Is Planet X Real?

In January 2016, California Institute of Technology (Caltech) astronomers Konstantin Batygin and Mike Brown announced research that provided evidence for a planet about 1.5 times the size of Earth in the outer solar system. The existence of this distant world remains theoretical at this point. Batygin and Brown did not actually observe a planet.

This hypothetical Neptune-sized planet would circle our Sun on a highly elongated path, far beyond Pluto. It could have a mass about 5 to 10 times that of Earth and orbit about 20 to 30 times farther from the Sun on average than Neptune. It would take between 10,000 and 20,000 Earth years to make one full orbit around the Sun.

If it exists, this planet could help explain the unique orbits of some smaller objects in the distant Kuiper Belt, a region of icy debris that extends far beyond the orbit of Neptune.

The existence of this far-off world rests on gravitational patterns in the outer solar system, and again — the planet remains theoretical at this point.

Batygin and Brown nicknamed the hypothetical object “Planet Nine.” However, the notion of an undiscovered large planet in the realm beyond Neptune has been investigated multiple times in the past century, based on distinct lines of evidence. Sometimes it’s also referred to as “Planet X.”

NASA does not take a position on the name used. The information on this page includes both names in the interest of helping the public to find it via online search.

To be clear, Planet X (Planet Nine) has not yet been discovered, and there is debate in the scientific community about whether it exists.

As mentioned above, Batygin and Brown nicknamed their predicted object “Planet Nine,” but the formal naming rights of an object go to the person who actually discovers it. If the predicted world is found, the name must be approved by the International Astronomical Union. Planets are traditionally named for mythological Roman and Greek gods.

The names Planet X and Planet Nine are sometimes used interchangeably, but the term Planet X has been in use for more than a century. Planet Nine, meanwhile, refers to the prediction of a specific hypothesis published in 2016.

The name Planet X was initially used for a hypothesis developed by astronomer Percival Lowell in 1915 to explain what might be perturbing the orbit of Uranus. The “X” is the letter x, not the Roman numeral for 10.

Astronomers eventually determined that the perturbations to Uranus’ orbit were errors in measurement, and were not real. But the search for a giant, distant planet that Lowell kicked off led indirectly to the discovery of Pluto, in 1930.

Pluto was considered the ninth major planet in our solar system until the definition of “planet” was changed by the International Astronomical Union (IAU) in 2016. This new definition reclassified Pluto as a dwarf planet. Even before the IAU action, back when it was discovered, it was thought that Pluto was as massive as Earth. As time went on, and more was learned about Pluto, scientists corrected the mistake.

Because Pluto was now labeled a dwarf planet, and no longer considered our solar system’s ninth planet, Caltech astronomers referred to their hypothesized planet as Planet Nine when they announced it in 2016.

Astronomers studying the Kuiper Belt have noticed some of the dwarf planets and other small, icy objects in that region tend to follow orbits that cluster together. By analyzing these orbits, the Caltech team predicted the possibility that a large, previously undiscovered planet may be hiding far beyond Pluto. They proposed that gravitational interactions with a potential giant planet might explain the unusual orbits of those Kuiper objects.

It could help explain:

1. Why long-period objects in the Kuiper Belt are, on average, tilted by about 20 degrees with respect to the plane within which the planets orbit the Sun

2. Why these long-period orbits cluster in their orientations

3. Why the solar system hosts a distant population of highly inclined trans-Neptunian bodies

4. The existence of objects that reside between the giant planets and orbit the Sun in a retrograde direction

5. The persistence of long-period Kuiper Belt objects whose orbits cross the orbit of Neptune

It could also make our solar system seem a little more “normal.” Surveys of planets around other stars in our galaxy have found the most common types to be “super Earths” and their cousins — bigger than Earth, but smaller than Neptune. Yet none of this kind exist in our solar system. Planet Nine would help fill that gap.

Some of the world's most powerful telescopes, including the Hawaii-based Keck and Subaru telescopes, are searching for the planet. A NASA-funded citizen science project called Backyard Worlds: Planet 9, encourages the public to help search using images captured by NASA's Wide-field Infrared Survey Explorer (WISE) mission. WISE surveyed the full sky in four infrared wavelength bands. (WISE was renamed and repurposed as NEOWISE in 2013 to hunt for near-Earth asteroids and comets. Its mission ended on July 31, 2024).

A new observatory also may help in the hunt for Planet 9 — the Rubin Observatory on top of Cerro Pachón, a mountain in Northern Chile. The observatory is expected to begin operations in 2025. It will conduct a 10-year survey of the Southern Hemisphere sky to help answer some of astronomers' biggest questions about the universe. The observatory will search for more Kuiper Belt objects. If the orbits of these objects are systematically aligned with each other, it may give more evidence for the existence of Planet X (Planet Nine), or at least help astronomers know where to search for it. Another possibility is that Planet X (Planet Nine) does not exist at all. Some researchers suggest the unusual orbit of those Kuiper Belt objects can be explained by their random distribution.