[This diagram, produced by NASA's Jet Propulsion Laboratory,

shows the nine planets as a small inset within the much larger Oort Cloud,

extending trillions of miles out from the sun.

The hypothetical planet or brown dwarf would lie about halfway out from the center of the cloud.]

Two teams of researchers have proposed the existence of an unseen planet or a failed star circling the sun at a distance of more than 2 trillion miles, far beyond the orbits of the nine known planets. The theory, which seeks to explain patterns in comets' paths, has been put forward in research accepted for publication in two separate journals.

Speculation about the existence of unseen celestial companions dates back far before the discovery of Pluto in 1929 -- and even figures in more recent fringe phenomena such as the 1997 "Heaven's Gate" tragedy and talk of a new "Planet X". This latest hypothesis, however, is aimed at answering nagging scientific questions about how particular types of comets make their way into the inner solar system. Some comets, like Halley's Comet, follow relatively short-period orbits -- circling the sun in less than two hundred years. These comets are thought to originate in the Kuiper Belt, a disk of cosmic debris that lies beyond Neptune's orbit.

The best way to think of the distances involved is in terms of Astronomical Units. One AU is the distance from Earth to the sun (93 million miles or 149.6 million kilometers). Pluto, the most distant of the planets, is at 39 AU. The Kuiper Belt extends from 30 AU to perhaps 1,000 AU.

Even further out is the Oort Cloud, a spherical haze of comets surrounding the solar system at distances between 10,000 AU and more than 50,000 AU. That's where long-period comets such as Hale-Bopp are thought to come from. For some time, astronomers have noticed that the directional patterns of these comets are not completely random. And after years of study, some researchers are reporting that the patterns hint at something big out there perturbing the cometary paths.

WHAT COULD IT BE?

No telescope has yet detected this object. But on the basis of its gravitational effect, John B. Murray, a planetary scientist at Britain's Open University, speculates that the object could be a planet larger than Jupiter, the biggest of the solar system's known planets. Murray puts the object's orbit at 32,000 AU, or 2.98 trillion miles from the sun. His proposal appears in the Oct. 11 issue of the Monthly Notices of the Royal Astronomical Society.

Meanwhile, researchers at the University of Louisiana at Lafayette say the object could be a planet or brown dwarf -- that is, a dark, failed star -- roughly three times the size of Jupiter and orbiting at 25,000 AU. The researchers, led by physicist John Matese, say their paper is to be published by the journal Icarus.

Both studies acknowledge that other factors could influence the pattern seen in long-period comets: for example, the Milky Way's gravitational tidal effects. But the Louisiana researchers say the cometary patterns are best explained by the existence of "a perturber, acting in concert with the galactic tide."

Matese said the proposed object should make one orbit around the sun every 4 million to 5 million years. Murray said the object he had in mind would make one orbit every 6 million years, circling the sun in a direction counter to that followed by the nine traditional planets.

The two researchers said they were familiar with each other's work but hadn't taken a close look at each other's studies. They acknowledged that their estimates for the mass and orbit of a mysterious object were similar, but couldn't say whether they were talking about the same object.

MORE QUESTIONS

How could such a massive object exist so far from the sun? The researchers say a planet or dark star could have coalesced during the formation of the solar system billions of years ago, but more probably would be a passing celestial body that was captured by the sun's subtle gravitational pull.

Another question: Why hasn't such an object been seen? Murray says that even a Jupiter-scale planet could not be observed at the immense distances involved. Matese and his colleagues say that their hypothetical brown dwarf wouldn't have been detected even by the Infrared Astronomical Satellite, which surveyed the heavens in 1983 -- but that the yet-to-be-launched Space Infrared Telescope Facility just might be able to pick it up.

All this may sound like science fiction, but an expert in the field notes that the hypothesis has been a subject of serious speculation for years.

"We've all wondered whether there was something out there," said Brian Marsden, who heads the International Astronomical Union's Central Bureau for Astronomical Telegrams as well as the Minor Planet Center at the Smithsonian Astrophysical Observatory.

However, Marsden also expressed some skepticism about the evidence behind the latest research. "I'm not convinced it is not due to chance," he told MSNBC in an e-mail message. "In any case, the data may not be as good as one would like."

SPECULATIVE SCENARIO

If the research holds up, it could open the door for renewed speculation on even spookier questions: Some theorists have proposed that the gravitational effect of a massive unseen object in a distant orbit -- nicknamed "Nemesis" or the "Death Star" - could set off periodic cometary storms, which would increase the chances of a catastrophic impact with Earth. Indeed, physicist Daniel Whitmire, a colleague of Matese's who is a co-author of the new research, laid out just such a scenario in 1985 to explain mass extinctions on Earth, such as the demise of the dinosaurs.

Matese also speculated back then about such an effect, but he emphasized that the newly detected object didn't fit the doomsday profile. "This object is not a Nemesis," he told MSNBC. "It does not create comet storms."

He said his proposed object appeared to have an influence on about 25 percent of the long-period comets coming in from the Oort Cloud.

Matese noted that theories proposing a correlation between extinctions on Earth and celestial orbits had fallen out of scientific favor in recent years. But he said there could be a "much more gentle" effect that links periodic changes in cratering to the solar system's oscillating motion through the galactic plane.

As the solar system moves around the Milky Way's center, it bobs slowly up and down through the galactic disk, Matese explained. The gravitational effects could cause changes in the number of comets sent into the inner solar system, he said.

"We don't know the precise period of that motion" through the plane of the galaxy, he said. "If we discover that it's closer to a 35-million-year period, then a case can be made that it causes periodic changes in cratering."

Old Greek Samian Riddle