– Xochielt Sanchez
Answer by Robert Frost:
The galaxies Hubble sees from so long ago are huge. Pluto is small. Their respective distances don't compensate for that difference.
Hubble's Wide Field Camera (WFC) has a resolution of about 0.05 arc seconds.
We can figure out what that means with a little trigonometry.
Tan(theta/2) = opposite side (o) / adjacent side (a)
Pluto is about 4.71 billion km away from Earth, right now. That gives us a value for (a). So solving for (o) we get 570.8 km. That means the smallest resolvable object is 1141.7 km across.
Pluto is 2400 km across.
That means that Pluto would be 2400/1141.7 = 2.1 pixels across – in other words, a smudge.