This is a Glory. It looks similar to a rainbow, but forms in a different way. Where a rainbow reflects and refracts the light back towards you, forming an arc, the glory is formed as sunlight hits the tiny droplets in a cloud or fog. The light is scattered, as well as refracted and reflected multiple times. This gives you the colors of a rainbow, but depending on the size and concentration of the droplets, the rings are very close to the center (5 to 20 degrees), instead of 40 to 42 degrees from the center, as they would be with a rainbow.

The center of the glory is directly in line with the Sun and the viewer, so you can tell where my seat was on the plane by finding the center.