I was watching a documentary on a mission to study Pluto. At 21:11, an image is presented with an arrow mark pointing a small dot which is now identified as Pluto.

As there are hundreds of dots on the image (I'm not sure what kind of an image this is as I'm not an astrophysicist), how can stars, planets, asteroids and other objects be distinguished in this image? And what kind of an image is this?

Enter image description here

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    $\begingroup$ You don't identify it from a single plate. $\endgroup$ – dmckee Mar 9 '14 at 4:26
  • $\begingroup$ @dmckee I think I get it now. $\endgroup$ – user6123723 Mar 9 '14 at 4:26
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    $\begingroup$ Well, one of our astronomers will no doubt be able to give you the gory details. $\endgroup$ – dmckee Mar 9 '14 at 4:29
  • $\begingroup$ @dmckee Feel like a dunce right now as the documentary a minute later explains how it is identified. But I'll keep this question open as I may find out more details (technical) about how it is done. Thanks again! $\endgroup$ – user6123723 Mar 9 '14 at 4:34

One point doesn't tell you much of anything, other than that there is probably something in that direction. Pluto was discovered the same way many asteroids are, by comparing pictures of the same star field at different times, seeing if anything changes.

Below is the discovery image from Wikipedia.

Pluto discovery images

The dot moved across the image, while all the stars stayed the same. Of course this is not conclusive proof. More followup was done to ascertain that we really were seeing something orbiting the Sun far away. After all, it is possible that the two dots are unrelated - they could be fleeting glimpses of two different asteroids, or two unfortunate cosmic rays (though the photographic plates used are less susceptible to this than CCD's).

By the way, they had a special machine back in the old days for doing this sort of search - a blink comparator. Astronomers would mount the (glass) photographic plates in the machine, and rapidly switch the view from one to the other. They simply relied on their eyes to detect differences between the frames.

Once you know the orbit, you can predict precisely where Pluto will be at any given time. This enables you to figure out which "dot" it is in other photographs.

I should also mention that with more information than a single broadband image, one can infer more things about point sources. If you can take images in multiple bands, or better yet do spectroscopy, you can see if the light emitted by the object is consistent with various types of stars, for instance.

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    $\begingroup$ At the Lowell observatory, they have the blink_ comparator set up, with what appears to be original glass plates. You can look through the eye piece and see Pluto move. This is very, very cool to see with your own eyes. $\endgroup$ – Wayne Conrad Mar 9 '14 at 9:42
  • $\begingroup$ Unless near opposition (retrograde motion), the rate of apparent movement is an indication of how far out it is, right? $\endgroup$ – Peter Mortensen Mar 9 '14 at 10:48
  • $\begingroup$ OK, this may not be too useful for the outer planets (Neptune is in retrograde motion 43% of the time), but couldn't the Earth's motion be compensated for? $\endgroup$ – Peter Mortensen Mar 9 '14 at 11:01
  • $\begingroup$ @PeterMortensen Assuming circular, coplanar orbits, then I think measuring just the position of the planet and the rate of change of that angle (i.e. using just two images) gives you enough information to find its distance. But eccentricity adds another parameter, so you need more observations. Of course, most of Pluto's motion on sky is actually due to us moving, so in the limit that it can be taken to be still, you can again approximate its distance with just two measurements. $\endgroup$ – user10851 Mar 9 '14 at 17:45
  • $\begingroup$ @PeterMortensen IIRC, they used two plates to identify "things that are not stars" because the stars stay fixed relative to each other. Once they have identified an object of interest, they use more plates to try to construct potential orbits/hyperbola, from which the trajectory can be inferred. $\endgroup$ – RBarryYoung Mar 9 '14 at 17:46

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