Like a child on a merry-go-round, the solar system bobs up and down as it orbits the center of the Milky Way galaxy.
Unlike the child, two scientists suggest in a major new interpretation of forces shaping the Earth and the course of evolution, the solar system and its planets experience cataclysms every time they pass "up" or "down" through the plane of the disk-shaped galaxy.
This happens roughly every 33 million years, and the researchers said they have found evidence that it triggers showers of comets that bombard the Earth.
They said the comet impacts, in turn, set off a variety of geological events. The Earth's crust suddenly buckles, thrusting up mountains. The slowly spreading sea floor lurches. The polarity of Earth's magnetic field reverses. And, perhaps most significant, mass extinctions occur, suddenly opening the door for many new species to evolve.
"I don't know if it's true or not but it certainly looks good on paper," said Michael R. Rampino, one of the scientists who helped develop the theory.
Rampino, a geologist at Columbia University's Lamont-Doherty Geological Observatory, developed the theory along with Richard B. Stothers, an astronomer at the National Aeronautics and Space Administration's Goddard Institute for Space Studies in New York. Their report is published in Friday's issue of the journal Science.
The paper on which the evidence "looks good" is a printout from a sophisticated statistical analysis of the known dates of various geological events. The method, developed by Stothers, suggests that several diverse phenomena on the Earth have occurred at regular intervals.
For example, the researchers analyzed 65 craters known to have been caused by impacts from extraterrestrial objects, comets or asteroids, over the last 600 million years. About 100 such craters are known, all measuring several miles in diameter and most are apparent only from the air now that erosion and other processes have blurred their ancient features.
Of the 100 or so, 65 have been date reliably as to how long ago the impact occurred. An analysis of the dates suggests that crater formation occurs in 32-million-year cycles.
Rampino and Stothers did similar analyses of the timing of six other types of well-dated ancient geological events, including episodes of rapid mountain-building, sudden changes in the rate at which sea floors spread, periods of extreme low sea levels, and reversals of the Earth's magnetic poles.
Magnetic reversals are known to have happened many times in the past but their cause is unknown. After the next reversal, a compass would point its needle toward the South Pole. All the phenomena seemed to occur in cycles of 33 million, 34 million or 35 million years.
The scientists also applied their statistical methods to the evidence for mass extinctions and found a 30-million-year cycle.
This finding differs from previous calculations made by some paleontologists who favor a 26-million-year cycle based on another method of analysis.
This earlier indication of long cycles led some astronomers earlier this year to speculate that the sun has an unseen companion star with an elliptical orbit that brings it near the solar system every 26 million years, dislodging some of the billions of comets normally in orbits on the outer fringes of the solar system.
If the comets come too close to Earth, they would be pulled in to collisions that would raise vast dust clouds, blackening the sky long enough to induce an artificial winter that could wipe out many forms of life.
According to the speculation, there was one especially massive comet, or asteroid, that hit the Earth 65 million years ago, causing a mass extinction that wiped out the dinosaurs.
Rampino said that when the same evidence on extinctions was analyzed according to Stothers' method, a period of 30 million years emerged.
"We were quite surprised when all these periods started coming out to be so close together," Rampino said. "There's something going on here but I'm not sure exactly what it is."
Rampino said that when the margins of error are taken into account, it is not unreasonable to speculate that all these phenomena have a common cause that appears roughly every 33 million years, give or take a million or two. Rejecting the companion star idea as too contrived (there is no independent evidence for it), Stothers looked for known astronomical cycles that might correlate.
He found one in the "up" and "down" motion of the solar system as it wheels around the center of the galaxy.
Every 33 million years, astronomers have calculated, the solar system passes through the plane of the galaxy. As it happens, this is occurring now. The solar system entered the region of the galactic plane about five million years ago and will take another few million years to pass through.
During the last 5 million years, Rampino said, the Earth has sustained at least three major impacts and experienced a speed-up in the rates of sea floor spreading and continental drift. These years also have seen a general cooling of climate, bringing on a period of repeated ice ages (the current period is believed to be only a lull between ice ages).
Rampino said that while some peaks of geological activity seem to be going on now, the theory does not require that all the cycles peak at the same time. Various geologic effects may follow different lag times.
Stothers also found a second astronomical cycle that seems to correlate with a second geological cycle that emerged from the earlier analyses.
In addition to a 33-million-year cycle, the calculations suggested a cycle of 260 million years, give or take 25 million years.
This happens to be about the same as the "galactic year," the time it takes the solar system to complete one orbit around the center of the galaxy.
Stothers and Rampino have no idea how this might cause upheavals on Earth, but they do offer an explanation of the 33-million-year cycle.
Since intragalactic dust clouds are denser in the galaxy's plane, Stothers speculated that when the solar system passes through, the gravitational pull of the vast dust clouds may be strong enough to dislodge comets and send them toward Earth. Like an Earth-orbiting spacecraft that, when slowed down, dives into the atmosphere, freed comets would fall toward the sun.
If many comets are dislodged, as astronomers think would be the case, a swarm could cross Earth's orbit and be pulled in to collisions of incredible force.
It has been calculated that an asteroid of about six miles in diameter, the size estimated for the object that may have wiped out the dinosaurs, would hit the Earth with a million times more energy than all the earthquakes of an average year.
"It's not hard to think that that kind of a knock would have effects that went deep inside the Earth," Rampino said. He suggested its shock waves might suddenly alter the forces driving sea floor spreading or cause a portion of continental crust to rupture and buckle, pushing up mountains.
"There's good reason to believe," Rampino said, "that the kind of planet we have, even our own existence, is the result of extraterrestrial forces."