This case of a "UFO" submitted to Mufon, is described as:
The first impression was correct. However the resultant photo seems at little confusing.
The problem is that you naturally interpret this as a set of three lights. But what it actually is is a single light, the red collision avoidance strobe. This is a very bight flashing light, and illuminates other parts of the plane, even in daytime.
In this case it's illuminating the engines.
Another clue are the two faint bright red spots under the main light
This matches quite well with the wheel wells of a 737
The time given is 2018-05-08 8:59PM, that's 2018-05-09 (next day) 2:59AM in UTC. The camera exif time gives the same UTC time - maybe that's common for astrophotography? Looking at that location a likely candidate is SWA1988 from Phoenix to Omaha, which was passing just to the South at 39,000 feet.
Another thing that throws people off here is the thin lines. These are the long exposure traces of navigation lights, but seem odd as you can only see two of them, and they don't seem to line up with the other lights. However if you look carefully, or just boost the levels, then you can see there actually are three lines.
What's more the one of the left is green and the on on the right is red, which matches standard wingtip lights.
They don't line up for a somewhat unintuitive reason - planes do not fly in the direction they are facing. The fly in the direction they are facing at their airspeed PLUS the velocity of the wind relative to the ground. So if there's a crosswind the motion of the plane can be 5° or more off from the direction it is facing.