 Because of Gliese 581 g 's proximity to its parent star , it is predicted to be tidally locked to Gliese 581 . Just as Earth 's Moon always presents the same face to the Earth , the length of Gliese 581 g 's sidereal day would then precisely match the length of its year , meaning it would be permanently light on one half and permanently dark on the other half of its surface . Tidal locking also means the planet would have no axial tilt and therefore no seasonality in a conventional sense . 
