Only those forming toward the end, when very little protoplanetary material remains, survive and don't spiral into their stars. However, protoplanetary disks are thought to last for only 10-100 million years. Under current models, there is simply not enough time for these multiple cycles of planetary formation to occur. Another suggestion is that the magnetic field of a star clears out the protoplanetary material in its immediate vicinity. Once the gas giants reaches this empty region it stops spiraling in, just prior to being gobbled up.
Many questions remain unanswered.
This is far more difficult, due to their much smaller size. But plans are underway. The Kepler 1-meter class scope, is scheduled to launch in 2005. But the program still expects to find signatures of 640 earth size planets out of 100,000 stars surveyed. And the award goes to .