surface brightness and resolution.<br /><br />First, we can detect planets around other stars, usually, only by indirect evidence (star wobbles, eclipses etc). When we do actually see the planet (which we've done what, only a couple of times?) it's as a very faint point. We see no real features, any information we have about the actual planet comes from spectroscopy.<br /><br />We can see features and such on pluto. But there is one thing: it isn't very bright. Those planets we see reflect a lot of light from the parent star, their surface brightness is high. Pluto, doesn't have much light to reflect, and so it's surface brightness is low (even if it is closer). <br /><br />surface brightness, IIRC doesn't really change with distance, and is a key factor in how well objects show up in photographs.<br /><br />For instance, the andromeda galaxy puts out a LOT of light, and covers a large area of the sky (something like 8x the area of the moon?)..but we can't see it much, since the light is spread out over the entire disk of the galaxy. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>