Peanuts are botanically legumes like beans and peas. Peanuts are one of the famous food items people fight about on pesach. Some people have the custom to treat peanuts as kitniot because they are legumes, some people think this is one of the more stupid halachic discussions because they are no way like beans.
Rabbi Moshe Feinstein (Iggerot Moshe, OC III:63) famously allows peanut on the reasoning that they were not known at the time of the original degree. When they became known, there were authorities that considered kitniot a "foolish custom" and refused to add new items to the list (kashrut.com, myjewishlearning).
The OU is too careful to say "folks you are just plain wrong" so they write "There are some people who have the custom to not eat peanuts on Pesach, and they should continue to keep the minhag, but those who do not have this custom may eat peanuts on Pesach." (OU) – read: your parents were mistaken, but now you’re stuck with it.
So, many people agree that peanuts are not kitniot. One should think that this carries over to peanut products being certified as kosher lePesach. But the OU has stopped "in recent years" to certify peanut oil as kosher for pessach (OU Guidelines Nuts). So even people who would eat peanuts cannot.
Quick fact check for our real-life kitniot dangers: Crop rotation could be done with peanuts, don’t know if people really do that. Peanut flour seems to exists, though like lentil flour and the like I doubt it is used that often. When peanuts are ground to flour there might of course be a danger that it is done on the same equipment as real chametz. This is where certification would be useful. In my daily life I use peanuts only whole – as a snack or as a fancy ingredient for a fancy dish. In my view, there is absolutely no possibility to confuse whole peanuts with grains.
Like we discussed before with soy beans, oil made out of peanuts definitely falls under the reasoning of two doubts (1 – peanuts were not part of the original minhag, 2 – liquids cannot be confused with chametz), which would be reason enough to permit peanut oil (Machon shilo).
- Danger of chametz traces: Crop rotation?
- Danger of confusion with chametz (raw): No.
- Danger of confusion with chametz (processed): Yes (peanut flour).
- Botanical categorization: Legume.
- Verdict: Not kitniot.
photo (c) Pollinator