In Harry Potter's third year, Harry had to restrain himself from buying a perfect moving model of the Milky Way galaxy. Perfect model implies that wizards know everything that is in the galaxy and what it entails. Could that perhaps mean all of its stars and possibly ones that sustains life?
Well, what it really says is "galaxy", not "Milky Way." Although it's likely the model is of the Milky Way, the fact that wizards might know more than Muggles about different galaxies and universes does leave room for a shadow of a doubt that the model was of the Milky Way. Again, I think it probably is, but it's not stated outright.
He was sorely tempted, too, by the perfect, moving model of the galaxy in a large glass ball, which would have meant he never had to take another Astronomy lesson.
Prisoner of Azkaban - page 43 - Bloomsbury - chapter 4, The Leaky Cauldron
J.K. Rowling also doesn't explain why owning such a model would enable Harry to never have to take another Astronomy lesson. In the 10 canon books, Pottermore, and J.K. Rowling interviews, I have never heard J.K. Rowling address whether wizards would know more about the galaxy than Muggles.
When I read "perfect, moving model..." it sounds to me that perfect is just a way of saying "absolutely-awesome-looking model" rather than "100% precise model".
As for the reason of not having to take another lesson seems more of a hyperbole indicating that the model is so nicely done that it provides lots of info making the student's life too easy. Like the best guidebook ever written, which helps a lot, but does not justify missing classes.
Note that this was just my interpretation of that part, and I can't really tell whether wizards truly know more than muggles about astronomy.
One thing to consider is that the "perfect, moving model" was a comment driven by Harry's POV. Harry isn't somebody that would have a perfect enough understanding of the universe that he could identify the model that way at a glance and be correct in his identification. The description, therefore, is more likely to be coming from the packaging.
"Buy this perfect, moving model of the galaxy! Only 6 easy payments of $19.99!"
I suspect a better question than whether wizards have a perfect understanding of the Milky Way is whether wizards have any laws controlling truth in advertising. :)
Muggles are bound by the universal speed limit (i.e. the speed of light), but Wizards can travel instantaneously using Apparition (a.k.a. teleportation). Hence they are not bound by the speed of light. Neither do they need rocket or rocket fuel. These are the factors restricting our space explorations. And since these factors do not exhibit any problem for wizards, we can simply assume that they would have a greater understanding of the Universe (and that includes our Milky Way).