Planet Nine is a hypothetical ninth planet in the outer reaches of the Solar System, believed to exist far beyond the orbit of Neptune. Its existence was initially posited in 2016 by Caltech researchers Konstantin Batygin and Michael Brown to explain the unusual clustered orbits of a group of distant minor planets known as extreme trans-Neptunian objects (ETNOs).
The Clues: Trans-Neptunian Objects (TNOs)
The primary evidence for Planet Nine comes from the peculiar orbital characteristics of a handful of small, icy bodies that orbit the Sun primarily beyond Neptune in the Kuiper Belt and scattered disk. These ETNOs, which include objects like Sedna, share unexpected orbital alignments:
Clustering of Perihelia: The ETNOs tend to make their closest approach to the Sun (perihelion) clustered within a small sector of the sky, instead of being randomly distributed as would be expected.
Similar Orbital Tilt: Their orbital planes are similarly tilted—about $15^\circ$ to $25^\circ$ from the plane of the eight known planets (the ecliptic).
Gravitational Dominance: Astronomers argue that the probability of these alignments occurring randomly is extremely low, suggesting that an undiscovered planet's gravitational influence is shepherding these objects into their observed configuration.
Latest Data and Characteristics
Recent data and analysis continue to refine the possible characteristics and orbital path of Planet Nine, though it remains undiscovered through direct observation.
Estimated Mass and Size: Planet Nine is generally theorized to be a Super-Earth or mini-Neptune, with a predicted mass of approximately five to ten times that of Earth ($5-10 M_{\oplus}$). Some newer analyses, however, suggest it could be more massive, possibly greater than Neptune's mass, based on candidate sightings in infrared data.
Orbital Parameters: The hypothetical planet is thought to follow a highly elongated, elliptical orbit far from the Sun. Initial estimates placed its semi-major axis between 400 and 800 astronomical units (AU), which is roughly 13 to 26 times the distance of Neptune from the Sun (Neptune is about 30 AU). More recent model refinements have varied the semi-major axis estimate, with some proposals suggesting it could be around $290\pm 30$ AU or up to $460\pm 100$ AU. It could take 10,000 to 20,000 years to complete a single orbit.
Supporting and Alternative Hypotheses
The Planet Nine hypothesis is a leading but not universally accepted explanation for the ETNO clustering.
Additional Evidence: Researchers have proposed that Planet Nine's gravity could also explain the presence of some TNOs on orbits that cross Neptune's path, arguing that a planet is needed to "slingshot" them inward. Furthermore, new lines of evidence, including the analysis of asteroid orbits inside Neptune's orbit, also appear to support the Planet Nine hypothesis.
Skepticism and Alternatives: Some astronomers remain skeptical, suggesting the apparent clustering of TNO orbits may be an observational bias—we only find objects where our surveys are most effective. Alternative theories propose that the clustering could be caused by the collective gravitational force of a massive, but unseen, disk of small icy bodies beyond Neptune, possibly with a combined mass up to ten times that of Earth.
The Ongoing Search
The hunt for Planet Nine is a race among international teams, utilizing some of the world's most powerful telescopes:
Current Search Efforts: Dedicated searches have been conducted using powerful sky surveys like Pan-STARRS and the Subaru Telescope.
Infrared Candidates: In May 2025, a team of astronomers reported finding a possible candidate for Planet Nine in archival infrared data from the IRAS (Infrared Astronomical Satellite) and AKARI surveys, taken 23 years apart. This candidate, if confirmed, would be more massive than Neptune.
Future Prospects: The upcoming Vera C. Rubin Observatory (formerly LSST) is expected to be a game-changer. Its powerful, wide-field camera will scan the southern sky nightly over the next decade, significantly increasing the chances of either directly observing Planet Nine or collecting enough TNO data to statistically refute its existence.
The existence of Planet Nine remains purely conjectural until it is directly observed, but the continued anomalies in the outer Solar System keep the search alive, much like the original prediction of Neptune's orbit led to its discovery in the 19th century.