QUESTION:
Why do we have to use plutonium? Why not use solar panels? 

ANSWER from FAQ:
Spacecraft that travel at or within Earth's orbit can use solar energy
to power their instruments. However, at the great distance of
Jupiter, the only feasible power source means using Galileo's
Radioisotope Thermal Generators (or RTGs for short). Galileo
would need a minimum of 700 to 1,600 square feet of solar
panels--a solar panel about the size of a house! 

Unlike other power sources, the RTGs are insensitive to the
freezing cold of space, and are virtually invulnerable to high
radiation fields, such as Earth's Van Allen belts and Jupiter's
magnetosphere.