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My question is which of the outer planets you could see with the naked eye if you were in close proximity to them. It's pretty obvious the human eye can see the inner planets through to Mars, but I know that space probes to the outer planets need sensors designed for extremely low light conditions, and the pictures we see are often enhanced to make them brighter than they would actually appear. So:

  • Could you tell if a planet was there against the blackness of space?
  • Could you see details?
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  • $\begingroup$ "close proximity" It depends how close you mean by 'close proximity'. So, are we talking 1 km, 1000 km, 1,000,000 km..? $\endgroup$ – Andrew Thompson Jun 25 '16 at 14:54
  • $\begingroup$ By close proximity I mean in orbit or a flyby, think the orbit of Ganymede around Jupiter $\endgroup$ – GdD Jun 25 '16 at 15:15
  • $\begingroup$ The stars will maintain the same brightness and pattern (visually) no matter where you are in the solar system, so even if you couldn't detect reflected light from the planet, you might notice a chunk of the sky where the stars disappear. But it would depend on how large the planet's disk is as seen from your position. If you went 'behind' the planet, the eclipse of the Sun may be noticeable as well, again depending on distance to sun and to planet. $\endgroup$ – uhoh Jun 26 '16 at 0:49
  • $\begingroup$ This may contain information slightly helpful for someone attempting an answer: http://www.telescope-optics.net/eye_intensity_response.htm. $\endgroup$ – uhoh Jun 26 '16 at 1:03
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    $\begingroup$ three of the outer planets can be seen with the naked eye even from the earth $\endgroup$ – szulat Jun 29 '16 at 23:22
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You can try for yourself how bright it is on Pluto. High noon on Pluto is as bright as a certain point during dusk/dawn on Earth. I tried this, at that point it's bright enough to make out details. My conclusion is that you can see the daylight side of every planet incl. Pluto with the naked eye if you're close enough.

This does depend on the planet's albedo. At 14.5%, Pluto isn't very bright, but some bodies have lower albedos (comets like 99P Chury are around 5%) so you might not be able to see some of the smaller bodies in the solar system.

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  • $\begingroup$ Dawn/Dusk is far more light than you need to see it--you'll be able to see objects considerably farther out than Pluto even without using stellar occlusion. $\endgroup$ – Loren Pechtel Jun 27 '16 at 2:44
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    $\begingroup$ The human eye has an astonishing dynamic range, and can adapt to extremely low light levels easily. $\endgroup$ – Russell Borogove May 29 '18 at 16:29
  • $\begingroup$ I always remembered it as light on pluto being approximately the same as indoor lighting at home. $\endgroup$ – Ingolifs Oct 16 '19 at 18:26
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Bright sun light of a clear sky is about 100,000 Lux, room light at night about 100 Lux, street light at night about 10 Lux. We are thus able to see with 1/10000 of the brigthness of summer day with clear sky. If the distance to the sun is 10 times greater, brightness is 1/100 smaller. Seeing an outer planet in a distance of 100 au to the sun (1 au is the distance between sun and earth) is therefore possible if we are close enough to see it without a telescope.

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  • $\begingroup$ I suspect your last conditional should be "close enough to resolve it with the naked eye", since otherwise it's rather self-referential. $\endgroup$ – Nathan Tuggy Jun 29 '16 at 19:40

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