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Naval Observatory Astronomers Search for Theoretical 10th Planet

January 11, 1990

ARLINGTON, Va. (AP) _ Astronomers at the U.S. Naval Observatory are narrowing their solar system search for a 10th planet, a mysterious, phantom giant that has long captured the speculation and interest of stargazers.

Speculation about the existence of another planet evolved because something, perhaps a massive object orbiting the sun on the outer edge of the solar system, is giving a gravitational nudge that disrupts the predicted orbits of Uranus and Neptune, said astronomer R.S. Harrington.

″We are still incapable of predicting the location of Uranus,″ Harrington said Wednesday. ″It’s clear something is wrong in the outer solar system.″

Astronomers have theorized since the beginning of the century that some additional planets existed beyond the orbits of Neptune. The theory sprang from observations that Uranus failed to orbit the sun with the majestic and mathematical symmetry experts expected.

That led to a search that resulted in the 1930 discovery of Pluto, the ninth known planet in the solar system. Further searching revealed in 1978 that a moon was orbiting Pluto.

But Harrington said calculations later showed the combined mass of Pluto and its moon are about 1,000 times too small to account for the detected, but unexplained irregularity of the Neptune and Uranus orbits.

″Something is there and we’ve got to look,″ Harrington said.

Some astronomers have attributed the abnormal orbit of Uranus and Neptune to errors in calculation.

However, Harrington said a model of the solar system created by the Jet Propulsion Laboratory, which guided the Voyager spacecraft near both Uranus and Neptune, still is not consistent with recent observations on the position of Uranus. He said the JPL data seemed to be in agreement for a time, but new observations show Uranus is deviating from the expected path.

″Our current evidence is that the data is still off,″ he said. ″We still do not understand the outer solar system.″

A search for Planet X is under way in Washington, the main office of the Naval Observatory, and at the Black Birch Astrometric Observatory near Blenheim, New Zealand.

Harrington said the search has been narrowed using a computer. A number of theoretical 10-planet solar systems were simulated in a Naval Observatory computer to determine where Planet X had to be to produce the effects seen on the orbits of Uranus and Neptune.

That relatively small area of the heavens will be searched in April and May by an astrographic telescope at Black Birch. The telescope will take pictures of the target area on successive nights and the photos will be sent to Washington for study.

Harrington said he would use a machine called the blink comparator to look for the phantom planet. A blink comparator flashes back and forth between two photo plates, causing an apparent motion to the observer if any point of light in the twin photos has changed position from one night to the next.

Planet X, Harrington said, is thought to be three to five times larger than Earth and moving in an orbit about three times farther from the sun than Neptune or Pluto. That would make the phantom planet about as visible as Pluto, a planet seen only by sophisticated telescopes.

A planet in such a distant orbit would take about 1,000 years to circle the sun.

Even with the new techniques, Harrington said he was far from certain that Planet X will be found.

″I have no better than a 50-50 confidence level that this thing is there,″ he said.

Harrington discussed the Planet X search on the opening day of the 175th national meeting of the American Astronomical Society.

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