A new astronomy theory says the solar system's main asteroid belt is littered with icy invaders from far away.

The so-called invaders are asteroids that seem more like primitive frozen comets than the baked rocks that make up the overwhelming majority of asteroids between Mars and Jupiter.

A theory proposed by scientists in the journal Nature suggests the icy rocks are "contamination" from the Kuiper ("KY-per") asteroid belt beyond Neptune.

When Saturn and Jupiter scooted farther from the sun nearly 4 billion years ago, the shock sent the tiny distant meteorites spraying about. Planetary scientist Hal Levison suggests some of these got stuck with the asteroids floating between Jupiter and Mars, while others bombarded Earth and the moon.