The chunks of ice and dust that make their home in the Oort cloud, far beyond the orbit of Pluto, sometimes become dislodged and head into the solar system as streaky comets. Some disruptions, caused by passing stars and other interactions with the Milky Way galaxy, are severe enough to send Oort comets into orbits that buzz or even collide with Earth. New simulations have revealed a novel mechanism for their entry into our part of the solar system, a method that also suggests that comet showers may not have been strongly involved in major extinctions on Earth.

Comet dynamics depend heavily on Jupiter and Saturn: their huge gravitational fields tend to keep objects away from Earth. Comets that manage to skirt Jupiter and Saturn, the conventional thinking goes, 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.

A computer simulation by Nathan Kaib and Thomas Quinn, both at the University of Washington, have upended this thinking. They have found that the comets that manage to cross the Jupiter-Saturn barrier do in fact originate in large numbers in the inner Oort cloud, even in the absence of a large disruption causing a comet shower. Specifically, they found that the relatively nearby objects of the inner Oort cloud can be kicked into the reaches of the outer cloud via interactions with the massive planets. 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, they slip past the gas giants. “They can basically hop over the Jupiter-Saturn barrier,” Kaib says.

Kaib and Quinn estimate that more than half of the comets we observe streaking in from the Oort cloud reach our neighborhood via this route, and other researchers agree the simulation appears valid. “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 the NASA Jet Propulsion Laboratory.

The new research presents a route for comet production “that goes some way” toward resolving discrepancies between the standard model and the observations, says Scott Tremaine, an astrophysicist at the Institute for Advanced Study in Prince­ton, N.J. “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.

Kaib and Quinn used their newfound mechanism, as well as the number of observed comets, to estimate an upper limit on how much material could be in the inner Oort cloud. They then produced a statistical model of how many comets would have hit Earth in comet showers in the past several hundred million years. Their conclusion: large cometary showers were few and hence probably did not cause more than one extinction event.

Using cometary dynamics to unwind the extinction history on Earth will likely meet with some controversy. Weissman notes that the extinction implications of Kaib and Quinn’s analysis would involve comet showers, not comets in general, and that even a diminished profile of showers does not rule out the role of comets in extinctions. One big strike, rather than a shower of small ones, is all that’s needed to trigger extinctions, he points out.

Note: This article was originally printed with the title, "Not Set in Stone (or Ice)."