The articles below, detailing a search for a Planet X, or the 10th planet in our solar system, are speaking of the same planet Sitchin calls the 12th Planet. In his book, The 12th Planet, Sitchin explains that the ancient Sumerians counted the Sun and the Earth's moon as planets, and thus the Sun, Earth, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto added up to 11 planets. Modern astronomy excludes the Sun and the Earth's Moon, counting only 9 planets in our known solar system.
Search for the Tenth Planet
Astronomers are readying telescopes to probe the outer reaches of our solar system for an elusive planet much larger than Earth. Its existence would explain a 160-year-old mystery. ... The pull exerted by its gravity would account for a wobble in Uranus' orbit that was first detected in 1821 by a French astronomer, Alexis Bouvard. Beyond Pluto, in the cold, dark regions of space, may lie an undiscovered tenth planet two to five times the size of Earth. Astronomers at the U.S. Naval Observatory (USNO) are using a powerful computer to identify the best target zones, and a telescopic search will follow soon after. ... Van Flandern thinks the tenth planet may have between two and five Earth masses and lie 50 to 100 astronomical units from the Sun. (An astronomical unit is the mean distance between Earth and the Sun.) His team also presumes that, like Pluto's, the plane of the undiscovered body's orbit is tilted with respect to that of most other planets, and that its path around the Sun is highly elliptical.
New York Times
June 19, 1982
A pair of American spacecraft may help scientists detect what could be a 10th planet or a giant object billions of miles away, the national Aeronautics and Space Administration said Thursday. Scientists at the space agency's Ames Research Center said the two spacecraft, Pioneer 10 and 11, which are already farther into space than any other man-made object, might add to knowledge of a mysterious object believed to be beyond the solar system's outermost known planets. The space agency said that persistent irregularities in the orbits of Uranus and Neptune "suggest some kind of mystery object is really there" with its distance depending on what it is. If the mystery object is a new planet, it may lie five billion miles beyond the outer orbital ring of known planets, the space agency said. If it is a dark star type of objet, it may be 50 billion miles beyond the known planets; if it is a black hole, 100 billion miles. A black hole is a hypothetical body in space, believed to be a collapsed star so condensed that neither light nor matter can escape from its gravitational field.
Does the Sun Have a Dark Companion?
June 28 1982
When scientists noticed that Uranus wasn't following its predicted orbit for example, they didn't question their theories. Instead they blamed the anomalies on an as yet unseen planet and, sure enough, Neptune was discovered in 1846. Now astronomers are using the same strategy to explain quirks in the orbits of Uranus and Neptune. According to John Anderson of the Jet Propulsion Laboratory in Pasadena, Calif., this odd behavior suggests that the sun has an unseen companion, a dark star gravitationally bound to it but billions of miles away. ... Other scientists suggest that the most likely cause of the orbital snags is a tenth planet 4 to 7 billion miles beyond Neptune. A companion star would tug the outer planets, not just Uranus and Neptune, says Thomas Van Flandern of the U.S Naval Observatory. And where he admits a tenth planet is possible, but argues that it would have to be so big - a least the size of Uranus - that it should have been discovered by now. To resolve the question, NASA is staying tuned to Pioneer 10 and 11, the planetary probes that are flying through the dim reaches of the solar system on opposite sides of the sun.
Searching for a 10th Planet
The hunt for new worlds hasn't ended. Both Uranus and Neptune follow irregular paths that observers can explain only by assuming the presence of an unknown body whose gravity tugs at the two planets. Astronomers originally though Pluto might be the body perturbing its neighbors, but the combined mass of Pluto and its moon, Charon, is too small for such a role. ... While astronomers believe that something is out there, they aren't sure what it is. Three possibilities stand out: First, the object could be a planet - but any world large and close enough to affect the orbits of Uranus and Neptune should already have been spotted. Searchers might have missed the planet, though, if it's unusually dark or has an odd orbit. ...
NASA has been recording velocities for a year now and will continue for as long as necessary. This past spring, it appeared that budget cuts might force the end of the Pioneer project. The space agency now believes that it will have the money to continue mission operations. Next year, the JPL group will begin analyzing the data. By the time the Pioneer experiment shows results, an Earth-orbiting infrared telescope may have discovered the body. ... Together, IRAS and the Pioneers will allow astronomers to mount a comprehensive search for new solar system members. The two deep space probes should detect bodies near enough to disturb their trajectories and the orbits or Uranus and Neptune. IRAS should detect any large body in or near the solar system. Within the next year or two, astronomers may discover not one new world, but several.
New York Times
January 30, 1983
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. ... 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 last 19th century, two American astronomers, Willian H. Pickering and Percival Lowell, predicted the size and approximate location of the trans-Neptunian body, which Lowell called Planet X. 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 observation 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 1/5 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. Richard 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. ...
US News World Report
Planet X - Is It Really Out There?
Sept 10, 1984
Shrouded from the sun's light, mysteriously tugging at the orbits of Uranus and Neptune, is an unseen force that astronomers suspect may be Planet X - a 10th resident of the Earth's celestial neighborhood. Last year, the infrared astronomical satellite (IRAS), circling in a polar orbit 560 miles from the Earth, detected heat from an object about 50 billion miles away that is now the subject of intense speculation. "All I can say is that we don't know what it is yet," says Gerry Neugesbeuer, director of the Palomar Observatory for the California Institute of Technology. Scientists are hopeful that the one-way journeys of the Pioneer 10 and 11 space probes may help to locate the nameless body.