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Planets beyond Neptune

Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and continued at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune,[1] speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.[2]

This article is about hypothetical planets. For known objects beyond Neptune, see Trans-Neptunian object.

Clyde Tombaugh's discovery of Pluto in 1930 appeared to validate Lowell's hypothesis, and Pluto was officially named the ninth planet. In 1978, Pluto was conclusively determined to be too small for its gravity to affect the giant planets, resulting in a brief search for a tenth planet. The search was largely abandoned in the early 1990s, when a study of measurements made by the Voyager 2 spacecraft found that the irregularities observed in Uranus's orbit were due to a slight overestimation of Neptune's mass.[3] After 1992, the discovery of numerous small icy objects with similar or even wider orbits than Pluto led to a debate over whether Pluto should remain a planet, or whether it and its neighbours should, like the asteroids, be given their own separate classification. Although a number of the larger members of this group were initially described as planets, in 2006 the International Astronomical Union (IAU) reclassified Pluto and its largest neighbours as dwarf planets, leaving Neptune the farthest known planet in the Solar System.[4]


While the astronomical community widely agrees that Planet X, as originally envisioned, does not exist, the concept of an as-yet-unobserved planet has been revived by a number of astronomers to explain other anomalies observed in the outer Solar System.[5] As of March 2014, observations with the WISE telescope have ruled out the possibility of a Saturn-sized object (95 Earth masses) out to 10,000 AU, and a Jupiter-sized (≈318 Earth masses) or larger object out to 26,000 AU.[6]


In 2014, based on similarities of the orbits of a group of recently discovered extreme trans-Neptunian objects, astronomers hypothesized the existence of a super-Earth or ice giant planet, 2 to 15 times the mass of the Earth and beyond 200 AU with possibly a highly inclined orbit at some 1,500 AU.[7] In 2016, further work showed this unknown distant planet is likely to be on an inclined, eccentric orbit that goes no closer than about 200 AU and no farther than about 1,200 AU from the Sun. The orbit is predicted to be anti-aligned to the clustered extreme trans-Neptunian objects.[8] Because Pluto is no longer considered a planet by the IAU, this new hypothetical object has become known as Planet Nine.[9]

An analysis of mid-infrared observations with the have ruled out the possibility of a Saturn-sized object (95 Earth masses) out to 10,000 AU, and a Jupiter-sized or larger object out to 26,000 AU.[6] WISE has continued to take more data since then, and NASA has invited the public to help search this data for evidence of planets beyond these limits, via the Backyard Worlds: Planet 9 citizen science project.[99]

WISE telescope

Using modern data on the of the perihelia of Saturn, Earth, and Mars, Lorenzo Iorio concluded that any unknown planet with a mass of 0.7 times that of Earth must be farther than 350–400 AU; one with a mass of 2 times that of Earth, farther than 496–570 AU; and finally one with a mass of 15 times that of Earth, farther than 970–1,111 AU.[100] Moreover, Iorio stated that the modern ephemerides of the Solar System outer planets has provided even tighter constraints: no celestial body with a mass of 15 times that of Earth can exist closer than 1,100–1,300 AU.[101] However, work by another group of astronomers using a more comprehensive model of the Solar System found that Iorio's conclusion was only partially correct. Their analysis of Cassini data on Saturn's orbital residuals found that observations were inconsistent with a planetary body with the orbit and mass similar to those of Batygin and Brown's Planet Nine having a true anomaly of −130° to −110°, or −65° to 85°. Furthermore, the analysis found that Saturn's orbit is slightly better explained if such a body is located at a true anomaly of 117.8°+11°
−10°. At this location, it would be approximately 630 AU from the Sun.[102]

anomalous precession

Using public data on the orbits of the , it has been confirmed that a statistically significant (62σ) asymmetry between the shortest mutual ascending and descending nodal distances does exist; in addition, multiple highly improbably (p < 0.0002) correlated pairs of orbits with mutual nodal distances as low as 0.2 AU at 152 AU from the Solar System's barycentre or 1.3 AU at 339 AU have been found.[103] Both findings suggest that massive perturbers may exist at hundreds of AUs from the Sun and are difficult to explain within the context of a uniform distribution of orbital orientations in the outermost Solar System.[104]

extreme trans-Neptunian objects

As of 2023 the following observations severely constrain the mass and distance of any possible additional Solar System planet:

Fictional planets of the Solar System

List of hypothetical Solar System objects

Ken Croswell (1997). . New York: The Free Press. ISBN 978-0684832524.

Planet Quest: The Epic Discovery of Alien Solar Systems

Mark Littman (1990). Planets Beyond: Discovering the Outer Solar System. New York: Wiley.  978-0471510536.

ISBN

Govert Schilling (2009). The Hunt for Planet X: New Worlds and the Fate of Pluto. New York: Springer.  978-0387778044.

ISBN

Clyde W. Tombaugh (1946). "The Search for the Ninth Planet, Pluto". Astronomical Society of the Pacific Leaflets. 5 (209): 73–80. :1946ASPL....5...73T.

Bibcode

"Planet X Discovered??" by PBS SpaceTime

SEDS on Planet X

E. Myles Standish Jr. (May 1993). "Planet X: No Dynamical Evidence in the Optical Observations". Astronomical Journal. 105 (5): 2000–2006. :1993AJ....105.2000S. doi:10.1086/116575.

Bibcode

Gerald D. Quinlan (May 6, 1993). . Nature. 363 (6424): 18–19. Bibcode:1993Natur.363...18Q. doi:10.1038/363018b0. S2CID 29058579.

"Planet X: A Myth Exposed"

Daniel P. Whitmire & John J. Matese (January 3, 1985). "Periodic Comet Showers and Planet X". Nature. 313 (5997): 36–38. :1985Natur.313...36W. doi:10.1038/313036a0. S2CID 7658694.

Bibcode

Jones, R. L.; M. E. Brown; P. A. Abell; A. C. Becker; et al. (2009). (PDF). Submission to the National Academy of Sciences Board on Physics and Astronomy. Retrieved 2009-04-10.

"Next steps in understanding the outer solar system: A whitepaper submitted to the 2010 Decadal Survey Committee"

Andrew Coates provides a simplified summary of the for Planet X

history behind the search & claims

Raymond, Sean N.; Izidoro, Andre; Kaib, Nathan A. (2023). "Oort cloud (Exo)planets". Monthly Notices of the Royal Astronomical Society: Letters. 524: L72–L77. :2306.11109. doi:10.1093/mnrasl/slad079.

arXiv