In January 2016, astronomers Konstantin Batygin and Mike Brown from the California Institute of Technology (Caltech) announced research suggesting the existence of a planet about 1.5 times the size of Earth in the outer solar system. This theoretical world, dubbed “Planet Nine,” has not been directly observed, but its possible presence is inferred from the unusual orbits of distant celestial objects.
If it exists, Planet Nine would be a Neptune-sized planet with a mass between 5 and 10 times that of Earth. It would follow a highly elongated orbit around the Sun, located roughly 20 to 30 times farther out than Neptune. A single orbit might take between 10,000 and 20,000 Earth years to complete.
The potential planet could help explain several puzzling features of the Kuiper Belt—a distant region of icy debris beyond Neptune. These include:
- Why long-period Kuiper Belt objects are tilted about 20 degrees relative to the plane of the solar system.
- Why these objects have orbits that appear to cluster in orientation.
- The presence of distant, highly inclined trans-Neptunian objects.
- The existence of some objects between the giant planets that orbit the Sun in a retrograde direction.
- The continued existence of long-period Kuiper Belt objects whose orbits intersect with Neptune’s.
This hypothetical planet could also make our solar system appear more typical. Observations of exoplanetary systems show that “super Earths”—planets larger than Earth but smaller than Neptune—are the most common type. Strangely, our solar system doesn’t have one. If Planet Nine exists, it could fill that gap.
The nickname “Planet Nine” refers specifically to Batygin and Brown’s 2016 hypothesis. The term “Planet X,” first coined by astronomer Percival Lowell in 1915, has historically been used to describe any unknown massive planet beyond Neptune. Lowell believed such a planet was perturbing Uranus’s orbit, but it was later determined those perturbations were due to measurement errors. However, Lowell’s search indirectly led to the discovery of Pluto in 1930.
Pluto was considered the ninth planet until 2006, when the International Astronomical Union (IAU) redefined what constitutes a planet. Pluto was reclassified as a “dwarf planet,” leaving the ninth-planet slot vacant—hence the modern use of “Planet Nine” to describe the predicted world.
Search efforts for Planet Nine involve some of the world’s most powerful telescopes, such as the Keck and Subaru observatories in Hawaii. Additionally, a NASA-funded citizen science project, Backyard Worlds: Planet 9, allows the public to help identify candidates using data from NASA’s Wide-field Infrared Survey Explorer (WISE) mission, which ended on July 31, 2024.
A promising new player in the search is the Rubin Observatory, located on Cerro Pachón in northern Chile. Scheduled to begin operations in 2025, it will conduct a 10-year survey of the Southern Hemisphere sky. By cataloging more Kuiper Belt objects, the observatory may find patterns in their orbits that either strengthen the case for Planet Nine or suggest alternative explanations.
Some astronomers propose that the observed orbital clustering of Kuiper Belt objects may be a statistical fluke or the result of observational bias, rather than the influence of a hidden planet. As research continues, the mystery of Planet Nine remains one of the most intriguing in modern astronomy.
