On Aurelia, an Earth-sized planet half shrouded in perpetual darkness, vast floodplains give way to groves of treelike stinger fans that use ambulatory roots to creep across the muddy surface. On Blue Moon, a lunar orb in an adjacent solar system light-years from Aurelia, winged skywhales gulp aerial plankton suspended in the dense atmosphere, while balloon plants float beneath the canopies of massive pagoda tree forests, buoyed by hydrogen gas-filled membranes like miniature Hindenburgs.Sounds like a pair of scenes ripped from your standard off-world fantasy novel, except the science behind these alien planets isn't fiction. Aurelia and Blue Moon are based on computer models created by NASA and SETI Project researchers to help identify which stars among the universe's 70 sextillion are most likely to support life. CGI representations of the worlds first appeared in a National Geographic documentary; the film and related interactive simulations are on display through February at the London Science Museum. A US tour is planned for this fall.Scientists began with the essential ingredient for life: They assumed both worlds in the exhibit contained water. They then used as blueprints two scenarios formulated by the SETI Project. The first is a planet orbiting a sun close enough to keep water from freezing out, yet far enough away to avoid evaporation. The other is a moon orbiting a gas giant and warmed by twin suns.To make the worlds as realistic as possible, SETI astrophysicist Laurance Doyle and NASA researcher Manoj Joshi ran detailed climate simulations on a desktop Linux box. The sims allowed the scientists to observe the consequences on habitability of a range of complex atmospheric variables like thermal circulation and precipitation levels. Next, a group of life scientists, led by University of Cambridge paleobiologist Simon Conway Morris, applied the principles of natural selection and adaptation to populate the planets. They determined creature leg lengths and wingspans using biomechanics algorithms, and they established vegetation height and characteristics according to factors like available light and the amount of carbon dioxide in the atmosphere.