# Planet Nine <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Extreme trans-Neptunian objects in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=etno&embed=1" data-title="Extreme trans-Neptunian objects in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=etno`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: press l to label the three real sednoids among the illustrative points and note how far beyond Neptune their closest approaches lie; switch the scale to true and scroll out until Neptune's orbit shrinks to a small ring inside their long ovals; then set the speed to 100 years/s and watch Neptune lap again and again while the distant bodies barely move along their orbits.* **Planet Nine** is a hypothetical planet in the far outer [[PORTAL_Solar_System|Solar System]], proposed to explain why the orbits of the most distant known [[Trans-Neptunian_object|trans-Neptunian objects]] appear to be clustered in orientation.[^batygin2016][^batygin2019] These [[Extreme_trans-Neptunian_object|extreme trans-Neptunian objects]] (ETNOs), with semi-major axes of hundreds of [[Astronomical_unit|astronomical units]], tend to reach perihelion in one sector of the sky and to share a similar tilt, a pattern that a massive unseen planet could maintain through its [[Gravity|gravity]].[^batygin2016][^trujillo2014] Konstantin Batygin and Michael Brown proposed the planet in its current form in 2016. Their estimates put it at about five to ten Earth masses on an eccentric orbit hundreds of AU from the [[Sun]]; later estimates have ranged from 380 AU and 460 AU to 290 AU, the last with a mass of about 4.4 Earth masses.[^batygin2019][^brown2021][^brown2022][^siraj2025] No planet has been seen. Surveys have excluded part of the sky where it might be, but a body of this size and distance would be faint enough to have been missed, and the search continues.[^brown2016][^meisner2018] Several teams argue that the clustering is an artefact of where and when surveys look, and alternative explanations range from a massive disc of small bodies to a primordial black hole.[^napier2021][^shankman2017a] The explorer at the top of this page is locked on the ETNOs: a cloud of ILLUSTRATIVE points with semi-major axes of 150 AU and more and perihelia beyond [[Neptune]]'s reach, among which the three sednoids are drawn on their real catalogued orbits.[^jpl-sbdb] It does not draw Planet Nine, which has not been found. ## History The idea of a planet beyond Neptune is older than Pluto. After Neptune's discovery in 1846, apparent irregularities in the motion of [[Uranus]] suggested another massive body, and Percival Lowell calculated an orbit for a "Planet X" and began a search in 1906. Clyde Tombaugh continued it and found [[Pluto]] in 1930, which proved far too small to be Lowell's planet.[^grosser1964][^croswell1997] After Voyager 2 passed Neptune in 1989, the residuals in Uranus's orbit turned out to come from an inaccurate value for Neptune's mass, and the original case for Planet X disappeared.[^browne1993] The modern argument rests on a different kind of evidence, the shapes and orientations of distant orbits. [[Sedna_(dwarf_planet)|Sedna]], found in 2003, has a perihelion of about 76 AU, too far for Neptune to have lifted it there; its discoverers and others proposed that an unseen planet, a star of the Sun's birth cluster, or a passing star had shaped its orbit.[^brown2004][^jpl-sbdb] In 2014 Chad Trujillo and Scott Sheppard reported 2012 VP113, a second [[Sednoid|sednoid]], with a perihelion of about 80 AU.[^trujillo2014] They noted that the ETNOs with perihelia beyond 30 AU and semi-major axes above 150 AU had arguments of perihelion clustered near 0°, and suggested that a planet of several Earth masses at 200–300 AU was holding them there.[^trujillo2014] Rodney Gomes had already argued that a distant, inclined, Neptune-mass planet could explain [[Centaur_(small_Solar_System_body)|centaurs]] with large semi-major axes.[^gomes2015] Carlos and Raúl de la Fuente Marcos proposed that more than one planet was needed.[^delafuente2014] ## Batygin and Brown hypothesis In January 2016 Batygin and Brown, both at Caltech, showed that six ETNOs with semi-major axes above 250 AU are aligned in physical space, not only in argument of perihelion. Their perihelia point in roughly the same direction and their orbital planes are tilted together, and they estimated the chance of that combination arising at random at 0.007%.[^batygin2016] Because the six were found by different surveys, a single pointing bias seemed unlikely to explain it.[^batygin2016] Differential precession should smear such an alignment out within a few hundred million years, so something must maintain it now; the authors proposed a distant planet on an eccentric orbit anti-aligned with the ETNOs.[^batygin2016] ### Orbit The 2016 paper modelled a planet on an eccentric orbit several hundred AU across, roughly in the plane of the ETNOs but with its perihelion pointing the opposite way.[^batygin2016] The 2019 review gave a semi-major axis of about 400–800 AU, an eccentricity near 0.2–0.5 and an inclination of about 15–25° to the [[Ecliptic|ecliptic]].[^batygin2019] A 2021 fit that allowed for survey biases gave a semi-major axis of about 380 AU.[^brown2021] At that distance one orbit takes about 7,400 years, from [[Kepler's_laws_of_planetary_motion|Kepler's third law]] (derived). ### Mass and radius The 2019 review estimated five to ten Earth masses, which would make it a super-Earth or a small ice giant with two to four times Earth's radius.[^batygin2019] A 2025 set of simulations by Amir Siraj, Christopher Chyba and Scott Tremaine, using 51 ETNOs, gave 4.4 ± 1.1 Earth masses.[^siraj2025] Jean-Luc Margot has noted that a body this massive would pass his test for [[Clearing_the_neighbourhood|clearing its orbit]] and so would count as a planet.[^margot-nine] For a ten-Earth-mass body with a hydrogen–helium envelope, Esther Linder and Christoph Mordasini estimated a radius of about 3.7 Earth radii and an effective temperature near 47 K.[^linder2016] ### Origin Batygin and Brown suggested that the planet formed among the giant planets and was scattered outward by [[Jupiter]] or [[Saturn]] early on, after which gas drag or a passing star in the Sun's birth cluster raised its perihelion.[^batygin2016][^batygin2019] Dynamical friction with a massive, distant belt of planetesimals could also have circularised such an orbit.[^eriksson2018] Alternatively the Sun could have captured a planet from another star while still in its birth cluster.[^mustill2016][^li2016] ### Evidence Besides the clustering, the planet would explain three other features: the high perihelia of [[Detached_object|detached objects]] such as [[Sedna_(dwarf_planet)|Sedna]], ETNOs whose orbits are nearly perpendicular to the planets', and trans-Neptunian objects with high inclinations and semi-major axes under 100 AU.[^batygin2019] Brown's analysis of discovery biases in 2017 put the chance of the observed clustering of perihelion longitudes arising from a uniform population at 1.2%, and a later analysis of fourteen ETNOs gave 0.2%.[^brown2017][^brown2019] ### Reception Brown and Batygin have been careful to call the planet a hypothesis until it is imaged. In 2016 Brown put the odds of its existence at about 90%.[^achenbach2016] Alessandro Morbidelli, who refereed the original paper, said he saw no alternative explanation.[^burdick2016] Samantha Lawler and co-authors of survey analyses have argued that the case does not survive a larger sample.[^lawler2020] ## Alternative hypotheses ### Missing planet from Nice model Simulations of the [[Nice_model|Nice model]] of early planetary migration reproduce the present Solar System more often when they start with a fifth giant planet, an ice giant that is later ejected.[^nesvorny2011][^grossman2011] Planet Nine has been proposed as that planet, left on a distant, eccentric orbit rather than lost entirely.[^tillman2017] ### Temporary or coincidental clustering The Outer Solar System Origins Survey (OSSOS), whose biases are well characterised, found eight objects with semi-major axes above 150 AU spread across many orientations, and after correcting for bias saw no significant clustering.[^shankman2017a] The Dark Energy Survey found the same for its ETNOs, and a combined analysis of three surveys concluded that the data are consistent with a uniform distribution, while noting that this does not rule the planet out.[^bernardinelli2020][^napier2021] Simulations by Cory Shankman and colleagues with a Planet Nine included produced large unobserved populations and ETNO perihelia at distances where none are seen, and they judged the planet unlikely.[^shankman2017b] ### Inclination instability in a massive disk Ann-Marie Madigan and Michael McCourt showed that a disc of eccentric bodies can become unstable and tilt into a cone, aligning arguments of perihelion without a planet, if it holds on the order of 1–10 Earth masses.[^madigan2016] ### Shepherding by a massive disk Antranik Sefilian and Jihad Touma found that a self-gravitating disc of about ten Earth masses of moderately eccentric bodies could hold the ETNOs' perihelia in place.[^sefilian2019] ### Planet in lower eccentricity orbit Renu Malhotra, Kathryn Volk and Xianyu Wang proposed that the longest-period detached objects are in mean-motion resonances with a planet on a less eccentric orbit, with an eccentricity below 0.18 and an inclination of about 11° or 48°.[^malhotra2016] ### Alignment due to the Kozai mechanism Trujillo and Sheppard's original suggestion was that a planet at 200–300 AU drives the ETNOs' arguments of perihelion to oscillate around 0° through the Kozai mechanism, so that they cross the planet's orbital plane only near perihelion and aphelion.[^trujillo2014] Batygin and Brown argued that this requires the planet's and the ETNOs' semi-major axes to be nearly equal, and so several finely tuned planets.[^batygin2016] ### Primordial black hole Jakub Scholtz and James Unwin suggested that the perturber could be a primordial black hole of planetary mass, which for five Earth masses would be only about 4–5 cm across (derived) and would explain why nothing has been seen.[^scholtz2020] ### Modified Newtonian dynamics Katherine Brown and Harsh Mathur showed in 2023 that modified Newtonian dynamics, in which the external field of the [[Milky_Way|Milky Way]] acts on the distant Solar System, would align the ETNOs' orbits toward the Galactic Centre.[^brown-mathur2023] ## Detection attempts ### Visibility and location A planet at hundreds of AU reflects very little sunlight; its apparent magnitude would be fainter than 22, some 600 times fainter than [[Pluto]].[^brown2016] By [[Kepler's_laws_of_planetary_motion|Kepler's second law]] it would spend most of its time near aphelion, where it would be faintest.[^batygin2019] Linder and Mordasini estimated a V magnitude near 21.7 for a ten-Earth-mass planet heated from inside, and a 2019 revision toward a smaller, closer orbit implied magnitude 21–22.[^linder2016][^batygin2019] ### Searches of existing data Batygin and Brown used archival data from the Catalina Sky Survey, Pan-STARRS and the Wide-field Infrared Survey Explorer to rule out much of the predicted path, leaving mainly the aphelion region and the dense star fields of the galactic plane.[^brown2016] A search of three years of Zwicky Transient Facility data excluded 56% of the remaining parameter space.[^brown2022] A combined WISE and NEOWISE search at 3.4 µm could have found a ten-Earth-mass planet out to about 800–900 AU away from the galactic plane, and found none.[^meisner2018] Searches of TESS images and of Atacama Cosmology Telescope maps also came up empty.[^rice2020][^naess2021] A 2025 study of IRAS and AKARI far-infrared data reported one candidate, which awaits confirmation.[^phan2025] ### Ongoing searches Batygin and Brown, and Trujillo and Sheppard, have searched with the 8-metre Subaru Telescope, whose wide field and large aperture suit a faint, slowly moving target.[^hand2016] A parallax-based search from the Javalambre observatory in 2022–2023 found nothing.[^socas2026] The Vera C. Rubin Observatory's survey is expected to cover much of the predicted region.[^siraj2025] ### Radiation The planet would still radiate the heat of its formation, peaking in the infrared at an effective temperature near 47 K, so millimetre-wave telescopes built to map the cosmic microwave background can also look for it.[^linder2016][^naess2021] ### Citizen science A Zooniverse project, the Catalina Outer Solar System Survey, ran from August 2020 to April 2023, using Catalina archive images to search for distant objects; it found no new bodies but added astrometry for known ones.[^zooniverse] ## Attempts to predict location ### Measurements of Saturn's orbit by the Cassini probe Ranging to the Cassini spacecraft fixed [[Saturn]]'s position very precisely. Agnès Fienga and colleagues found that Planet Nine's pull would be inconsistent with the data over parts of its orbit, with a best fit near a true anomaly of 118° and a distance near 630 AU, in Cetus.[^fienga2016] Matthew Holman and Matthew Payne tightened this to within about 20° of RA 40°, Dec −15°.[^holman2016a] JPL stated that the spacecraft itself showed no unexplained deviations.[^jpl2016] ### Analysis of Pluto's orbit Holman and Payne also analysed [[Pluto]]'s astrometry and found perturbations larger than Batygin and Brown's planet would produce, which they attributed to systematic errors, an unmodelled closer mass, or a different planet.[^holman2016b] ### Orbits of nearly parabolic comets Yuri Medvedev and colleagues identified [[Comet|comets]] whose orbits became hyperbolic, possibly after close passes of the planet, and inferred a position near aphelion in Orion.[^medvedev2017] ### Occultations by Jupiter trojans Malena Rice and Gregory Laughlin proposed a network of telescopes to time occultations of stars by [[Jupiter_trojan|Jupiter trojans]], whose orbits would record the planet's tidal pull.[^rice2019] ### Possible encounter with interstellar meteor In 2022 Héctor Socas-Navarro suggested that the meteor CNEOS 2014-01-08, a possible [[Interstellar_object|interstellar object]], reached [[Earth]] after a swing-by of Planet Nine, which would place the planet in Aries.[^socas2023] ## Attempts to predict the semi-major axis If the ETNOs sit in mean-motion resonances with the planet, their period ratios should reveal its semi-major axis, just as Neptune's resonances order the [[Kuiper_belt|Kuiper belt]]. Sarah Millholland and Gregory Laughlin found a pattern of commensurabilities fitting a planet at about 654 AU, with [[Sedna_(dwarf_planet)|Sedna]] in 3:2 and 474640 Alicanto in 3:1 resonance, and an eccentricity near 0.5 and inclination near 30°.[^millholland2017] Carlos and Raúl de la Fuente Marcos found commensurabilities consistent with a planet near 700 AU.[^delafuente2016] Three objects near 172 AU would fit resonances with a planet at 315 AU or 505 AU.[^khain2018] Elizabeth Bailey, Brown and Batygin then showed that an eccentric, inclined planet captures ETNOs into high-order resonances and moves them chaotically between them, so that current data cannot fix its semi-major axis by this route.[^bailey2018] The estimates from direct orbit fitting have instead moved inward: about 380 AU in 2021, about 460 AU after the Zwicky search, and 290 AU in the 2025 fit, with an eccentricity of 0.29 ± 0.13 and an inclination of about 6°.[^brown2021][^brown2022][^siraj2025] A planet on the last orbit would take about 4,900 years per revolution (derived). ## Naming Planet Nine has no official name. Only two planets, [[Uranus]] and [[Neptune]], have been discovered in recorded history,[^mittr2019] whereas many thousands of minor bodies have been named under established rules. If it is found, the International Astronomical Union will approve a name, normally giving priority to the discoverers' proposal.[^iau-naming] In their 2016 paper Batygin and Brown called the object only "the perturber"; the phrase "Planet Nine" came from their press material.[^batygin2016][^caltech2016] The phrase also echoes Ed Wood's film *Plan 9 from Outer Space*, a play on words picked up in the title of a 2016 *Scientific American* article.[^lemonick2016] Alan Stern objected to the name in 2018 as an erasure of Tombaugh's discovery of [[Pluto]], and he and 34 other scientists signed a statement urging neutral terms such as "Planet X" or "Giant Planet Five".[^mosher2018][^abell2018] Brown replied that Planet X was a specific, failed prediction by Lowell, unrelated to the present one.[^mosher2018] ## See also - [[Extreme_trans-Neptunian_object]] - [[Sednoid]] · [[Detached_object]] - [[Nice_model]] - [[Oort_cloud]] - Hypothetical planets of the Solar System (plain text: not yet on Wikitube) ## Notes Orbital periods marked "(derived)" come from Kepler's third law, P = a^{3/2} years with *a* in AU for a body of negligible mass orbiting the Sun: 380^{1.5} ≈ 7,400 years and 290^{1.5} ≈ 4,900 years. Perihelion distances for Sedna and 2012 VP113 are osculating values from the JPL Small-Body Database. The black hole's size is its Schwarzschild diameter, 4GM/c², for M = 5 Earth masses ≈ 3.0 × 10²⁵ kg. ## References [^batygin2016]: Batygin, K.; Brown, M. E. (2016). "Evidence for a distant giant planet in the Solar System". *The Astronomical Journal* 151: 22. https://doi.org/10.3847/0004-6256/151/2/22 [^batygin2019]: Batygin, K.; Adams, F. C.; Brown, M. E.; Becker, J. C. (2019). "The Planet Nine hypothesis". *Physics Reports* 805: 1–53. https://doi.org/10.1016/j.physrep.2019.01.009 [^trujillo2014]: Trujillo, C. A.; Sheppard, S. S. (2014). "A Sedna-like body with a perihelion of 80 astronomical units". *Nature* 507: 471–474. https://doi.org/10.1038/nature13156 [^brown2021]: Brown, M. E.; Batygin, K. (2021). "The orbit of Planet Nine". *The Astronomical Journal* 162: 219. https://doi.org/10.3847/1538-3881/ac2056 [^brown2022]: Brown, M. E.; Batygin, K. (2022). 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"A candidate location for Planet Nine from an interstellar meteoroid: The messenger hypothesis". *The Astrophysical Journal* 945: 22. https://doi.org/10.3847/1538-4357/acb817 [^millholland2017]: Millholland, S.; Laughlin, G. (2017). "Constraints on Planet Nine's orbit and sky position within a framework of mean-motion resonances". *The Astronomical Journal* 153: 91. https://doi.org/10.3847/1538-3881/153/3/91 [^delafuente2016]: de la Fuente Marcos, C.; de la Fuente Marcos, R. (2016). "Commensurabilities between ETNOs: A Monte Carlo survey". *Monthly Notices of the Royal Astronomical Society: Letters* 460: L64–L68. https://doi.org/10.1093/mnrasl/slw077 [^khain2018]: Khain, T.; Becker, J. C.; Adams, F. C.; Gerdes, D. W.; Hamilton, S.; et al. (2018). "Dynamical analysis of three distant trans-Neptunian objects with similar orbits". *The Astronomical Journal* 156: 273. https://doi.org/10.3847/1538-3881/aaeb2a [^bailey2018]: Bailey, E.; Brown, M. E.; Batygin, K. (2018). "Feasibility of a resonance-based Planet Nine search". *The Astronomical Journal* 156: 74. https://doi.org/10.3847/1538-3881/aaccf4 [^iau-naming]: International Astronomical Union. "Naming of astronomical objects". https://iauarchive.eso.org/public/themes/naming/ [^mittr2019]: "There's probably another planet in our solar system" (5 March 2019). *MIT Technology Review*. https://www.technologyreview.com/2019/03/05/136785/theres-probably-another-planet-in-our-solar-system/ [^caltech2016]: Fesenmaier, K. (20 January 2016). "Caltech researchers find evidence of a real ninth planet". California Institute of Technology. https://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523 [^lemonick2016]: Lemonick, M. D. (2016). "Planet Nine from outer space". *Scientific American* 314 (5): 36. https://doi.org/10.1038/scientificamerican0516-36 [^mosher2018]: Mosher, D. (7 June 2018). "Is it Planet 9 or Planet X? Scientists spar over what to call the solar system's hypothetical missing world". *Business Insider*. http://www.businessinsider.com/planet-nine-x-name-argument-2018-6 [^abell2018]: Abell, P.; et al. (29 July 2018). "On the insensitive use of the term 'Planet 9' for objects beyond Pluto". *Planetary Exploration Newsletter* 12 (31). http://planetarynews.org/archive18/pen_v12_n31_180729.txt ## External links - Batygin, K.; Adams, F. C.; Brown, M. E.; Becker, J. C. (2019). "The Planet Nine hypothesis". https://doi.org/10.1016/j.physrep.2019.01.009 - NASA Science: Hypothetical Planet X. https://science.nasa.gov/solar-system/planets/hypothetical-planet-x/ - JPL Small-Body Database Lookup. https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Planet_Nine) : [Wikitube](https://en.wikitube.io/wiki/Planet_Nine) · pinned revision [1374056612](https://en.wikipedia.org/w/index.php?oldid=1374056612) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-045 · explorer state `?obj=etno`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->