Comets From Edge of Solar System Unlikely to Hit Earth

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.