<P> In the 1980s and 1990s, Robert Harrington led a search to determine the real cause of the apparent irregularities . He calculated that any Planet X would be at roughly three times the distance of Neptune from the Sun; its orbit would be highly eccentric, and strongly 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 . This hypothesis was met with a mixed reception . Noted Planet X sceptic 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 . </P> <P> 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 . 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 known outer planets . </P> <P> Although 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". Further analysis revealed that of several unidentified objects, nine were distant galaxies and the tenth was "interstellar cirrus"; none were found to be Solar System bodies . </P> <P> 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 . </P>

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