Here is a link to the original article in the Washington Post, 1983 announcing Planet X. Link provided by Mike Chambers.
Possibly as Large as Jupiter;
Mystery Heavenly Body Discovered
By Thomas O’Toole
Washington Post Staff Writer
Friday, December 30, 1983 ; Page A1
A heavenly body possibly as large as the giant planet Jupiter and possibly so close to Earth that it would be part of this solar system has been found in the direction of the constellation Orion by an orbiting telescope aboard the U.S. infrared astronomical satellite.
So mysterious is the object that astronomers do not know if it is a planet, a giant comet, a nearby “protostar” that never got hot enough to become a star, a distant galaxy so young that it is still in the process of forming its first stars or a galaxy so shrouded in dust that none of the light cast by its stars ever gets through.
“All I can tell you is that we don’t know what it is,” Dr. Gerry Neugebauer, IRAS chief scientist for California’s Jet Propulsion Laboratory and director of the Palomar Observatory for the California Institute of Technology, said in an interview.
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The most fascinating explanation of this mystery body, which is so cold it casts no light and has never been seen by optical telescopes on Earth or in space, is that it is a giant gaseous planet as large as Jupiter and as close to Earth as 50 trillion miles. While that may seem like a great distance in earthbound terms, it is a stone’s throw in cosmological terms, so close in fact that it would be the nearest heavenly body to Earth beyond the outermost planet Pluto.
“If it is really that close, it would be a part of our solar system,” said Dr. James Houck of Cornell University’s Center for Radio Physics and Space Research and a member of the IRAS science team. “If it is that close, I don’t know how the world’s planetary scientists would even begin to classify it.”
The mystery body was seen twice by the infrared satellite as it scanned the northern sky from last January to November, when the satellite ran out of the supercold helium that allowed its telescope to see the coldest bodies in the heavens. The second observation took place six months after the first and suggested the mystery body had not moved from its spot in the sky near the western edge of the constellation Orion in that time.
“This suggests it’s not a comet because a comet would not be as large as the one we’ve observed and a comet would probably have moved,” Houck said. “A planet may have moved if it were as close as 50 trillion miles but it could still be a more distant planet and not have moved in six months time.”
Whatever it is, Houck said, the mystery body is so cold its temperature is no more than 40 degrees above “absolute” zero, which is 456 degrees Fahrenheit below zero. The telescope aboard IRAS is cooled so low and is so sensitive it can “see” objects in the heavens that are only 20 degrees above absolute zero.
When IRAS scientists first saw the mystery body and calculated that it could be as close as 50 trillion miles, there was some speculation that it might be moving toward Earth.