Ketu is generally referred to as a "shadow" planet. It has a tremendous impact on human lives and also the whole creation. Astronomically, Ketu and Rahu denote the points of intersection of the paths of the Sun and the Moon as they move on the celestial sphere.
Ketu (Sanskrit: IAST: Ketú) (U+260B.svg) is the descending lunar node in Vedic, or Hindu astrology. After the head of Svarbhānu, an Asura, was cut off by God Vishnu, his head and body joined with a snake to form 'Ketu', representing the body without a head, and Rahu, representing the head without a body. According to some accounts in Hindu mythology, Ketu belongs to Jaimini Gotra, whereas Rahu is from Paiteenasa gotra and hence both are totally different entities with distinct characteristics and not two parts of a common body. Ketu is generally referred to as a "shadow" planet. It is believed to have a tremendous impact on human lives and also the whole creation. In some special circumstances it helps someone achieve the zenith of fame. Ketu is often depicted with a gem or star on his head signifying a mystery light.
Astronomically, Ketu and Rahu denote the points of intersection of the paths of the Sun and the Moon as they move on the celestial sphere. Therefore, Rahu and Ketu are respectively called the north and the south lunar nodes. The fact that eclipses occur when the Sun and the Moon are at one of these points gives rise to the understanding of swallowing of the Sun and the Moon by the snake (Rahu).
In ancient Tamil astrological scripts, Ketu was considered as incarnation of Indra. During a war with Asuras, Indra was defeated and took a passive form and a subtle state as Ketu. Indra spent this time realizing his past mistakes, and failures and that lead to spirituality towards Lord Shiva.