Scientists at the University of Chicago, after analyzing data on 9,250 extinct life forms, have found evidence strongly supporting the theory that mass extinctions have swept the Earth every 26 million years for at least the last 250 million years.

The evidence bolsters the claim, first advanced two years ago by the same scientists, that the Earth is visited periodically by some unknown phenomenon that can kill as many as half the existing species.

Their findings suggest that the next mass extinction is due in about 14.7 million years.

It was the earlier evidence, based on a coarser study of the fossil record, that led to speculation that asteroids or comets periodically bombard the Earth and trigger sudden, brief climatic shifts that doom many species. The most widely accepted scenario envisions the fiery impacts blasting up so much dust and smoke that, as in the hypothetical phenomenon called nuclear winter, the sky is darkened over much of the Earth, bringing on a devastating freeze.

Because no earthly mechanism is known to operate on such a protracted cycle, many scientists speculated that some source of extraterrestrial gravitation was coming close to the solar system every 26 million years. They theorized that the sun could have an unseen companion star, dubbed Nemesis, or that there was an undiscovered 10th planet, Planet X, or that the solar system periodically passed through a massive cloud of dust in the plane of the Milky Way galaxy.

The effect, the speculations held, would be for the gravity of one of these periodically passing objects to dislodge some of the billions of comets that are known to orbit the sun beyond Pluto and send them falling toward the sun. If so, some may hit the Earth on the way.

The new evidence was published this week in the journal Science by David M. Raup and J. John Sepkoski. Their earlier study was based on a compilation of the best evidence for the time when each of 567 families -- units of taxonomic classification that include one or more species -- became extinct.

The new report expands the count to 2,160 families that include nearly 11,800 genera, of which 9,250 are extinct. A genus, the singular of genera, typically includes several species.

"We think the evidence is more compelling than ever," Raup said. "We're not saying these periodic events are the only cause of extinction. Extinctions are always happening for a whole variety of independent reasons. What we are saying is that when we look at all the extinctions we can assign a date to, the percentage that occur in a given interval tends to rise and fall surprisingly regularly."

Raup and Sepkoski said they think that when the percentage of species becoming extinct at one time rises far above the "background" rate, it suggests that some wide-ranging cause was at work, some major environmental shift that many species could not survive.

In the last 250 million years, the paleontologists found, there were eight significant peaks in the extinction rate. The most recent was 11.3 million years ago. Going back in time the others occurred 38, 65, 91, 144, 194, 219 and 248 million years ago.

While the most recent four happened close to 26 million years apart, the intervals seem more irregular in the past. Raup interprets the data to suggest that the Earth may have escaped a mass extinction twice, once in the 53-million-year interval between 91 and 144 million years ago and again in the 50-million-year interval between 144 and 194 million years ago.

Raup said it is surprising that there are only two missing mass extinctions. Astrophysicists have estimated that for the Earth to be struck by one comet dislodged from the swarm beyond Pluto, thousands would have to have been dislodged and sent spiraling toward the sun. Only chance, they say, would determine whether the Earth were hit by several comets or none.

Scientists generally agree that at least one mass extinction -- the event 65 million years ago that wiped out dinosaurs -- happened about the time that an extraterrestrial object collided with Earth. The evidence is in a thin layer of sediment deposited at the time in which geologists have found an unusually high concentration of iridium, an element that is rare on the Earth's crust but much more abundant in asteroids and other extraterrestrial rocks.

They speculate that an object at least six miles wide crashed into the Earth, vaporizing the object's iridium content. Atoms of iridium then spread worldwide and settled to the ground.

If extraterrestrial impacts caused the other mass extinctions, there should be iridium-rich layers in the rock strata corresponding to those times. There are preliminary reports of at least five other iridium layers, all fitting the 26-million-year cycle, but none is as widely confirmed as the one that marks the last days of dinosaurs.