The Spitzer Space Telescope will be able to study many different objects in our Solar System,
our Galaxy, and the Universe.
Spitzer was designed to study these science areas:
While these are Spitzer's main science themes, it will also be able to study many other things, like the outer solar system (Jupiter, Saturn, Uranus, Neptune, and Pluto) and early star formation.

What is the Spitzer Space Telescope?
What is the Purpose of the Spitzer Space Telescope ?
Building the Spitzer Space Telescope