It's possible for an asteroid impact to send rocks from other planets to Earth, which is how I got myself a Mars rock and why this guy thinks he's found aliens. New simulations show that it's possible for things to work in the other direction as well, sending rocks from Earth out into our solar system and on to interstellar space.
Of all the places in the solar system (besides Earth) that might be a good place for life to hang out, Europa (one of the moons of Jupiter) seems like one of the most likely. More likely than Mars, even. It's possible that Europa has an ocean underneath its icy surface, kept nice and warm by tidal friction. Whether or not life actually originated on Europa is still up for debate, but there's another possibility: microbes may have hitched a ride there all the way from Earth.
When a large enough asteroid hits a planet, rocks are blasted out into space. New computer simulations performed by Mexican researchers have shown that these rocks get much, much farther than we ever thought, and hardy ancient bacteria (or other biological material) may even now be wandering around out there, just waiting to land on a nice moon or planet somewhere (like Europa) so that they can start multiplying. Over the course of these simulations, a bunch of material makes it to Mars, but surprisingly, at high enough impact energies more rocks will travel all the way from Earth to Jupiter, making a Europa landing a real possibility.
But why stop at Europa? As it turns out, more ejected rocks will exit the solar system entirely than will re-impact all of the planets and moons in our entire solar system put together. This not only means that a bunch of Earth critters could be flying out towards distant solar systems, but a bunch of alien critters from distant solar systems could be flying in towards us. And not just right now, but a long time ago, and perhaps that's even how all life on Earth got jump-started.