Rav Moshe’s discussion of peanuts – which he spells phonetically with hebrew charcters – can be found in Iggrot Moshe OC 111:63. He reasons that kitniyot is a minhag and a minhag should not be stretched to ban things never intended to be banned – like peanuts, which were not really a commonly eaten food at that time.
Here is a translation of the Rav’s famous tshuvah on peanuts translated into English:
“Concerning peanuts, which were called stashkes in Europe—they have been accepted as being permitted on Pesach and are not considered kitniyos (legumes that are forbidden on Pesach) because all the reasons for the prohibition of kitniyos do not apply to peanuts. Peanuts are not sown in fields (with grain), and even if they were there is no fear that grain would be mixed together with the peanuts; bread is not baked from peanuts; and generally speaking though they are vegetables they have the appearance of nuts rather than kitniyos. And even though I have heard that in some places they were considered kitniyos, peanuts should not be forbidden in places where it is not known for certain what the custom had been in their city, because, with reference to kitniyos, when in doubt one should be lenient.
Therefore you may give certification for peanuts and the oil derived from them, and they will be permissible to the majority of persons. Those who know for certain that the custom of their city was not to eat peanuts on Passover should not eat them; others are permitted to eat them.”