We know that planets revolve around their parent star (or specifically Barycenter), they maintain a specific speed of revolution so that they don't end up smashing into their planet star. Which force or energy do that, so that they maintain their speed? If we have infinite amount of time, would planets smash into their parent star ?
Newton's First Law of Motion states that "an object in motion stays in motion with the same speed ... unless acted upon by an unbalanced force". On earth, an object in motion will slow down and stop due to air resistance (the unbalanced force), but in space, there is no atmosphere to cause air resistance, so the object will maintain its speed.