Circular orb photographed since an airplane

On December 15, 2014, a Mexican was flying in aircraft over the Serra Madras. He used this opportunity to take some photos of landscapes. Later, he noticed with amazement an unidentified circular orb.

Witness report:

I was flying into POV Mexico... descending over the Serra Madras into POV.
Taking shots of the mountain peaks, the rivers/roads and rock formations.
I didn't notice it at all until I was reviewing my photos from my trip.
I thought it might have been an ice crystal, or a spot until I reviewed the other shots taken seconds before and after. There were no other marks similar to the circle.