Galileo discovered that the planet had four satellites.
jupiter
mars
venus

The way I understand it, Galileo was the first one to use one of
Leeuwenhoek's new-fangled "telescope" things to look at
astronomical objects.  Fortunately for Galileo, his telescope was
so poor that he didn't instantly blind himself when he looked at the
sun with it.  He had a lot of fun looking at craters and mountains
on the moon, and when he turned it toward the planets, it was
good enough to let him see the four brightest moons of Jupiter. 
(You can see them too, with only binoculars.)
This was the first time that anybody ever saw anything in the sky
orbiting anything else but the Earth.  He soon realized that if those
little things could orbit Jupiter, then anything could orbit anything,
and the Earth didn't need to be the center of the universe. 
When he started telling other people about this, he got into big trouble.