And how exactly did we find out how they move?
In: Planetary Science
Its theoretically possible, but it would need so perfect finely-tuned conditions to stay circular or even become it in the first place. Even a slight variation in gravity due to, lets say another body maybe an asteroid or another planet, would shift the planets trajectory enough so it deteriorates to an elliptical orbit.
Its something we would call metastable, in the sense that it only needs one tiny change to make it not circular anymore. In reality there is nothing so perfect in nature.
Eli5: imagine balancing your phone on its side. If you could find the almost-impossibly perfect balancing point it would stay there, its somewhat stable. Theoretically it could stay there forever, however, as soon as something moves it ever so slightly, even just the air around it or the ground underneath your building vibrating ever so slightly, it falls back to the real stable position of lying there. So its not fully stable, its metastable. Here the balanced position is the circular orbit, and the stable fallen down position is the elliptical orbit.
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