Comets From Edge of Solar System Unlikely to Hit Earth
A long-period comet called 2001 RX14 (Linear) turned up in images captured in 2002 by the Sloan Digital Sky Survey telescope in New Mexico.
Credit: Mike Solontoi/University of Washington

Some of the comets that make their way to Earth?s neighborhood from the frigid outer reaches of the solar system likely follow a different route than previously thought, new modeling suggests.

The study's findings, detailed in the July 31 issue of the journal Science, are good news for our planet (especially in light of Jupiter's recent impact): Comets from this region should rarely cross Earth's orbit, and so aren't a collision concern.

In turn these rare encounters mean that these comets are unlikely to be the causes of past mass extinction events.

Oort cloud origins

So-called long-period comets (those with highly elongated orbits that take them hundreds or thousands of years to circle the sun) were long thought to come from the outer region of the Oort Cloud.

The Oort Cloud is a remnant of the nebula from which the solar system formed some 4.5 billion years ago. It encircles the solar system from a point about 93 billion miles from the sun (1,000 times the distance from Earth to the sun) and extends to about three light-years away (a light-year being the distance it takes light to travel in one year, about 5.9 trillion miles).

The Oort Cloud is thought to contain billions of comets, most of which are far too small and distant to be seen ?even with powerful telescopes.

But gravitational nudges from a passing star can send the comets on a path to the inner solar system, where astronomers can finally get a glimpse of these long-exiled bodies.

There are about 3,200 known long-period comets (the most well-known of which was Comet Hale-Bopp, which was visible for much of 1996 and 1997).

Scientists thought that very few of these comets came from the inner Oort Cloud, and that they only did so when a passing star made a particularly close fly-by, setting off a comet shower in events that play out over millions of years.

"It was thought the long-period comets we see just tell us about the outer Oort Cloud," said lead author of the study Nathan Kaib, a graduate student of the University of Washington in Seattle.

Kaib's work suggests this isn't the case.

Innies, not outies

Scientists had thought that most of the comets coming from the inner Oort Cloud would be ejected from the solar system by gravitational interactions with Saturn and Jupiter, which act like body guards for the inner solar system planets.

"They cut down on the number of bodies reaching Earth-crossing orbits," Kaib said.

But after running computer models of the evolution of comet clouds for 1.2 billion years, Kaib and his colleague found that comets from the inner Oort Cloud could slip past the protective barrier of Jupiter and Saturn and reach an Earth-crossing orbit.

The new modeling suggests that a substantial portion of observable long-period comets actually come from the inner, not the outer, Oort Cloud.

Unlikely impactors

While the actual number of comets in the inner Oort Cloud is unknown, Kaib and his colleague were able to make an estimate of the highest possible number of comets in the region.

With this maximum, they could further estimate the number of comets likely to have struck Earth during the last 500 million years. They determined that it should be no more than two or three comets, which would have been part of the most powerful comet shower in that time span.

Three major impacts are known to have occurred nearly simultaneously (within a million or so years of each other) at around the same time as a mass extinction event about 40 million years ago. If that relatively minor extinction event was caused by a shower of inner Oort Cloud comets, it was likely the most intense comet shower since the fossil record began, and so it is unlikely that inner Oort Cloud comets were responsible for other extinction events (though other impactors may still be the culprits).

"That tells you that the most powerful comet showers caused minor extinctions and other showers should have been less severe, so comet showers are probably not likely causes of mass extinction events," Kaib said.

So while some comets slip through the Jupiter-Saturn barrier, most don't, and those that do aren't likely to hit Earth. Some might hit Jupiter and Saturn themselves though.

Whether or not Jupiter's bruise last week was caused by an impacting long-period comet isn't known for sure, but "it's certainly a possibility," Kaib told SPACE.com.

Kaib's work was funded by the National Science Foundation and NASA.