If planets, moons and asteroids are formed by clumps of irregular debris accreting and clumping together by the force of gravity, then of course there’ll be hollows in between the clumps.
The key word being "if" here... That theory is an old one and it has been around for a long, long time before any exoplanets were discovered and studied. And based on what we've learned in the last couple of decades seems to indicate that many extra-solar planetary systems could not have formed by the accretion of gases/elements, as previously believed (before we had any real data on the subject, I might add)... Which means, at this particular juncture, the bottom line is: We honestly don't know.
Post Edited (29-Jan-13 01:33)