Extrasolar planet migration, illustration. The theory of planetary migration explains why some extrasolar planets are found so close to their parent star. Some are gas giants, known as 'Hot Jupiters'. These cannot form close to their parent star under accepted theories of planetary formation, so they are thought to have formed further away (upper left) and then migrated inwards. This can be due to the gravitational influence of comets (upper centre), resulting in a gradual inwards spiral (upper right). The ultimate fate may be for the planet to be destroyed by the star (lower centre), or to stabilise in a close, tidally locked orbit (lower right).