Orbits of distant objects suggest an undiscovered planet 10 times Earth's mass. Extensive searches have found nothing despite strong orbital evidence.
nagualdesign; Tom Ruen, background taken from File:ESO - Milky Way.jpg, CC BY-SA 4.0, via Wikimedia Commons
The Gravitational Ghost
The address for this point of interest is the cold, dark expanse beyond Neptune, but the idea was born at the location pinned on your map: the California Institute of Technology. On January 20, 2016, Caltech astronomers Konstantin Batygin and Mike Brown published a paper presenting evidence for a giant, undiscovered planet. They nicknamed it Planet Nine.
The evidence is purely gravitational, inferred from the strange behavior of a handful of small, icy bodies in the Kuiper Belt. These extreme trans-Neptunian objects (eTNOs) follow bizarre, highly elongated orbits. Astronomers noticed was a distinct clustering: the long, oval-shaped paths of at least six of these objects all point in roughly the same direction, and their orbits are tilted at a similar angle to the plane of our solar system. The probability of this alignment occurring by chance is estimated to be about 0.2%. Something massive, they calculated, must be gravitationally guiding these smaller bodies into their correlated orbits.
Portrait of a Planet
Through mathematical modeling and computer simulations, Batygin and Brown developed a profile of the unseen object. Planet Nine is predicted to have a mass about 5 to 10 times that of Earth, making it a "super-Earth" or "mini-Neptune". Its existence would help explain why our solar system lacks a planet of this common size, while they are frequent around other stars.
The planet would follow a highly eccentric, or oval-shaped, orbit far beyond Neptune. Its average distance from the Sun is estimated to be between 400 and 800 Astronomical Units (AU), where 1 AU is the distance from Earth to the Sun. A single year on this distant world would be immense, lasting between 10,000 and 20,000 Earth years. The planet's gravity explains other long-standing solar system mysteries, including the slight 6-degree tilt of the Sun's rotational axis relative to the orbits of the known planets.
The search is difficult. Because of its extreme distance, Planet Nine would be exceptionally faint, with an apparent magnitude fainter than 22, making it at least 600 times dimmer than Pluto. Astronomers using powerful instruments like the Subaru Telescope in Hawaii are scanning the skies for it. The best chance for discovery lies with the Vera C. Rubin Observatory in Chile, expected to begin its survey in 2025.
💡Fun Facts
Mike Brown, a key proposer of Planet Nine, is the same astronomer whose discovery of the dwarf planet Eris led to Pluto's reclassification. His Twitter handle is @plutokiller.
The planet's immense, tilted orbit could be responsible for the 6-degree tilt of the Sun relative to the plane of the other planets, essentially twisting the entire solar system out of alignment over billions of years.
If it exists, Planet Nine was likely formed closer to the Sun and was ejected into the distant solar system by a gravitational encounter with Jupiter or Saturn during the system's early history.
The name "Planet Nine" is an informal nickname. If discovered, the formal naming rights go to the discoverer, with final approval from the International Astronomical Union.
