Alien planet forming. Computer model of planet formation in a more massive protoplanetary disc than that from which our Solar System formed. The planet (white) is at centre. Arrows (yellow) show disc components' movements. Protoplanetary discs comprise gas left from a star's formation. The planet has swept out a clear area (dark) in the gas. Computer models such as this have shown that the formation of one massive planet triggers the formation of other planets from the gas on the edges of the cleared area. This causes high-mass planets to form close to a star, as has been seen in several extrasolar planetary systems. This work was done at the University of Toronto, Canada.