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When we see a star it looks much bigger in diameter to us than it really is, this picture (extracted from here) explains what I mean:

Star as we see it and as it really is

Notice that the point we see in the skynight, represented by the yellow outer circle, would actually encompass even planets orbiting the actual star, represented by the black dot in the center. In this case we see one planet inside the white circle.

My question is, can the effect of distance and brightness be measured as to know how far would we must be to watch a star that encompasses neptune?, the earth? or does it depend on different factors?

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    $\begingroup$ I'm not sure I entirely understand your question. This is about diffraction, right? So, since the biggest factor would be the atmosphere itself (not accounting for any diffraction of the interstellar medium), where in the Orion belt would we be looking from? From a hypothetical planet similar to Earth with the atmosphere like ours, or in outer space and all we're interested in is diffraction of the ISM and our Sun's own heliosphere? $\endgroup$
    – TildalWave
    Commented Dec 24, 2013 at 21:16
  • $\begingroup$ I didn't know the difference was caused by diffraction, this would've been part of the answer I was looking for. Thanks. $\endgroup$
    – Eduardo Serra
    Commented Dec 25, 2013 at 2:07
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    $\begingroup$ The observed image of a star is not a sharp edged circle. There are two effects which prevent us from seeing the actual stellar edge (which can be seen for the Sun). First, atmospheric seeing and second diffraction. The first jiggles the image on very short time scales (causing a sparkling appearance) and is caused by turbulence in the upper atmosphere. This effect may be quantitatively and/or qualitative different when observing from any other planet, say in Orion. $\endgroup$
    – Walter
    Commented Dec 25, 2013 at 14:00

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Such observations are possible, see Fomalhaut b. It depends mainly on the diameter of the objective of the telescope (diffraction limit) and contrast. Telescopes can (at least in theory) be combined to an effective telescope of larger diameter (aperture) by interferomentry. Masks can help to occult bright stars to overcome the contrast limitatios.

There are indirect methods to "see" faint planets, as e.g. applied in the Kepler mission, or by Gaia.

And yes, it is calculable. With naked eye we wouldn't be able to distinguish the sun from the planet orbits from a distance of about 1000 lightyears, as the orion belt stars: 1000 lightyears are about 300 parsec. This means, that the earth orbit would be about 1/300 arc second. The resolution of the human eye is about 4 arc minutes, the 72,000-fold. For Pluto it's still more than 1000-fold.

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