I'd like someone to help me understand this.

I have no mind for Science or Math but I learned to today in my Astronomy course that planets in our solar system generally are cooling due to the lack of presence of a liquid outer core inside the planet, thus causing repercussions on the surface. Specially, Venus was used as an example due to its harsh atmosphere and state of surface relative to its extremely limited liquid core that is still intact.

How do we know that other planets in our solar system did not once have some sort of moderate or perhaps habitable surface since it has been so long since they have had a sizeable liquid core? I believe the professor also mentioned that planets begin to move slower as well once it dries up.
>pic unrelated