The object that hit Jupiter was definitely not Earth sized, btw, the debris cloud was.
The object could have been an asteriod that got ejected from the asteroid belt, or more likely, a visitor from the Ort cloud or Kuiper belt, out beyond the orbit of Neptune. Every once in while, a piece of rock from out there is perturbed into an orbit that approachs closer to the Sun. When that happens it will soon either be ejected by gravitational interaction with Jupiter and the other gas giants or hit something. The something most likely to get hit is Jupiter, because of his large cross section and because his gravitational field acts like a cometary vacuum cleaner, sucking in anything that comes too close.
We are only now beginning to catalog all the junk out there.
