Rahu, the 8th planet, in shadows and like Ketu, this planet is also not visible in the galaxy. A mine of information about Rahu is available in Brahma puraana, Mahabhagavata, Mahabharat, Matsya puraan, Vishnu puraan and Rigved.
Rahu was born to Simhika and Viprachitti. Rahu’s mother simhika (also known as Simhita) was the daughter of Hiranyakashyapa (Bhakta Prahlada’s father). There are several brothers to Rahu and they are Salya, Nabha, Vaataapi, Ilvala and Namuchi. It is also believed that Rahu is the eldest of the 100 brothers. He also has a sister by name Maahishmati.Born into paithenasa gotra, He was born in the year parthiva in bhaadrapada maasa, krishna paksha on the fourteenth day of the wanning moon on a Sunday. Two days before NAVRATRI. Visakha is his birth star.
It so happened that Sukracharya (Venus) was blessed with the knowledge of “Sanjivani Mantra” also known as the Maha Mrityunjay Mantra after great tapasya for the benefit of all people (universal benefit). Armed with this knowledge, Sukracharya could now revive all the dead people. However, Asuraguru as he was, he chose to use the knowledge for the benefit of the Asuras Contrary to the purpose for which he was blessed with this mantra. Armed with the powers of Sukracharya, the Asuras attacked Devas. Sukracharya would revive all the dead Asura soldiers while those on the other hand the Devas were getting killed. Eventually, being hugely outnumbered, Indra lost his kingdom.When the defeated Devas approached Lord Vishnu for his protection, he advised them to perform “Samudra Manthan” to attain Amrit (Nectar) consuming which would make them immortal.
The churning of the Ocean of Milk was an elaborate process. Mount Meru was used as the dasher (churning tool), and Vasuki, the king of serpents, became the churning rope. The gods held the tail of the snake, while the demons (Asuras) held its head, and they pulled on it alternately causing the mountain to rotate, which in turn churned the ocean. However, once the mountain was placed on the ocean, it began to sink. Vishnu in his second incarnation, in the form of a turtle Kurma, came to their rescue and supported the mountain on his back.
Note that the Mahabharata version of the story differs in many respects from the one in the various Puranas such as Bhagawat, Brahma-vaivarta, and Agni. For example, in Mahabharata, it was not Vishnu who took the Kurma avatara, but the Akupara, the king of tortoises, who did it on request from Devas and Asuras.Bhagavat makes a mention of Rahu in the cosmic churning( Samudra Manthan). As the Asuras and the Devas went on churning the milky ocean with Meru parvata, several interesting things like Gems, Poison etc surfaced and the Ambrosia (Amrita) is one among them. Poison was drank by Lord Shiva to save the universe and there after he was also named as Nilkanth (Blue Necked).
Lord Vishnu, disguised himself as Mohini, a celestial dancer and took upon himself the task of distributing the Amrita. In this process, Mohini kept Asuras in one row and Devas in the other. She used to give Liquor to the Asuras and Amrit to the Devas. Seeing this, Swarbhanu (Rahu) suspected her and came to know the reality about Vishnu being Mohini. He disguised as one of the gods and sits in between Surya and Chandra to drink the Amrita.