Once,there lived a asura demon called swarbhanu.His mother was simhika and father was viprachitta.The gods once asked demons for unity for churning the ocean togwther snd getting the immortialty nectar.The churning began.First kamadhenu appeared, then lakshmi, then airavata,celestial maidens and at last lord vishnu himself with pot of nectar appearead.He took a form of turtle called kurma and helped with churning.The demons and gods fighted and gods agreed that nectar should go to asuras as per vishnu's instructionsThen vishnu disguised himslef as mohini, a woman.SHe asked the asuras to take a bath before drinking the nectar of immortality and she would feed the nectar herself.Immbolised by love, They went.However, swarbhanu(a asura) judged something supicious and stayed back and disguised himself as one of the god.As lord vishnu realised, it was too late.He tried to kill him with his sudarshana chakra,but they aldready had drank the nectar, but because of the powerful effect of sudarshana chakra.They got divided into two men and are still alive.Rahu means the head and ketu means the rest of body.They then as asuras, cannot be called as gods, so they are called as planets.There are even some unlucky numbers, such as 13 etc.For one horoscope there is one unlucky no.But these are always unlucky, when add upto 4,7,8,9.