Planet X and the killer comets.

Another entry has just been added in the contest to devise an astronomical theory explaining the periodic showers of comets that are trought by many to wipe out life forms on earth every 26 million years or so (SN: 10/11/83, p. 212). Astrophysicist Daniel P. Whitmire and John J. Matese at the University of Southwestern Louisiana in Lafayette created the new theory by merging two ideas that were proposed in the past for reasons totally unrelated to periodic comet impacts and mass extinctions: a tenth planet, dubbed Planet X and envisioned to reside beyond the orbit of Pluto; and a disk or belt of comets throught to lie in the plane of the solar system beyond Neptune.

By fashioning a complex motion for Planet X, the theorists designed a model in which the planet periodically crosses near the belt of comets, disrupting their orbits and sending them to rain on the solar system.

So far their paper, published in the Jan. 3 NATURE and presented Jan. 11 at a symposium on the Galaxy and the Solar System (held just before the annual meeting of the american astronomical society) in Tucson, Ariz., has received limited scrutiny. Other scientists, including proponents of rival theories, call the Planet X concept imaginative, even ingenious, but say that it is too hard to judge the model iwthout more detailed calculations.

In the scenario developed by whitmire and Matese, Planet X would move at an average distance from the sun of 100 astronomical units (AU), or 100 times the distance between the earth and the sun, in a moderately elliptical orbit that is inclined from the plane of the solar system at an angle of about 45 degrees. Planet X is envisioned to complete one orbit every 1,000 years. But the orbit itself, like the orbits of other planets, would revolve, or precess, around the sun in this model because of the gravitational tugs from the other planets. Comet showers would be triggered every 28 million years -- whenever the orbit moves close to the comet belt.

Whitmire and Matese propose that the belt, which was orginally hypothesized in conjunction with theories on the origin of the solar system, extends from 35 to 70 AU. From 70 to 130 AU, Planet X would have cleared a gap in the belt, which then resumes beyond 130 AU. While this comet belt has never been seen sys Whitmire, it is widely thought to exist, especially the section of the belt closest to the sun. The gravitational pull of Planet X would dislodge comets near the gap when either the perihelion (point on the orbit closest to the sun) brushes by the inner edge of the gap, or the aphelion (point farther from the sun) graces the outer gap edge, although Whitmire believes the former effect is stronger than the latter since the comet belt is most dense closest to the sun. Whitmire sees two major advantages of the Planet X approach over the competing Nemesis theory, of which Whitmire was in fact one of the original creators. Nemesis is the name of the proposed sister star to the sun that is envisioned to intrude on the so-called Oort cloud of comets at distances much farther from the sun than the proposed orbit of Planet X (SN: 4/21/84, p. 250).

With Planet X, "we're not postulating the existence of anything that hasn't already been postulated before for other reasons," says Whitmire. The idea that there could be an extra planet cruising the periphery of the planetary system has been put forth a number of times over the last 100 years in order to account for the observed deviations in the motions of the known outermost planets from their predicted courses (SN: 1/31/81, p. 68). While other suns are known to have companion stars, there is no independent astronomical evidence that Nemesis exists, Whitmire says. Moreover, past studies have concluded that the "missing planet" should have 1 to 5 times the mass of the earth and should be found 50 to 100 AU from the sun, Characteristics consistent with Whitmire and Matese's Planet X theory for comet impacts.

The second advantage, according to Whitmire, is that the orbit of Planet X, being much closer to the sun than Nemesis, would be very stable. Recent calculations on the orbit of Nemesis, on the other hand, indicate that its period has changed by 15 percent over the last 250 million years because of the gravitational nudges from other bodies (SN: 11/3/84, p. 279). "This is not necessarily a fatal objection to Nemesis, but it's fthe one that's most often raised," he explains.

Both the Planet X and Nemesis ideas can accommodate a range of values for the period, which is an asset at present because there is some uncertainty and disagreement over the exact period for the fossil, crater and other geological records. But this flexibility is also a disadvantage, says Richard B. Stothers at the NASA Goddard Institute for Space Studies in New York, because the period can't be used to make testable predictions with either theory.

Stothers and co-worker Michael R. Rampino prefer a model in which the solar system oscillates through the galactic plane at the known time interval of 33 million years -- corresponding to periodicities the researchers claim to see in geological records (SN: 1/12/85, p.24).

All of the scientists involved in the debate do agree that the solution will depend on moe accurate dating of the geological and fossil records. Astronomers have also been looking for Nemesis. And, according to Ray Reynolds at NASA Ames Research Center in Moffett Field, Calif., who with co-workers had been planning to search for Planet X for a number of years, the data from the Infrared Astronomy Satellite (IRAS) have just been put in a form that can be analyzed to look for Planet X.

One of the reason why Planet X may not have been found in the past, says Whitmire, is that previous surveys concentrated on the Northern Hemisphere while recent calculations show that Planet X, if it exists, is more likely to be found in the Southern Hemisphere. The IRAS data cover both hemispheres.

In the meantime, comments Stothers, "I think we haven't seen the last of the astronomical mechanisms. I have a stack of preprints related to all this on my desk.... The field is full of flowers."