Planets beyond Neptune

Since the discovery of the planet Neptune in 1846, there has been considerable speculation that a ninth planet might exist beyond its orbit. The search began in the 19th century with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of Uranus and Neptune, speculating that the gravity of a large unseen planet could have perturbed their orbits enough to account for the irregularities.

Clyde Tombaugh's discovery of Pluto in 1930 initially appeared to validate Lowell's hypothesis; however in 1978, Pluto was found to be too small for its gravity to affect the gas giants, resulting in the 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 an incorrect measurement of Neptune's mass.^[1] After 1992, the discovery of numerous small icy objects within or near Pluto's orbit 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 many of the larger members of this group were initially described as planets, in 2006 the International Astronomical Union reclassified Pluto and its largest neighbours as dwarf planets, leaving only eight planets in the Solar System.^[2]

Today, the astronomical community widely agrees that Planet X, as originally envisioned, does not exist. However, the concept of Planet X has been revived by a number of astronomers to explain other anomalies observed in the outer Solar System. In popular culture, and even among some astronomers,^[3] "Planet X" has become a stand-in term for any undiscovered planet in the outer Solar System, regardless of its gravitational effect.

Planet X

Origin of the discrepancy

In the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus. Hypothesising that the perturbations were caused by the gravitational pull of another planet, Le Verrier sent his calculations to German astronomer Johann Gottfried Galle. On September 23, 1846, the night following his receipt of the letter, Galle and his student Heinrich d'Arrest found Neptune exactly where Le Verrier had predicted.^[4]

After the discovery of Neptune, however, some slight orbital discrepancies remained. These were taken to indicate the existence of yet another planet orbiting beyond Neptune. In 1905, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet,^[5] which he termed "Planet X". The "X" in the name represents an unknown and is pronounced as the letter, as opposed to the Roman numeral for ten, as it would not, at the time, have been the tenth planet. Lowell's hope in tracking down Planet X was to establish his scientific credibility, which had been dented by his widely derided belief that channel-like features visible on the surface of Mars were in fact canals constructed by an intelligent civilisation.^[6] He performed two searches for it without success, the first ending in 1909, and after revising his prediction for where it should be found, the second from 1913 to 1915, after which Lowell published his mathematical hypothesis of the parameters for Planet X.^[7]

Discovery of Pluto

Lowell and his observatory conducted his search from 1905 until his death in 1916, but to no avail. Lowell's disappointment at not locating Planet X, according to one friend, "virtually killed him".^[8] Constance Lowell, Percival Lowell's widow, subsequently embroiled the observatory in a decade-long legal battle to secure the observatory's million-dollar portion of Lowell's legacy for herself, which meant that its search for Planet X could not resume until 1929.^[9] In that year, the observatory's director, Vesto Melvin Slipher, summarily handed the job of locating the planet to Clyde Tombaugh, a 22-year-old Kansas farm boy who had only just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.^[9]

Tombaugh's task was to systematically capture sections of the night sky in pairs of images, each taken two weeks apart. The two images of each section were then placed in a machine called a blink comparator, which rapidly shifted them back and forth to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. A lesser-quality photograph taken on January 21 helped confirm the movement.^[10] Upon confirmation, Tombaugh walked into Slipher's office and declared, "Doctor Slipher, I have found your Planet X."^[10] The object lay just six degrees from one of two locations for Planet X Lowell had suggested, thus it seemed he had at last been vindicated.^[11] After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. The new object would later be found on photographs dating back to March 19, 1915.^[12]

Since astronomers thought Pluto was massive enough to perturb planets, they assumed that it may have a common asteroid albedo of 0.04, and thus a diameter of roughly 9000 km.^[13]^[14] Throughout the mid-20th century, estimates of Pluto's mass were often revised downward. James Christy's discovery of Pluto's moon Charon in 1978 allowed the direct measurement of the mass of the Pluto-Charon system for the first time. Its mass, roughly 0.2 percent that of the Earth,^[15] was far too small to account for the discrepancies in the orbit of Uranus. Lowell's "prediction" had been a coincidence; if there was a Planet X, it was not Pluto.^[16]

Further searches for Planet(s) X

After discovering Pluto, Tombaugh continued to search the ecliptic for other distant objects. He found hundreds of asteroids and variable stars, as well as two comets, but no more planets.^[17] However, a number of astronomers kept up the search for Lowell's Planet X, convinced that, since Pluto was no longer a viable candidate, some other object must be perturbing the outer planets.^[18] The existence of a tenth planet (as Pluto was by then the ninth planet) was speculated by astronomers and the general public alike.

In the 1980s and 1990s, astronomer Robert Sutton Harrington of the US Naval Observatory, who had first calculated that Pluto was too small to be Planet X, led a search to determine the real cause of the apparent irregularities.^[19] He calculated that any Planet X would be at roughly three times the distance from the sun of Neptune's orbit, highly elliptical, and far below the ecliptic (the planet's orbit would be at roughly a 90-degree angle from the orbital plane of the other known planets).^[20] This hypothesis was met with a mixed reception. Noted Planet X sceptic Brian Marsden of Harvard University's Minor Planet Center pointed out that these discrepancies were a hundred times smaller than those noticed by Le Verrier, and could easily be due to observational error.^[19]

In 1972, Joseph Brady of the Lawrence Livermore National Laboratory studied irregularities in the motion of Halley's Comet. Brady claimed that they could have been caused by a Jupiter-sized planet beyond Neptune that is in a retrograde orbit around the Sun. However, both Marsden and Planet X proponent P. Kenneth Seidelmann attacked the hypothesis, showing that Halley's Comet randomly and irregularly ejects jets of material, causing changes to its own orbital trajectory, and that such a massive object as Brady's Planet X would have severely affected the orbits of the outer planets.^[19]

While its mission did not involve a search for "Planet X", the IRAS space observatory made headlines briefly in 1983 due to an "unknown object" that was at first described as "possibly as large as the giant planet Jupiter and possibly so close to Earth that it would be part of this Solar System."^[21] However, further analysis revealed that of several unidentified objects, nine were distant galaxies and the tenth was "intergalactic cirrus"; none were found to be Solar System bodies.^[22]

Planet X disproved

Harrington died in 1993, having never found Planet X.^[19] That same year, Myles Standish used data from Voyager 2's 1989 flyby of Neptune, which had revised the planet's total mass downward by 0.5 percent (an amount comparable to the mass of Mars),^[23] to recalculate its gravitational effect on Uranus.^[24] When the newly determined mass was used in the Jet Propulsion Laboratory Developmental Ephemeris (JPL DE), the supposed discrepancies in its orbit vanished.^[1] Also, to date there are no discrepancies in the trajectories of any space probes (Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2) that can be attributed to the gravitational pull of a large undiscovered object in the outer Solar System.^[25] Today the overwhelming consensus among astronomers is that Planet X, as Lowell defined it, does not exist.

Observed trans-Neptunian objects

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After Pluto and Charon, no more trans-Neptunian objects were found until the discovery of (15760) 1992 QB1 in 1992.^[26] Since that time, hundreds of trans-Neptunian objects have been discovered. The objects are now recognised as mostly belonging to the Kuiper belt: a swarm of icy bodies orbiting in the ecliptic plane beyond Neptune, which are left over from the formation of the Solar System. Some of these distant trans-Neptunian objects, such as Sedna, were initially described in the media as "the tenth planet", though none were as large as Pluto.^[27]

In 2005, astronomer Mike Brown and his team announced the discovery of 2003 UB313 (later named Eris),^[28] a trans-Neptunian object larger than Pluto. Soon after the announcement, NASA's Jet Propulsion Laboratory put out a press release describing the object as the "tenth planet".^[29] However, the 2006 definition of planet redefined both Eris and Pluto as dwarf planets because they have not cleared the neighborhood of their orbits.^[2] In other words, they did not orbit the Sun alone, but as part of a population of similarly sized objects. Pluto itself is now recognized as being a member of the Kuiper belt and the second largest dwarf planet after Eris. A number of astronomers, most notably Alan Stern, head of NASA's New Horizons mission to Pluto, contend that the IAU's definition is flawed, and that not only Pluto and Eris, but all large trans-Neptunian objects, such as Sedna, Quaoar and Varuna, should be considered planets in their own right.^[30] Regardless, Eris is not Planet X, as it is far too small to have significantly affected the orbits of the outer planets.^[31]

Subsequent proposed trans-Neptunian planets

Though it is unlikely that Lowell's Planet X exists, the idea that a large unseen planet could create observable gravitational effects in the outer Solar System has been revived by a number of astronomers. These hypothetical objects are often referred to as "Planet X", although their link to that world is purely conceptual.^[32]^[33] The Kuiper belt terminates suddenly at a distance of 48 astronomical units (AU) from the Sun (for comparison, Neptune is 30 AU from the Sun),^[34] and there is some speculation this sudden dropoff, known as the "Kuiper cliff", may be caused by the presence of an object with a mass between that of Mars and Earth located beyond 48 AU. Computer simulations in 2008 have suggested that a body roughly the size of Earth, ejected outward by Neptune early in the Solar System's formation and currently in an elongated orbit between 80 and 170 AU from the Sun, could explain not only the Kuiper cliff but also the peculiar "detached" TNOs such as Sedna.^[35] While some astronomers have cautiously supported these claims, others have dismissed them as "contrived".^[33] An alternative hypothesis, proposed in 1999, has long period comets originating from specific regions of the sky, rather than coming from random directions as proposed by Jan Oort. This would result from comets being disturbed by an unseen object at least as large as Jupiter, and possibly a brown dwarf.^[36]^[37]

References

^ ^a ^b Tom Standage (2000). The Neptune File: A Story of Astronomical Rivalry and the Pioneers of Planet Hunting. Walker. pp. p. 188. ISBN 0802713637. {{cite book}}: |pages= has extra text (help)
^ ^a ^b "IAU 2006 General Assembly: Resolutions 5 and 6". IAU. 2006-08-24.
^ S. C. Tegler & W. Romanishin (24 May 2001). "Almost Planet X". Nature. 411: 423–424. doi:10.1038/35078164. Retrieved 2008-06-26.
^ K. Croswell (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. The Free Press. p. 43. ISBN 978-0684832524.
^ J. Rao (11 March 2005). "Finding Pluto: Tough Task, Even 75 Years Later". SPACE.com. Retrieved 2006-09-08. {{cite web}}: Check date values in: |date= (help)
^ Croswell p. 48
^ Mark Littman (1990). Planets Beyond: Discovering the Outer Solar System. Wiley. pp. p. 70. ISBN 047151053X. {{cite book}}: |pages= has extra text (help)
^ Croswell p. 49
^ ^a ^b Croswell, p. 50
^ ^a ^b Croswell p. 52
^ Croswell p. 54
^ W. G. Hoyt (1976). "W. H. Pickering's Planetary Predictions and the Discovery of Pluto". Isis. 67 (4): 551–564. doi:10.1086/351668. Retrieved 2007-06-27.
^ Dan Bruton. "Conversion of Absolute Magnitude to Diameter for Minor Planets". Department of Physics & Astronomy (Stephen F. Austin State University). Retrieved 2008-06-16. (Pluto: H=-0.7 and if Albedo=0.04 would be 9172km diameter)
^ Wm. Robert Johnston (June 28, 2003). "Asteroid albedos: graphs of data". Johnston's Archive (personal web site). Retrieved 2008-06-16. {{cite web}}: Check date values in: |date= (help)
^ Christy, J. W.; Harrington, R. S. (1978). "The satellite of Pluto" (PDF). Astronomical Journal. 83: 1005. Bibcode:1978AJ.....83.1005C.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Croswell p. 55
^ "Clyde W. Tombaugh". New Mexico Museum of Space History. Retrieved 2008-06-29.
^ Croswell p. 57
^ ^a ^b ^c ^d Croswell pp. 56-71
^ P. K. Seidelmann, R. S. Harrington (1987). "Planet X - The current status". Journa Celestial Mechanics and Dynamical Astronomy. 43: 55. doi:10.1007/BF01234554.
^ Thomas O'Toole (30 December 1983). "Mystery Heavenly Body Discovered". Washington Post. p. A1. Retrieved 2008-01-28. {{cite news}}: Check date values in: |date= (help); Unknown parameter |publisherlink= ignored (help)
^ Thomas J. Chester (CalTech). "No Tenth Planet Yet From IRAS". personal web site. Retrieved 2008-01-28.
^ Croswell, pg. 66
^ Ken Croswell (January 30, 1993). "Hopes Fade in hunt for Planet X". New Scientist. p. 18. Retrieved 2007-11-04. {{cite news}}: Check date values in: |date= (help)
^ Littmann, pg. 204
^ Minor Planet Center (1992). "Circular No. 5611". Retrieved 2008-07-02.
^ "Astronomers discover 'new planet'". BBC News. 2004. Retrieved 2008-06-20.
^ Central Bureau for Astronomical Telegrams, International Astronomical Union (2006). "Circular No. 8747" (PDF). Retrieved 2007-02-23.
^ "NASA-Funded Scientists Discover Tenth Planet". Jet Propulsion Laboratory. 2005. Retrieved 2007-02-22.
^ Alan Stern (2006). "Unabashedly Onward to the Ninth Planet". NASA. Retrieved 2008-06-25.
^ David Jewitt (University of Hawaii) (2006). "David Jewitt:Planet X". Personal web site. Retrieved 2008-05-21.
^ J. Horner and N. W. Evans (September 2002). "Biases in cometary catalogues and Planet X". Monthly Notices of the Royal Astronomical Society. 335 (3): 641–654. doi:10.1046/j.1365-8711.2002.05649.x. Retrieved 2008-06-27. {{cite journal}}: More than one of |work= and |journal= specified (help)
^ ^a ^b Schilling, Govert (2008-01-11). "The Mystery of Planet X" (subscription required). New Scientist. pp. 30–33. Retrieved 2008-06-25.
^ Williams, David R. (September 1, 2004). "Neptune Fact Sheet". NASA. Retrieved 2007-08-14. {{cite web}}: Check date values in: |date= (help)
^ Lykawka, P. S.; Mukai, T. (2008). "An Outer Planet Beyond Pluto and the Origin of the Trans-Neptunian Belt Architecture". Astronomical Journal. 135: 1161. doi:10.1088/0004-6256/135/4/1161. arXiv:0712.2198.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Murray, J. B. (1999). "Arguments for the presence of a distant large undiscovered Solar system planet". Monthly Notices of the Royal Astronomical Society. 309: 31. doi:10.1046/j.1365-8711.1999.02806.x. {{cite journal}}: Unknown parameter |laydate= ignored (help); Unknown parameter |laysource= ignored (help); Unknown parameter |laysummary= ignored (help)
^ Matese, J. J.; Whitman, P. G.; Whitmire, D. P. (1999). "Cometary Evidence of a Massive Body in the Outer Oort Clouds". Icarus. 141: 354. doi:10.1006/icar.1999.6177.{{cite journal}}: CS1 maint: multiple names: authors list (link)