"There are ten decades in a century."
Translation:Estas dek jardekoj en jarcento.
No - it's more like "Look, here are ten decades in a century (for you)".
It's when you point to something to show it to someone.
Compare to "voilà" or "voici" in French.
So you could "Jen mia libro" (Voici mon livre) for "This is my book (that I am showing to you)".
But "Jen ok planedoj en nia sunsistemo" might work if you are pointing to a diagram, but not for a general statement that there are eight planets.