......Something out there beyond the farthest reaches of the known solar system seems to be tugging at Uranus and Neptune. Some gravitational force keeps perturbing the two giant planets, causing irregularities in their orbits. The force suggests a presence far away and unseen, a large object that may be the long-sought Planet X.
......Evidence assembled in recent years has led several groups of astronomers to renew the search for the 10th planet. They are devoting more time to visual observations with the 200-inch telescope at Mount Palomar in California. They are tracking two Pioneer spacecraft, now approaching the orbit of distant Pluto, to see if variations in their trajectories provide clues to the source of the mysterious force. And they are hoping that a satellite-borne telescope launched last week will detect heat “signatures” from the planet, or whatever it is out there.

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.......The Infrared Astronomical Satellite was boosted into a 560-mile-high polar orbit Tuesday night from Vandenberg Air Force Base, CA. It represents an $80-million venture by the United States, Britain and the Netherlands. In the next six or seven months, the telescope is expected to conduct a wide-ranging survey of nearly all the sky, detecting sources not of ordinary light, but of infrared radiation, which is invisible to the human eye and largely absorbed by the atmosphere. Scientists thus hope that the new telescope will chart thousands or infrared-emitting objects that have gone undetected – stars, interstellar clouds, asteroids and, with any luck, the object that pulls at Uranus and Neptune.
.......The last time a serious search of the skies was made, it led to the discovery in 1930 of Pluto, the ninth planet. But the story begins more than a century before that, after the discovery of Uranus in 1781 by the English astronomer and musician William Herschel. Until then, the planetary system seemed to end with Saturn.

.......As astronomers observed Uranus, noting irregularities in its orbital path, many speculated that they were witnessing the gravitational pull of an unknown planet. So began the first planetary search based on astronomers’ predictions, which ended in the 1840’s with the discovery of Neptune almost simultaneously by English, French and German astronomers.
......But Neptune was not massive enough to account entirely for the orbital behavior of Uranus. Indeed, Neptune itself seemed to be affected by a still more remote planet. In the late 19th century, two American astronomers, William H. Pickering and Percival Lowell, predicted the size and approximate location of the trans-Neptunian body, which Lowell called Planet X.


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.......Years later, Pluto was detected by Clyde W. Tombaugh working at Lowell Observatory in Arizona. Several astronomers, however, suspected it might not be the Planet X of prediction. Subsequent observations proved them right. Pluto was too small to change the orbits of Uranus and Neptune; the combined mass of Pluto and its recently discovered satellite, Charon, is only one-fifth that of Earth’s moon.
.......Recent calculations by the United States Naval Observatory have confirmed the orbital perturbation exhibited by Uranus and Neptune, which Dr. Thomas C. Van Flandern, an astronomer at the observatory, says could be explained by “a single undiscovered planet.” He and a colleague, Dr. Robert Harrington, calculate that the 10th planet should be two to five times more massive than Earth and have a highly elliptical orbit that takes it some 5 billion miles beyond that of Pluto – hardly next-door but still within the gravitational influence of the Sun.
.......Some astronomers have reacted cautiously to the 10th-planet predictions. They remember the long, futile quest for the planet Vulcan inside the orbit of Mercury; Vulcan, it turned out, did not exist. They wonder why such a large object as a 10th planet escaped the exhaustive survey by Mr. Tombaugh, who is sure it is not in the two-thirds of the sky he examined. But according to Dr. Ray T. Reynolds of the Ames Research Center in Mountain View, CA, other astronomers “are so sure of the 10th planet, they think there’s nothing left but to name it.”
.......At a scientific meeting last summer, 10th-planet partisans tended to prevail. Alternative explanations for the outer-planet perturbations were offered. The something out there, some scientists said, might be an unseen black hole or neutron star passing through the Sun’s vicinity. Defenders of the 10th planet parried the suggestions. Material falling into the gravitational field of a black hole, the remains of a very massive star after its complete gravitational collapse, should give off detectable x-rays, they noted; no X-rays have been detected. A neutron star, a less massive star that has collapsed to a highly dense state, should affect the courses of comets, they said, yet no such changes have been observed.
.......More credence was given to the hypothesis that a “brown dwarf” star accounts for the mysterious force. This is the informal name astronomers give to celestial bodies that were not massive enough for their thermonuclear furnaces to ignite; perhaps like the huge planet Jupiter, they just missed being self-illuminating stars.
.......Most stars are paired, so it is not unreasonable to suggest that the Sun has a dim companion. Moreover, a brown dwarf in the neighborhood might not reflect enough light to be seen far away, said Dr. John Anderson of the Jet Propulsion Laboratory in Pasadena, CA. Its gravitational forces, however, should produce energy detectable by the Infrared Astronomical Satellite.
.......Whatever the mysterious force, be it a brown dwarf or a large planet, Dr. Anderson said he was “quite optimistic” that the infrared telescope might find it and that the Pioneer spacecraft could supply an estimate of the object’s mass. Of course, no one can be sure that even this discovery would define the outermost boundary of the solar system.