CLOUDY ORIGINS: A comet called 2001 RX14 likely originated from a hypothesized region called the Oort cloud, far outside the planetary region of the solar system. A new model of how comets wind up near the inner planets may revise estimates of the Oort cloud's properties. Image: Mike Solontoi/University of Washington
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.