Editing Planets beyond Neptune (section)

=== Further searches for Planet X ===
After 1978, a number of astronomers kept up the search for Lowell's Planet&nbsp;X, convinced that, because Pluto was no longer a viable candidate, an unseen tenth planet must have been perturbing the outer planets.<ref name=cros />

In the 1980s and 1990s, Robert Harrington led a search to determine the real cause of the apparent irregularities.<ref name=cros>Croswell, pp. 56–71</ref> He calculated that any Planet&nbsp;X would be at roughly three times the distance of Neptune from the Sun; its orbit would be highly [[orbital eccentricity|eccentric]], and strongly [[orbital inclination|inclined]] to the ecliptic—the planet's orbit would be at roughly a 32-degree angle from the orbital plane of the other known planets.<ref>{{Cite journal |last=Harrington |first=R. S. |date=October 1988 |title=The location of Planet X |journal=The Astronomical Journal |volume=96 |pages=1476 |bibcode=1988AJ.....96.1476H |doi=10.1086/114898}}</ref> This hypothesis was met with a mixed reception. Noted Planet&nbsp;X skeptic [[Brian G. Marsden]] of the [[Minor Planet Center]] pointed out that these discrepancies were a hundredth the size of those noticed by Le Verrier, and could easily be due to observational error.<ref>Croswell (1997), pp. 62–63.</ref>

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 at 59 AU that is in a [[retrograde orbit]] around the Sun.<ref name=Brady1972>{{cite journal |last=Brady |first=Joseph L. |title=The Effect of a Trans-Plutonian Planet on Halley's Comet |journal=Publications of the Astronomical Society of the Pacific |volume=84 |issue=498 |pages=314–322 |date=1972 |bibcode=1972PASP...84..314B |doi=10.1086/129290|s2cid=122053270 |doi-access=free }}</ref> However, both Marsden and Planet&nbsp;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&nbsp;X would have severely affected the orbits of known outer planets.<ref>Croswell (1997), p. 63.</ref>

Although its mission did not involve a search for Planet&nbsp;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".<ref>{{Cite news |last=O'Toole |first=Thomas |date=December 29, 1983 |title=Possibly as Large as Jupiter |url=https://www.washingtonpost.com/archive/politics/1983/12/30/possibly-as-large-as-jupiter/1075b265-120a-4d40-9493-a8c523b76927/ |access-date=2024-09-11 |newspaper=[[Washington Post]] |language=en-US |issn=0190-8286 |archive-date=2023-04-01 |archive-url=https://web.archive.org/web/20230401061239/https://www.washingtonpost.com/archive/politics/1983/12/30/possibly-as-large-as-jupiter/1075b265-120a-4d40-9493-a8c523b76927/ |url-status=dead }}</ref> Further analysis revealed that of several unidentified objects, nine were distant galaxies and the tenth was "[[infrared cirrus|interstellar cirrus]]"; none were found to be Solar System bodies.<ref>{{Cite journal |last1=Houck |first1=J. R. |last2=Schneider |first2=D. P. |last3=Danielson |first3=G. E. |last4=Neugebauer |first4=G. |last5=Soifer |first5=B. T. |last6=Beichman |first6=C. A. |last7=Lonsdale |first7=C. J. |display-authors=3 |name-list-style=vanc |date=March 1985 |title=Unidentified IRAS sources - Ultrahigh-luminosity galaxies |url=https://authors.library.caltech.edu/74762/1/1985ApJ___290L___5H.pdf |journal=The Astrophysical Journal |language=en |volume=290 |pages=L5 |bibcode=1985ApJ...290L...5H |doi=10.1086/184431 |issn=0004-637X}}</ref>

In 1988, A. A. Jackson and R. M. Killen studied the stability of Pluto's resonance with Neptune by placing test "Planet X-es" with various masses and at various distances from Pluto. Pluto and Neptune's orbits  are in a 3:2 resonance, which prevents their collision or even any close approaches, regardless of their separation in the [[z axis]]. It was found that the hypothetical object's mass had to exceed 5 Earth masses to break the resonance, and the parameter space is quite large and a large variety of objects could have existed beyond Pluto without disturbing the resonance. Four test orbits of a trans-Plutonian planet have been integrated forward for four million years in order to determine the effects of such a body on the stability of the Neptune–Pluto 3:2 resonance. Planets beyond Pluto with masses of 0.1 and 1.0 Earth masses in orbits at 48.3 and 75.5 AU, respectively, do not disturb the 3:2 resonance. Test planets of 5 Earth masses with semi-major axes of 52.5 and 62.5 AU disrupt the four-million-year libration of Pluto's argument of perihelion.<ref>{{Cite journal |last1=Jackson |first1=A. A. |last2=Killen |first2=R. M. |name-list-style=amp |date=October 1988 |title=Planet X and the stability of resonances in the Neptune-Pluto system |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=235 |issue=2 |pages=593–601 |bibcode=1988MNRAS.235..593J |doi=10.1093/mnras/235.2.593 |issn=0035-8711 |doi-access=free}}</ref>