Speaking as someone who had made a lot of orb photos with two different cameras, I have not a clue. I have pretty much ruled out dust particles, the popular Internet debunking method. I have shot orbs in the midst of a misting rain that has lasted a while. I really don't think dust particles could have survived. Besides orbs are perfect fractals and it's hard to believe that each dust particle is the same shape. I would like to say atmospheric conditions, but that doesn't seem to work either as I have two friends who are constantly trying to photograph orbs. Any time the orbs are around, I call them, but they capture no orbs. I have photographed them both in Arkansas and in Lousiana. I will say this, and this comes from a real skeptic: When I am in a very peaceful and spirtual frame of mind, I always find them -- or they find me. Most of the ones in my backyard are over where many of our pets from our 46-year marriage are buried. In Arkansas, some of them are over where the hunters skin the deer, but others are at the creek and down a trail I often walk. When we were staying at a downtown hotel during one of Joe's conventions, I photographed the Greater New Orleans Bridge from a patio at the Hilton. The left side of the photo had orbs.
So, after rambling here and there, I have no clue. I am convinced that science doesn't know or the answer would have been forthcoming. I am honored that I am able to capture them in photos, for whatever reason.