BALTIMORE: Sedna, though, still might have a moon that was hiding somewhere or too dark to be photographed by the Hubble Space Telescope, said astronomer Mike Brown, its discoverer. Given the planetoid’s slow rotation, the seeming lack of a moon surprises Brown. At 800 to 1,000 miles in diameter, Sedna is too small to qualify as a planet. It is only about three quarters the size of Pluto. Objects that size should complete one rotation in a matter of hours, but observations so far show it takes 20 to 40 days, Brown said. The images show Sedna with a faint, distant star in the background. There is a small chance a moon could have been behind or in front of the planetoid, Brown said. The planetoid, believed to be half rock and half ice, is named after the Inuit goddess said to have created the sea creatures of the Arctic.