Rahu and Ketu are considered shadow planets (chhaya graha). They are mathematical entities whose manifestation is physically evident only during solar and lunar eclipses.
References to Rahu and Ketu can be found in a lot of Hindu scriptures including Rigveda, Atharva-veda and Mahabharata. Per Vishnu Purana, Rahu was a demon (asura, rakshasa) who tasted amrit, the divine nectar of immortality, by deceit, which was being served only to Gods by Vishnu, under the guise of Mohini. Even though Vishnu tried to kill him, he did not die because by then he had already consumed amrit. The upper and lower body parts of that demon were named as Rahu and Ketu.
In the group of planets referred to in Indian astrology, Rahu and Ketu can be considered the only two demons. Every other planet is worshipped as a god in Indian mythology. They do not own any signs, but they do own nakshatras, and are hence assigned appropriate dasha periods in the Vimshottari dasha system. Rahu owns Ardra, Swati and Shatabisha and has 18 years of Vimshottari dasha. Ketu owns Ashvini, Magha and Moola nakshatras and has 7 years out of 120 years of Vinshottari dasha period.
In any of the journals of Indian astrology, no positive words have been used while describing either Rahu or Ketu. First and foremost, they come from a demonic background. Secondly, they are considered to be wile, deceitful and dishonest. After all, Rahu did succeed in tricking Vishnu in serving him amrit. Rahu is described as a liar, a patient of leprosy, foul mouth and full of criticism about others. Ketu is a scoundrel and one with low character. Ketu also has weak lungs because he smokes a lot. Both these planets cause smallpox, leprosy, worms in stomach and other diseases. Rahu especially is responsible for psychiatric disorders and other diseases of mind, especially when conjunct with Moon.
Given such negative characteristics, it would seem surprising that most of us survive the wrath of these planets because they are bound to affect one house or other in our horoscopes.
If placed well in a chart, these planets work to check the very bad qualities they represent. Rahu gives courage, prestige and fame, Ketu works towards providing knowledge and moksha, the final liberation from the cycle of birth and death. Rahu represents ghosts (pret and pishacha). Placed well in a chart, it can bring out extraordinary qualities in terms of supernatural in an individual.
The question is – what does good placement in a chart mean for Rahu and Ketu? Rahu and Ketu do well if they come under the influence of Jupiter and Venus, who are the gurus of Gods and demons respectively. As explained in Brihat Parashara Hora Shastra (BPHS), Rahu is exalted in Taurus and its moolatrikona sign is Cancer. Ketu is exalted in Scorpio and its moolatrikona signs (there are two of them) are Gemini and Sagittarius. Rahu’s negative tendencies can be kept in check by strengthening Mercury. Quite logical, given that Mercury represents intellect and Rahu works towards destroying it.
Rahu’s placement in ascendant gives money to an individual from unexpected sources. However, it is not considered good in house number two, which is the actual house of wealth. Rahu in sixth house gives a lot of power to conquer enemies in a battlefield. Ketu does well in third house, where it gives a lot of money to an individual. Although, this combination cannot be considered good for younger brothers. Ketu in sixth house is a good placement – it makes an individual generous and famous. Ketu’s placement in eleventh and twelfth house is considered extremely auspicious, where it gives results to make a person’s worth equivalent to that of kings.