The eyes have impressive dynamic range.

Moonlight is 250000x dimmer than sunlight, but you can still see things illuminated by moonlight once your eyes adjust.

Neptune is 29 AU from the Sun, so by inverse square law, the sunlight is 29^2=840x dimmer on Neptune than Earth. Visibility should be decent out there. Neptune will stand out pretty well against the perfect blackness of space.

> perfect blackness of space.

Eyes dynamic range is good enough that the "background" isn't black either, consider the view of the night sky from outside a city.

I figure the diffuse glow up there is light being reflected around by the atmosphere, which isn’t present in most of the solar system.

Hmm, I've always figured it's a bit of eyes blurring "bright" lights (the stars you can see), and a bit that there are lots of really dim stars that you can't really see, but add in some light.

I don't actually know though.

> lots of really dim eyes

Think you got your nouns mixed up there ;)

Oops, fixed, thanks :)