Purple UFO caught over Philadelphia, USA

On February 19, 2017, in night, a witness was in an airplane over Philadelphia (Pennsylvania, USA) when he has noticed some strange purple lights moving in the sky. He took photos of them. What is it?

Witness statement:

I was onboard a plane landing at the Philadelphia airport taking a photo of the city and then stars for my daughter. Upon checking the photo of the stars I notice the purple circles in the frame and looked out the window as I could not determine what I had observed. The purple circles were not visible to the naked eye. I took a second photo about 20 second later and observed the purple lights a lot further in the distance after which took a third photo after zooming in on the dots. The dots appeared to have moved quickly away from the plane and after initially been grouped together had separated as they moved away from the airplane.

I was pretty shocked to have seen the photos and showed a couple gents sitting next to me who also couldn't explain the event.

The object moved out of sight