system
Given the following post, respond as if you're commenting on Reddit:
human
How clearly can we see objects in other galaxies?

I have always been curious as to just what kind of details we can see, and what can only be theorized. For instance, we can count how many moons Saturn has, but can we tell the same for a planet in another solar system? To my understanding it would really come down to just how much light is being reflected from these planets and moons. I guess I don't really have a direct and clear cut question, so I suppose I'll just ask this, what criteria is needed for us to be able to know there is an object in space?
gpt
Our telescope resolution is what limits us to see detail on objects in other galaxies.  An object can be incredibly bright, but unless the optics you're using have the proper resolving power, it will look like a big bright blur.

Telescopes have specific resolution that depend on their size and ability to see beyond the turbulent atmosphere.  Hubble's resolution is about 0.1 arcseconds.  An arcsecond is the *angle* an object in space subtends -- that angle can be converted very easily to a "real size" if you know how far away the object is.  So for the galaxy M51, which is ~ 7 Mpc away, HST can resolve objects that are as small as (0.1 arcsec)(7 Mpc)/206265 = 3.394 x 10^-6 Mpc, or 7 x 10^5 AU, where 1 AU = the distance between the Sun and the Earth.

So HST would not be able to "separate" the Earth and the Sun (or the whole solar system for that matter), and in its optics the system would appear as a single dot.

EDIT: clarification to the last sentence...  "HST would not be able to "separate" the Earth and the Sun *at the distance of M51*"