The 9th house is the most auspicious house in the ascendant (Lagna) chart of the individual. The 9th house governs luck, fortune, destiny, long-distance travels, spiritual visits and pilgrimages, relations with one’s guru or preceptor, religious beliefs, Dharma or righteousness, prosperity, foresight, higher learning, father’s lifespan, father’s prosperity, relations with the father, educational institutions, etc.
Mercury’s placement in the 9th house is generally considered to be benefic or positive for the native unless it is afflicted by any malefic influences whatsoever.
A special note is to be made here that the 9th house is the house of Dharma or religion, and so, Rahu will almost always perform negatively or poorly here since Rahu stands for materialism. This holds true even for Taurus (meaning the ascendant would be Virgo), so Rahu Mercury conjunction in Taurus would end badly for the native as Rahu suffocates in the 9th house of the chart. The same can be said if the 9th house zodiac sign is Gemini (meaning the ascendant would be Libra) as Rahu would be able to provide no material benefits whatsoever in this house.
However, Mercury independently placed strongly in the 9th house will be immensely beneficial for the native. Not only will such an individual be religious, but he/she will also be interested in the study of religious scriptures and maybe perhaps even consider pursuing a career or livelihood in them. If Jupiter aspects Mercury, then this possibility increases manifold.
Additionally, a well-placed Mercury or Budha in the 9th house will give long life to the native’s father.
Some considerations to note here are:
Mercury in exaltation (Virgo) – this is applicable for Capricorn ascendants. Mercury in exaltation here gives full benefits to the individual including name, fame, fortune, prosperity, etc. However, Mercury in the 9th house Virgo would cast His 7th aspect or Drishti on the 3rd house, which is Pisces, or the debilitation sign for Mercury. As a result, the relations of the native would remain strained with his/her siblings.
Mercury in His own sign (Gemini) – this is applicable to Libra ascendants. Mercury in Gemini in the 9th house gives good fortune to the individual along with fame as well as growth and power in the house of destiny. Such an individual also enjoys good relations with his/her brothers and sisters as Mercury’s 7th aspect or Drishti would be in the 3rd house which in this case would be Sagittarius where Mercury would also perform well.
Mercury in debilitation (Pisces) – this is applicable to Cancer ascendants. Mercury in debilitation in the 9th house will make the person stray away from Dharma and they will struggle to maintain their faith in God (this will reverse if an exalted Jupiter aspects the placement). Additionally, the 7th aspect or Drishti of the debilitated Mercury on the 3rd house may also affect the native’s relations with his/her siblings.
Mercury Ketu conjunction – in the 9th house, Ketu will overpower Mercury in nearly any zodiac sign, even in exaltation or in Gemini where Mercury is His own sign. However, Ketu in the 9th house will make the individual undoubtedly spiritual.