So it's not a planet, is it?

NASA's New Horizons spacecraft is past Pluto, and pictures taken looking back show evidence of clouds on the ninth planet.  Scientists theorized that methane clouds could exist, and this might be what we are seeing.

Taking these rearward shots of Pluto shows things the spacecraft's approach photos don't, thanks to sunlight reflecting off the dwarf planet's surface and illuminating the haze in its thin sky. The top of the photo shows long wispy formations that could be clouds, the first evidence of any from New Horizons' photo collection and possible proof of the methane clouds predicted by models of Pluto's atmosphere. The bottom of the image shows a cleaner silhouette of mountain ranges and valleys.

This photo was taken from 13,400 miles past Pluto 19 minutes after New Horizon's closest approach. At that distance, the picture has a higher resolution at 1,400 feet per pixel than shots taken as New Horizons headed toward the dwarf planet.