The hottest planet yet discovered, KELT 9b [2], orbits its parent star from pole to pole, as seen in this NASA animation. KELT-9b is bigger and heavier than Jupiter, the giant of our own solar system. It orbits its star, which spins so fast that it's noticeably flattened, at just one-tenth of the distance from the Sun to its closest planet. So KELT-9b may top out at 7,800 degrees Fahrenheit, which is hotter than most stars. The planet’s atmosphere may be boiling off into space, forming a “tail” of super-hot gas. [NASA/JPL/Caltech]