Ninth house is the house of dharma and is governed by Jupiter. Mercury placement provides mixed results but with Sun or Jupiter as co-tenant it may give more favorable results. Mercury in ninth house is great for higher studies or for doing business in foreign countries. 

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     According to B. V. Raman: "Natives with Mercury in ninth house would acquire good education and wealth. They could also be scholars in their fields. They would have a scientific mind. They may travel abroad for seminars or lectures. They would have cordial relationship with father". Ninth house rules over faith. Mercury depending on astrological sign and co-tenant can make a native quite rigid and one who follows all the customs and practices with full devotion. But sometimes Mercury keeps true to his nature and promotes scientific and rational thinking. Such natives often question age-old traditions and customs if they don't find the logic relevant to current society. The ones who question everything are generally disliked but they are quite necessary cause change is essential with time.

   Mercury in ninth house is great for education. Education which involves logic, analytical skills, scientific reasoning, mathematics as well as law are places where natives with Mercury in ninth house excel. Similarly career as a lecturer is also likely to fetch a healthy income. Mercury in ninth house may give plenty of travels. Depending on other factors there is a good chance that native can go abroad for higher studies and settle there. Career may involve frequent short-distance or long-distance travels.

   Mercury in ninth house is a decent placement. It gives good chance to visit foreign countries for business related purpose. It can give good education and even let native settle abroad via opting for foreign education. Just in case there are troubles natives are advised to pray to Lord Vishnu. Depending on astrological signs and co-tenants native can either be quite rigid in following customs and traditions or follow a totally scientic approach to life which questions all matters of faith and spirituality. In both cases natives are advised to avoid imposing their beliefs on others or being vocal critics of others who don't align with their spiritual and religious views.

References:
Raman, Bangalore Venkata. How to Judge a Horoscope. Delhi: Motilal Banarsidass, 2014. Print.