The brightness of an astronomical unit is described as its 'magnitude', mag. for short. Magnitude is a logarithmic order of scale. All that means is it is not a geometric or linear relationship between magnitude one to magnitude two to magnitude three, etc. Roughly though, it means that magnitude 1 is 100X brighter than magnitude 6.
How does this affect what we see through the telescope? Well, you need to remember that a telescope collects light. The larger it is, the more it can collect, and in doing so it means that fainter and fainter objects can be seen as aperture increases. It has nothing to do with magnification, but, BUT, there are a few practical things that we can do using magnification to push things along to the limits of our instruments. This needs us to understand certain things about 'magnitude', our scopes, and atmospheric conditions.
I wrote 'magnitude' as it is not a straight forward thing to understand in astronomy. A star may be labeled as 'magnitude 8'. A galaxy may also be described as 'magnitude 8', but we can easily see the star, but not the galaxy! What's going one here??!!??
Thing is, extended objects, such as galaxies, nebulae, clusters, etc, have their magnitude determined as if the entire area of the object were put into a single point source, like a star. So that magnitude 8 galaxy apparently fainter than the star only because its light is spread out over an area.
The amount of light our telescopes can gather is determined by the size of the objective lens or primary mirror. Like I wrote earlier, the larger the aperture, the fainter the objects that can be seen. But it is not that simple either. Our very human eyes have a factor in this. The older we get, the smaller our pupils can dilate to. A 15 year old's pupils can dilate to 7mm. Me at over 40, closer to 6mm. This has an influence on how faint we can see. And only further complicating things, as we increase magnification, we reduce the size of the exit pupil coming through the eyepiece, meaning that once the exit pupil shrinks to below 6mm, I can now see things just as faint as that 15 year old.
Then there is the atmosphere. Thermal currents in the atmosphere will degrade the quality of the seeing. In turn, this reduces how faint we can see as more of that precious stellar light is scattered by the atmosphere than reaches our eyes.
Yes, a lot of material to digest here. And this is not all of it!
There are other definitions that need to be understood, but can be covered later as one's experience increases, and not to confuse things even more. But it is helpful to know the theoretical limits of our scopes so we can plan viewing sessions and select targets. No use selecting a magnitude 15 object if the limiting magnitude of one's scope is mag 15. You just won't see it.
This following link will take you to a magnitude calculator into which you can put in your set of parameters. You will notice that the limiting magnitude will vary a little as certain parameters are changed - quality of optics, age of observer, magnification being use, aperture of telescope:
http://www.cruxis.com/scope/limitingmagnitude.htm
And for what it's worth, this last link will give a list of magnitude values for various objects, from the Sun through to camera limits:
http://www.icq.eps.harvard.edu/MagScale.html
Jemmo, to finish answering your question, if Pluto is at magnitude 17 (as an example), it will be totally invisible in a 10" scope visually.
