The concept of a large, hidden planet or planets lurking in the most distant regions of our solar system has been known since before Pluto’s discovery on 1930s. Originally named “Planet X,” it had been proposed to account for irregularities in Uranus’s orbit. That mystery was eventually resolved by recalculating Neptune’s mass. But in 2016, Caltech astronomers Konstantin Batygin and Mike Brown resurrected the puzzle with a new idea — Planet Nine — which was based on the bizarre orbits of distant Kuiper Belt objects. The explanation is that a huge planet far away is gravitationally pulling on these objects.

The Case for Planet Nine

According to Batygin and Brown’s observation, many of the Kuiper Belt objects don’t travel on orbits as expected. This suggests that, like the Moon (whose outward spiral from Earth is due to earth’s gravity), these distant objects are somehow being perturbed by something other than our Sun. The hypothesised Planet Nine is believed to be several times the size of Earth and orbiting out beyond Neptune. Supporting evidence are the discovery of recently detected trans-Neptunian-object with elliptical-orbits (such as 2017 OF201 ) which could have been sculpted by a massive planet.
In 2024, Brown reaffirmed confidence in the theory, stating, “There are currently no other explanations for the effects that we see.” More trans-Neptunian discoveries keep pointing toward an unknown gravitational force.

Challenges and New Clues

However, the theory faces hurdles. Other astronomers contend that there is not enough data on the Kuiper Belt to warrant a Planet Nine. Others suggest alternative explanations, such as a debris ring or even something more exotic like a small black hole. One is reduced observing time; tens of thousands of years are required for these objects so far from our planet to orbit.

Recent finds, such as the sednoid 2023 KQ14 — so elongated that it looks stable in an empty solar system — complicate the scenario even more. If Planet Nine is out there, it could be at least 500 AU away from the Sun. In the meantime, astronomers are continuing to search those huge, remote edges of our solar system using ground- and space-based telescopes.