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A retrograde hot Jupiter is a hot Jupiter with a retrograde orbit, i.e., opposite the direction of the star's rotation. Since systems are assumed to form from rotating clouds, forming both the star's rotation and the planets' orbits from the initial cloud rotation, this is a mystery. They were of considerable interest when exoplanet discoveries began: early discoveries were generally giant planets near their host (the easiest planets to detect), and finding retrograde orbits so soon had not been anticipated. Hot Jupiters have also been found with other highly unusual orbits, e.g., polar. It is assumed that either the planet's orbit was disturbed or the star flipped its rotation direction.