Planet X: 1980's SEARCH Rationale

However the story might have changed SINCE the 1983 (gasp!) discovery,
here is what was reported when BEFORE the discovery, when panic in the
public was not a concern.  

Astronomy
Search for the Tenth Planet
Dec 1981

    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.

Astronomy
Searching for a 10th Planet
Oct 1982

    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. ...

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. ... 
    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. ...