Comet Exchange 
Most of the comets that pass near the Sun are leftover building blocks from the birth of the planets. But a few may be the offspring of other star systems that were stolen away by the Sun. In fact, the Sun may have swapped comets with quite a few stars during its long lifetime.
Most of the Sun’s comets reside in the Oort Cloud — a vast “bubble” that extends more than a light-year away from the Sun. This reservoir could contain half a trillion comets. Most of them probably formed near the Sun, but were thrown into extended orbits by the gravity of the giant outer planets.
But many of the comets in the Oort Cloud today may not have been there when it formed.
Catherine Gosmeyer, a graduate student at Indiana University, simulated close encounters between the Sun and other stars. The Sun probably has had thousands of such encounters. In most of them, the two stars are moving so fast that not much happens. But the simulations show that in a few, the stars are moving slowly enough that they swap comets. The heavier star — which is usually the Sun — takes more than it gives. Over time, the exchanges add up, so a fraction of the comets in today’s Oort Cloud may be refugees from other stars.
The encounters also stir up the Oort Cloud. That sends a few comets plunging toward the Sun. As they approach the Sun they grow long, glowing tails — beautiful interlopers from the deep freeze of the outer solar system — and perhaps from other stars.
Script by Damond Benningfield, Copyright 2012