Cometary knots are structures observed in several nearby planetary nebulae, including the Helix Nebula (NGC 7293), the Ring Nebula (NGC 6720), the Dumbbell Nebula (NGC 6853), the Eskimo Nebula (NGC 2392), and the Retina Nebula (IC 4406). It is believed that they are a common feature of the evolution of planetary nebulae, but can only be resolved in the nearest examples. They are generally larger than the size of the Solar system (i.e. the orbit of Pluto), with masses comparable to that of the Earth. The knots represent "the ionized 'skin' of a dense, dusty molecular globule" that forms a crescent-shaped structure where it is ionized and illuminated by the central star, with a trailing spoke or tail. The central globule is at least 1000 times denser than the surrounding material that stream