It probably is happening all the time. And by all the time I mean as many as .0001% of solar systems may have that happening (completley pulling that number from my ass). Now, tell me, how many planets in our own solar system have we studied in real detail, I mean with a probe or manned mission. Now expand that to the number of planet systems in the Milky Way? Now, how many probes or manned miisions have we sent to the millions upon millions of galaxies outside our Milky Way?
As far as space exploration goes, we are basically like the 1800's were to medicine. We have an ok enough understanding, but we really don't have the tools, experience, ability, or the breakthrough to jump ahead yet.