Neptune is one of two planets not visible to Earth by the naked eye. This is perhaps the main reason why it was the first planet to be discovered by mathematical prediction. It was separately discovered in the mid-19th century by English astronomer John C. Adams and French mathematician Urbain Le Verrier. The planet is covered by thick clouds that move rapidly. NASA reports that Neptune's winds move at speeds up to 700 mph. This color-enhanced photo taken by the Hubble telescope in 2005 shows Neptune as never seen before.