A few times a year, a visitor from deep space swings by Earth's neighborhood. Usually coming in peace, these interlopers pass by close enough to be seen, then continue on their way.

The uninvited guests are comets, streaky globules of ice and dust dislodged from one of their usual haunts far from the sun and planets: the Oort cloud. Named for Dutch astronomer Jan Oort, who hypothesized its existence in 1950, the theorized cloud is thought to contain billions or even trillions of comets that range out a few thousand to tens of thousands of times as far from the sun as Earth is. Oort cloud comets are occasionally nudged onto trajectories carrying them into the inner solar system by the passing of nearby stars or other interactions with the rest of the Milky Way Galaxy.

During rare extreme stellar encounters, numerous Oort comets are sent flying, with some winding up in orbits that buzz or even collide with Earth. Some theories hold that these comet showers could explain some of Earth's extinction events, much as an asteroid or comet impact 65 million years ago is widely thought to be behind the demise of the dinosaurs.

The conventional wisdom of comet dynamics has long held that comets that manage to skirt Jupiter and Saturn without being thrown clear by those massive planets' gravitational influence had to have originated in the outer reaches of the Oort cloud, where perturbations from outside the solar system can be felt most strongly and are writ large across vast cometary orbits that take hundreds of years to complete. Only during comet showers caused by close stellar passages, the theory holds, have extreme gravitational disruptions brought inner Oort cloud comets into the mix.

But a new paper published online today by Science contends that the comets that manage to cross the solar system's so-called Jupiter–Saturn barrier do in fact originate in large numbers in the inner Oort cloud, even in the absence of a massive disruption causing a comet shower. What is more, the authors say, this newly unveiled mechanism implies that comet showers are likely not to blame for major extinctions in the past.

The relatively nearby objects of the inner Oort cloud can be temporarily kicked into the reaches of the outer cloud via interactions with the massive planets, according to a simulation carried out by graduate student Nathan Kaib and his doctoral adviser, Thomas Quinn, both of the University of Washington in Seattle. Those newly far-flung comets, suddenly enjoying a longer orbit and greater gravitational perturbations from interstellar space, can find their orbits so changed that by the time they pass through the planetary region again, the gauntlet of the massive planets has been cleared. "They can basically hop over the Jupiter–Saturn barrier," Kaib says.

"This mechanism…could work"
Kaib and Quinn estimate that more than half of the comets we see streaking in from the Oort cloud reach our neighborhood via this route, and at least two researchers in the field agree the simulation appears to hold water. "This mechanism, this dynamical path, as we call it, could work and could be a significant contributor," says Paul Weissman, a senior research scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., who did not contribute to the Kaib and Quinn study.

The new research presents a route for comet production "that sort of had not been thought about before and that goes some way toward resolving at least one of the issues...where there are discrepancies between the [standard] model and the observations," says Scott Tremaine, an astrophysicist at the Institute for Advanced Study in Princeton, N.J., who also did not work on the new Oort cloud study.

"One of the issues is that [the conventional view of] the cometary formation process is so inefficient; in order to produce the number of comets that we see, you'd need a really massive protoplanetary disk, one that appears to be incompatible with our best estimates from other sources," Tremaine says. "This could help solve this problem."

Implications for extinction events
Kaib and Quinn used their newfound mechanism, as well as the number of observed comets, to set an estimated upper limit on how much material could be in the inner Oort cloud. Given the efficiency of the process by which inner Oort cloud comets can reach the inner solar system, "it would be hard to fit more bodies in there without producing a larger comet flux than we see today," Kaib says.

Using that upper limit, the researchers produced a statistical model of how many comets would be expected to have impacted Earth in comet showers over the past several hundred million years. What Kaib and Quinn found was that a suspected bombardment at the end of the Eocene epoch, some 35 million years ago, which some have theorized caused a partial extinction, was likely the largest seen in the past 500 million years.

"Statistically speaking, there should be these close stellar passages every 50 million or 100 million years, so that's been proposed as a possible mechanism to produce many of these extinction events," Kaib says. "And so basically we showed that you might be able to produce one minor event, but beyond that they're really not a robust mechanism to produce multiple extinction events."

The extension of the findings to unwinding the extinction history on Earth will likely meet with more controversy than will the new mechanism for comet production. "Of course, any extrapolation is dicey," Tremaine says. "I think that's an interesting result but not the most interesting result of the paper because...these sorts of calculations always involve an extrapolation of what we know."

Weissman notes that the extinction implications involve comet showers, not comets in general, and that even a diminished profile of showers does not rule out their role in extinctions. "Just because the 'biggest' observed comet shower did not cause a major extinction, that does not mean that other showers will not cause a major extinction," he says, adding it is likely that not the multiplicity of strikes but rather the magnitude of the largest strike can wipe out species.

With enough impacts both from showers and from the odd comet strike during other times, "combined with the expected size distribution of comets, the probability is that there will be several big impacts included in the mix, and those big impacts can each cause an extinction," Weissman says.