Planet Nine

• 1. Hypothesis Origins And History

  ›

  In 1846, astronomers discovered Neptune after noticing irregularities in Uranus's orbit. This discovery sparked speculation that another planet might exist beyond its orbit. Percival Lowell began an extensive search for this hypothetical trans-Neptunian planet in 1906. He called the object Planet X, a name previously used by Gabriel Dallet. Clyde Tombaugh continued Lowell's search and found Pluto in 1930. However, Pluto was soon determined to be too small to qualify as Lowell's Planet X. After Voyager 2 flew by Neptune in 1989, scientists realized the difference between Uranus' predicted and observed orbit resulted from using an inaccurate mass of Neptune. George Forbes postulated two trans-Neptunian planets in 1880. One would have an average distance from the Sun of 100 AU, while the second would have a semi-major axis of 300 AU. The discovery of Sedna in 2004 led to speculation about a massive body influencing its peculiar detached orbit. In March 2014, the announcement of a second sednoid with a perihelion distance of 80 AU renewed speculation about an unknown super-Earth. Rodney Gomes proposed at a 2012 conference that an undetected planet was responsible for orbits of some extreme trans-Neptunian objects.

• 2. Orbital Mechanics And Predictions

  ›

  Batygin and Brown described how similar orbits of six extreme trans-Neptunian objects could be explained by Planet Nine in early 2016. They proposed a possible elliptical orbit around the Sun with an eccentricity of approximately 0.6. The semi-major axis was estimated to be roughly 700 astronomical units, taking the planet between 10,000 to 20,000 years to complete one full orbit. Its inclination to the ecliptic was projected to be around 30 degrees. The aphelion, or farthest point from the Sun, would lie near Taurus, while the perihelion, nearest point, would face Serpens Caput, Ophiuchus, and Libra. Brown thinks a probe could reach it in as little as 20 years using a powered slingshot trajectory. In August 2021, Batygin and Brown reanalyzed data accounting for observational biases. They predicted characteristics including a semi-major axis of 300, 520 AU and a mass of 6.2 Earth masses. Simulations showed that swarms of scattered disk objects began with random orientations were sculpted into collinear groups by a massive distant planet. Objects with semi-major axes greater than 250 AU are strongly anti-aligned with Planet Nine, with perihelia opposite its own.

• 3. Mass Composition And Physical Properties

  ›

  Planet Nine is estimated to have five to ten times the mass and two to four times the radius of Earth. Astronomer Jean-Luc Margot stated that if detected, it would satisfy criteria qualifying it as a planet. Esther Linder and Christoph Mordasini assumed its internal composition would resemble Uranus and Neptune. They calculated a hydrogen-helium atmosphere averaging 47 kelvins with an iron core and magnesium silicate mantle filled with water ice. Later simulations by Amir Siraj and colleagues in 2025 refined estimates of the planet's mass to 4.4 ± 1.1 times that of Earth. These new models suggest its composition might be closer to a rocky planet like Earth rather than an ice giant. At its estimated temperature, peak emissions occur at infrared wavelengths. Its apparent magnitude in the V filter would be 21.7, making it at least 600 times fainter than Pluto. If close to perihelion, astronomers could identify it based on existing images from surveys like WISE or NEOWISE.

• 4. Alternative Explanations For Clustering

  ›

  Several hypotheses challenge the existence of Planet Nine. The Outer Solar System Origins Survey documented over 800 trans-Neptunian objects without finding evidence for clustering. Samantha Lawler argued the hypothesis does not hold up to detailed observations given the larger sample size. She suggested gravitational occultation from Neptune during its outward migration explains extreme orbits. Ann-Marie Madigan and Michael McCourt postulate an inclination instability in a distant massive belt called a Zderic-Madigan belt. This process requires an extended time and significant mass of about one billion years for a disk containing 1, 10 Earth masses. Pedro Bernardinelli found orbital elements of ETNOs from the Dark Energy Survey showed no evidence of clustering. In 2019, Jakub Scholtz and James Unwin proposed a primordial black hole was responsible for orbit clustering. Their analysis revealed planetary mass objects more numerous than local stars in the galactic bulge direction. Modified Newtonian dynamics offers another alternative by predicting major axes align with the Galactic Center direction.

• 5. Observational Searches And Detection Efforts

  ›

  Searches of existing data have excluded much of the sky along predicted orbits. Remaining regions include directions near the Milky Way plane where distinguishing from stars is difficult. Surveys included archival data from Catalina Sky Survey to magnitude 21, 22 and Pan-STARRS to magnitude 21.5. In 2021, searches of Zwicky Transient Facility data ruled out 56% of parameter space for possible positions. The primary search uses the Subaru Telescope which has both large aperture and wide field of view. Brown and Batygin initially narrowed the search to roughly 2,000 square degrees near Orion. Subsequent refinements reduced this to 600, 800 square degrees. They spent four half-nights and three full nights observing with Subaru in December 2018. A search combining WISE and NEOWISE data covered regions away from the galactic plane at the W1 wavelength. This method could detect a 10-Earth mass object out to 800, 900 AU. By 2022, comparisons between IRAS and AKARI data yielded no detection due to contamination from galactic nebulae.

• 6. Scientific Reception And Skepticism

  ›

  Batygin was cautious interpreting results saying until Planet Nine is caught on camera it does not count as being real. All they have now is an echo. In 2016, Brown put odds for existence at about 90 percent while Greg Laughlin estimated 68.3 percent. Jim Green director of NASA's Science Mission Directorate said evidence is stronger than before but cautioned against other explanations. Tom Levenson concluded that for now Planet Nine seems the only satisfactory explanation known about outer Solar System regions. Renu Malhotra remains agnostic noting orbits seem tilted in ways difficult otherwise to explain. Ethan Siegel suggests previously observed clustering resulted from observational bias claiming most scientists think Planet Nine does not exist. Hal Levison speculates chance of ejected object ending up in inner Oort cloud is about 2 percent. Skepticism rose further in 2020 based on Outer Solar System Origins Survey results documenting over 800 trans-Neptunian objects. Both surveys adjusted for observational bias concluding practically all objects' orbits can be explained by physical phenomena rather than a ninth planet.