Critics, most prominent among them, British astronomer Dr. Brian Marsden with the International Astronomical Union, say that inadvertent data error is the real culprit behind the residuals, not a missing planet. In fact, says Dr. Marsden, Planet X is not a scientific problem as much as it is a psychological problem.

Nevertheless, some astronomers believe there may as yet be something we don't know about our own solar system -- an undiscovered planet, or feeble star, a million times further away than Pluto. The justification for their belief stems from an apparent orderly arrangement of certain comets in the sky.

In the October 11 issue of the Monthly Notices of the Royal Astronomical Society, Dr. John Murray, an astronomer from the Open University in the United Kingdom proposed that a large object in the extreme outer realms of the solar system may be gravitationally affecting the orbits of long-period comets. He theorizes that the object would have to orbit the sun 32,000 times farther away than Earth (about 3 trillion miles) and would have to be at least as massive as Jupiter, if not more so. Given its distance, it would also be extremely faint and slow moving.

In other research, a professor of physics at the University of Louisiana at Lafayette, Dr. John J. Matese, is making a case for the existence of a 2- to 3-Jupiter mass object orbiting some 2.3 trillion miles from the sun. In a paper soon to be published in the planetary journal, Icarus, Dr. Matese asserts that this object, too, has created a "concentration" of Oort cloud comets and is responsible for sending a significant number of them - perhaps as much as 25 percent - into the inner solar system.

None of this speculation would be possible if it weren't for a family of billions of comets that hardly ever divert into the inner solar system. Known as long-period comets, these gravelly, mountainous icebergs are thought to inhabit a vast sphere known as the Oort cloud, which surrounds the solar system between 900 billion and 4.5 trillion miles from the sun.

The intrusive gravity of some massive object could disturb these objects like a fat man diving into a school of fish, sending them off into other orbits. But the question of what disturbs them and why some of them appear to be regularly distributed like foam in the crest of a standing wave still remains.

Murray's research suggests that the some of the incoming comets include a group coming from directions in space that are aligned in an arc across the sky. This arc, he asserts, could mark the wake of some large body moving through space in the outer part of the Oort cloud. A similar theme arises in Dr. Matese's research. His study of 82 Oort cloud comets indicates that approximately 25 percent of these have an "anomalous distribution" in the sky that can best be understood if there exits some perturbing force in the Oort cloud, i.e., a large, as yet undetected, body.

Dr. Murray rules out the notion that the object might be a heretofore undiscovered brown dwarf since, being brighter than a planet, it would probably have been detected by now. He does not, however, rule out other possible explanations for the observed entrainment of comet orbits.

Not everyone agrees with the two researchers that these results suggest the existence of a mysterious tenth planet. Dr. Anita Cochran, astronomer and comet specialist at the University of Texas at Austin, says Dr. Murray's idea doesn't hold much promise. "First of all, the long-period comet orbits ARE randomly oriented," says Dr. Cochran, "especially if one takes into account the selection effects of discovery."

Most prominent among these, she says, is the fact that there are more observers in the Northern Hemisphere to discover comets than in the Southern Hemisphere, thus a number of long-period comets are probably escaping detection and analysis. Moreover, Dr. Cochran adds that anything that would perturb the periods of comets would leave a significant gravitational signature on the outer planets in the form of slight orbital changes, and that has not been seen, or at least acknowledged.

"I am not sure what [Dr. Murray] is getting at but I think he has neglected the body of previous solutions," she says.

Dr. Matese's theory focuses on different aspects of long-period comet orbits, but nevertheless begs the question: could the darkest corner of our solar system harbor a tenth planet or a brown dwarf? A brown dwarf, he contends, would not have been detected in the previous infrared searches, such as the one conducted by the Infrared Astronomical Satellite (IRAS) in the early 1980s, because the alleged planet/brown dwarf is too near the galactic plane. To ferret out such an object in that busy IR region requires greater sensitivity than IRAS possessed at the time.

For now, it seems the mysteries of the remote solar system will remain largely hidden, as will the truth about whatever exists out there. Perhaps more telling is the fact that astronomers still don't know everything there is to know about our solar neighborhood. That may change with the advent of a new generation infrared searches of bodies in the outer solar system. If a large warm planet or brown dwarf is skulking about stirring up comets, astronomers will find it.