There has been much speculation on the possibility that showers of comets have periodically wiped out many of Earth's life forms, including the dinosaurs. Some scientists wonder if our sun has an undiscovered dark companion -- the so-called Nemesis ``death star'' -- which sends cometary showers earthward when its orbit takes it through the reservoir of comets believed to surround the solar system. Now actual evidence of this hypothetical Nemesis may have been found.
There is a touch of irony in this. It comes at a time when the evidence for periodic mass extinctions has gone mushy. This would be another of the many occasions in science where an ill-founded notion stimulates fruitful research.
Armand H. Delsemme of the University of Toledo (Ohio) reported this research at the recent annual meeting of the Astronomical Society of the Pacific (ASP). What he has shown is that the orbits of 126 young comets reflect the gravitational influence of a massive body that could fit the Nemesis prescription. In other words, he has found what he considers to be evidence of a dark body with a mass 10 to 90 times that of Jupiter and which orbits the sun once every 5 million to 50 million years at distances approaching those of the nearest stars.
The large uncertainties in these estimates reflect the fact that Delsemme is not dealing with observations of the object itself. He is working backward from anomalies in the comet orbits to infer the causal object.
Luis Alvarez, a physicist and Nobel laureate, and his son Walter (a geologist), Frank Asaro, and Helen Michel sparked such a search six years ago by suggesting that an asteroid did in the dinosaurs. This University of California, Berkeley, team proposed that dust thrown up by the impact changed the climate drastically enough to kill off many species in the mass extinction that occurred 65 million years ago.
A major comet hitting Earth would likely cause even more ecological disruption. Thus, in 1983, when David Raup and John Sepkoski Jr. of the University of Chicago claimed the geological record shows mass extinctions occurring regularly about every 26 million years, comets were suspected.
Cometary scientists generally accept the theory developed in the early 1950s by Dutch astronomer Jan Oort that a cloud containing many trillions of incipient comets envelops the solar system. This Oort cloud extends out to interstellar distances. Passing stars can deflect objects out of the Oort cloud. Some of these objects become periodic comets.
If some other influence also deflected comets out of the Oort cloud every 26 million years or so, this might explain periodic extinctions. In such showers involving billions of comets, there would be a few that would hit Earth. Some scientists suggest that an undiscovered Planet X may cause such showers; others speculate that the periodic motion of the solar system that takes it through the plane of our galaxy every 32 million years or so may be involved. A UC Berkeley team has proposed that the sun has a dark companion star -- Nemesis.
All of these suggestions, including periodic mass extinctions, are highly controversial. Many geologists question whether comets or asteroids are needed at all to explain major extinctions. They contend that changes of climate due to less drastic causes are involved. Other scientists have shown that, even if comets could cause mass extinction, a series of such extinctions occurring periodically cannot be found in the geological record.
The case for Planet X or for passage through the galactic plane causing comet showers has been largely discredited, as Paul Weissman of the NASA Jet Propulsion Laboratory explained at the ASP meeting.
It is against this background of mounting skepticism about the whole comet-shower, mass-extinction scenario that Delsemme has come forward with some actual evidence that a Nemesis-like body may exist, whether or not it has anything to do with life on Earth.
Briefly, what he has done is to look at the orbits of 126 very ``young'' comets that have not yet been strongly influenced by encounters with the planets. He finds that these orbits show clearly that the comets were ejected from the Oort cloud by passage of a slow-moving, massive body that is bound gravitationally to the solar system. In other words, he says, he has found observational evidence that a body like Nemesis probably exists.
Astronomers, Delsemme says, should stop arguing about speculative notions and search for the object whose signature he believes he has found in the sky.
A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.