A Tenth Planet?

It has been 42 years since Clyde Tombaugh, at Arizona's Lowell Observatory, discovered the last and outermost of the solar system's nine known planets. But many astronomers have never given up hope of finding a tenth planet even farther from the sun. They have been encouraged in their search by irregularities in the orbit of the eighth planet, Neptune, which some suspect could be caused by the gravitational tug of a mysterious "Planet X." Until now, however, all efforts to sight Planet X have failed.

This week a University of California scientist announced that he may finally have found that elusive target. Writing in the journal of the Astronomical Society of the Pacific, Joseph L. Brady of the Lawrence Livermore Laboratory gave a description of the long-sought tenth planet, complete with its distance from the sun and its current position in the heavens. His "discovery" was made, not by scanning photographic plates, but by analyzing the erratic behavior of Halley's Comet, which comes into view every 76 years (next appearance: 1986), as it nears the sun in its elliptical and far-ranging orbit.

Checking historical observations dating back nearly 1,700 years, Brady found a peculiar irregularity: on each approach to the sun, Halley's Comet shows up as many as four days earlier or later than its predicted arrival date. That variation seemed to indicate that some unknown force must be influencing the comet's motion. Could it be the gravitational tug of a planet beyond Pluto?

For four years, Brady fed into a computer mathematical models of a ten-planet solar system, seeking the characteristics of a still undiscovered planet that would cause the irregularities in the comet's orbit. Gradually the description of Planet X emerged: it would be three times as massive as Saturn (second largest of the planets) and nearly 6 billion miles from the sun (more than half again as far as Pluto). It would take 464 years to complete a single trip around the sun, and the plane of its orbit would be tilted an angle of approximately 60DEG from the general orbital planes of the planets. Strangest of all, its motion would be retrograde; that is, it would travel around the sun in the opposite direction from all the other planets.

To convince skeptics, Brady has already begun additional computations to check the gravitational effects that Planet X would have on the known orbits of the outer planets. Still, the real test must be visual--a photograph of Planet X. At its great distance from the sun. however, Planet X would reflect only a modicum of light. Furthermore, Brady's calculations indicate that the planet is now located in the Constellation Cassiopeia, which is cluttered with so many stars that the planet would be hard to find. Nonetheless, Brady is hopeful that a sharp-eyed astronomer, scanning photographic plates, will some day detect a dim pinpoint of light reflected from far-off Planet X.

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