When an asteroid crashes into a planet, then in addition to a crater, fragments and dust are created that begin to move freely in space.

If there are extraterrestrial microorganisms on the planet that has been hit, then the fragments created, will transfer them to other planets or galaxies.

Astrobiologists study this debris and dust to see if they carry life from other parts of the universe.

Although the chances of these organisms surviving on these pieces due to long distance and long time, radioactivity and temperature, scientists confirm that many such end up on Earth.

[Source: http://www.space.com]