The Century-Long Hunt for a Hidden 'Planet Nine' Is Still Not Over

For more than a century, astronomers have wondered whether an undiscovered planet lurks in the outer solar system. The idea dates to the early 20th century, when American astronomer Percival Lowell proposed that orbital irregularities in Uranus and Neptune pointed to a distant unknown world he called "Planet X" — a search that led to the discovery of Pluto in 1930, before Pluto was reclassified as a dwarf planet in August 2006.

Voyager 2's encounters with Uranus and Neptune in the 1980s later showed their orbits could be explained without an extra planet. But in 2016, Caltech astronomers Konstantin Batygin and Michael Brown revived the idea in a different form, arguing that odd orbital patterns among distant icy bodies beyond Neptune could be explained by an undiscovered planet roughly the mass of Neptune, which they call Planet Nine.

In 2024, the Caltech team presented additional findings focused on long-period objects that cross Neptune's orbit. "These objects are dynamically unstable, so the population has to be continuously replenished," Batygin told Space.com. "Planet Nine naturally produces the observed population, whereas the model without Planet Nine is strongly inconsistent with the data once observational biases are accounted for." Batygin said the theory could also explain orbital clustering, large perihelion distances, retrograde orbits and highly inclined orbits seen among distant solar system objects.

The only piece still missing is a direct detection of the planet itself. Batygin said the Vera C. Rubin Observatory in Chile could soon provide the additional observations needed, though he cautioned that not every distant object is expected to show clustering: "The picture has become more complicated, rather than simply stronger or weaker."