After all the hoopla about Pluto being demoted to "Dwarf Planet" status a few months back, I've gotten a lot of questions about what qualifies a hunk of rock as a planet. One of the criteria which Pluto failed to satisfy, was that of having attained sufficient mass and thereby gravitation pull so as to have cleared its orbit of other debris.
Well it turns out that a very nice image illustrating the principle turned up a few days ago. NASA's Cassini spacecraft just turned in a very nice closeup of Saturn's tiny moon Pan in the midst of the planets network of rings.
The tiny moon, little more than 16 miles in diameter, has swept clean a 200 mile-wide gap in the rings which we now call the Encke gap. Incidentally, we saw the gap first, and deduced that there must have been some sort of gravitational object to have caused the sweep, and then later found the moon.
Original Source at NASA.