Register for an account


Enter your name and email address below.

Your email address is used to log in and will not be shared or sold. Read our privacy policy.


Website access code

Enter your access code into the form field below.

If you are a Zinio, Nook, Kindle, Apple, or Google Play subscriber, you can enter your website access code to gain subscriber access. Your website access code is located in the upper right corner of the Table of Contents page of your digital edition.

The Sciences

#23: Comets Are Interstellar Visitors

By Tim FolgerDecember 16, 2010 6:00 AM
Comet C/2011 N3, seen in a series of UV images just before its death. | Courtesy Wei Liu/Karel Schrijver/SDO/AIA/Lockheed Martin's Advanced Technology Center


Sign up for our email newsletter for the latest science news

Astronomers have always assumed that everything in our solar system formed around the sun some 4.5 billion years ago. But comets may be a notable exception, says Hal Levison, an astronomer at the Southwest Research Institute in Boulder, Colorado. “Hale-Bopp, Halley, all the bright guys you can think of—most of them may be from other stars,” he says.

Levison reached this conclusion after puzzling over the number of objects in the Oort cloud, the spherical collection of frozen comets and other icy debris that surrounds our solar system. Observations suggest that the Oort cloud contains as many as 400 billion objects, 100 times as many as theoretical models can account for. To resolve this discrepancy, Levison and his team simulated the dynamics of a star-forming area containing hundreds of stars packed within a region a few light-years across—the kind of setting in which our sun was probably born. Their results, published in June, show that the infant sun’s gravity could have pulled in enough comets that originated with its stellar siblings to produce the dense Oort cloud we see today. “A conservative estimate is that 90 percent of the material in the Oort cloud came from other stars,” Levison says.

3 Free Articles Left

Want it all? Get unlimited access when you subscribe.


Already a subscriber? Register or Log In

Want unlimited access?

Subscribe today and save 70%


Already a subscriber? Register or Log In