of the Kuiper Belt. The planet has not been directly detected but its effect on the orbits of six dwarf planets was observed. Based on theoretical models, the most likely explanation for the unexpected orbits of these icy bodies is a planet, 5 to 10 times the mass of Earth. They're calling it Planet Nine, but I remember when I first became interested in Astronomy at 7 years old, reading about the mysterious "Planet X" that many believed orbited somewhere in the Kuiper Belt. The story of the search for Planet X and other far orbiting bodies is apparently far from over.
Anomalies in Uranus’ orbit led astronomers to calculate the theoretical presence of another large planet. These theoretical calculations were verified with the discovery of Neptune in the 1840s. However, the initially measured mass of Neptune could not completely account for the perturbations in Uranus’ orbit. A decades long hunt for “Planet X” commenced at the end of the 19th century at Lowell Observatory. A few locations for Planet X were proposed but no candidate bodies revealed themselves until 1930, when Clyde Tombaugh noticed a faint blob that moved across the background of distant stars. The new body was named Pluto and many astronomers attempted to determine its mass. None of their calculations allowed them to claim Pluto as the long searched for Planet X. In 1989, Voyager 2 flew close enough to Neptune to allow astronomers to accurately measure its mass. The new mass measurement turned out to be adequate to describe the fluctuations in Uranus’ orbit, which removed the need for Planet X. Pluto's discovery turned out to be a happy coincidence.
In an interesting twist, one of the Caltech astronomers proposing the existence of Planet Nine is Michael Brown, author of the book "How I Killed Pluto and Why It Had It Coming". For over 70 years, the status of Pluto as a major planet was not questioned. However, as telescope sizes increased and the ability to place telescopes in orbit around Earth became feasible, more information about Pluto came to light. This amassed evidence showed that bodies orbiting past Neptune were very different than the eight major planets in our Solar System. In August of 2006, eight months after the launch of NASA's New Horizons satellite, the International Astronomical Union (IAU) changed the classification of Pluto from planet to dwarf planet. Many still question the criteria set by the IAU to define a planet. Data provided by the New Horizons mission after it makes its fly-by of the dwarf planet MU69 in 2019 may help the IAU further modify the definition.
It is increasingly obvious that we are still learning what is lurking in the outer reaches of our Solar System. Much like the search for Planet X led us to discover the icy worlds in the Kuiper Belt, the search for Planet Nine is guaranteed to teach us more about our Solar System.